Measurement of charm production at central rapidity in proton-proton collisions at $\sqrt{s}$ = 7...

$: Measurement of charm production at central rapidity in proton-proton collisions at $\sqrt{s}$ = 7 TeV$
EUROPEAN ORGANIZATION FOR NUCLEAR RESEARCH

CERN-PH-EP-2011-181November 5, 2011

Measurement of charm production at central rapidityin proton–proton collisions at

√s = 7 TeV

The ALICE Collaboration∗

Abstract

The pt-differential inclusive production cross sections of the prompt charmed mesons D0, D+, andD∗+ in the rapidity range|y| < 0.5 were measured in proton–proton collisions at

√s = 7 TeV at

the LHC using the ALICE detector. Reconstructing the decaysD0 → K−π+, D+ → K−π+π+,D∗+ → D0π+, and their charge conjugates, about 8,400 D0, 2,900 D+, and 2,600 D∗+ mesons with1< pt < 24 GeV/c were counted, after selection cuts, in a data sample of 3.14×108 events collectedwith a minimum-bias trigger (integrated luminosityLint = 5 nb−1). The results are described withinuncertainties by predictions based on perturbative QCD.

∗See Appendix A for the list of collaboration members

CERN-PH-EP-2011-18103 November 2011

Charm production at central rapidity in proton–proton collisions at√

s = 7 TeV 3

1 Introduction

The measurement of the production of hadrons containing heavy quarks, i.e. charm and beauty, inproton–proton (pp) collisions at LHC energies provides a way to test, in a new energy domain, calcula-tions of quantum chromodynamics (QCD) processes based on the factorization approach. In this scheme,cross sections are computed as a convolution of three terms:the parton distribution functions of the in-coming protons, the partonic hard scattering cross sectioncalculated as a perturbative series in the stronginteraction coupling constant, and the fragmentation function, parametrizing the relative production yieldand momentum distribution for a charm quark hadronizing to aparticular species of D meson. Recentimplementations of such calculations, at the perturbationlevel of next-to-leading order or at fixed orderwith next-to-leading-log resummation (FONLL) [1] describe well the beauty production cross sectionmeasured in pp collisions at

√s = 1.96 TeV at the FNAL Tevatron collider [2–4] and in pp collisions at√

s = 7 TeV at the CERN Large Hadron Collider (LHC) [5,6]. The production cross section of charmedhadrons (D mesons) at Tevatron is reproduced within the theoretical uncertainties of the calculations aswell [7–9]. However, the comparison suggests that charm production is underestimated by the resultsobtained with the central values of the calculation parameters, as observed also in pp collisions at theBNL RHIC collider at the lower energy of

√s = 200 GeV [10, 11]. In this context, it is particularly

interesting to perform the comparison for charm productionat the LHC energy, which is more than threetimes higher than at the Tevatron. Furthermore, at LHC energies, the measurement of charm produc-tion in the low transverse momentum (pt) region probes the parton distribution functions of the protonat small values of parton fractional momentumx and squared momentum transferQ2. For illustration,using a simplified 2→ 2 kinematics at leading order, c quarks (mc ≈ 1.5 GeV/c2) with pt ∼ 2 GeV/cand rapidityy ∼ 0 probe the parton distribution functions atx ∼ 7×10−4 andQ2 ∼ (5 GeV)2, where thegluon component is dominant. In this kinematic regime, the gluon distribution may reach the level ofsaturation, leading to a measurable departure of the observed cross sections from the expectations basedon the factorization approach (see e.g. [12]).

We report on the measurement of the production cross sectionof the prompt charmed mesons D0, D+,and D∗+, in pp collisions at

√s = 7 TeV, reconstructed in the range 1< pt < 24 (16 for D0) GeV/c and

|y|< 0.5 with the ALICE experimental apparatus [13], using data collected in 2010. The detector layoutand the data sample used for the measurement are described insection 2. The D meson reconstructionprocedure, the raw yield extraction, and the corrections applied to obtain the production cross sections,including the estimation of the prompt D meson fraction, arepresented in sections 3 and 4. Finally, theresultingpt-differential cross sections are reported in section 4.3 and compared to QCD predictions.

2 Detector layout and data collection

D0, D+, and D∗+ mesons, and their charge conjugates, are reconstructed from their decays into chargedhadrons in the central rapidity region utilizing the tracking and particle identification capabilities of theALICE central barrel detectors.

The ALICE apparatus is described in detail in [13]. It consists of a central barrel covering the pseu-dorapidity interval|η | < 0.9, a forward muon spectrometer, and a set of small detectors in the forwardregions for trigger and event characterization purposes. Only the detector features that are relevant forthe D meson analysis are presented here. The ALICE global coordinate system is right-handed, with theorigin coinciding with the geometrical centre of the central barrel, thez axis directed along the beamline, thex axis in the LHC (horizontal) plane, pointing towards the centre of the accelerator, and theyaxis pointing upward. The central barrel detectors are positioned within a large solenoid magnet, with afield B= 0.5 T, parallel to the beam line. Data collected with both magnet polarities were used for thisanalysis.

The innermost detector of the ALICE central barrel is the Inner Tracking System (ITS) which is made

4 The ALICE Collaboration

of six cylindrical layers of silicon detectors, with radialdistance to the beam-line between 3.9 cm and43.0 cm. The two innermost layers, with average radii of 3.9 cm (about 1 cm from the beam vacuumtube) and 7.6 cm, are equipped with Silicon Pixel Detectors (SPD), comprising 9.8×106 pixels of size50(rφ)×425(z) µm2, with intrinsic spatial resolution of 12(rφ)×100(z) µm2. The signals of the 1,200SPD readout chips provide a fast trigger through a programmable logic. The two intermediate layers,at radii of 15.0 and 23.9 cm, are made of Silicon Drift Detectors (SDD). They allow one to measurethe hit position alongz with resolution better than 30µm from the centroid of signals collected on theanodes, and to determine therφ coordinate from the drift time with a resolution that depends on thelevel of calibration, as discussed below. The two outermostlayers are made of Silicon Strip Detectors(SSD), located at radii of 38.0 and 43.0 cm, consisting of double-sided silicon strip sensor modules,with an intrinsic spatial resolution of 20(rφ)× 830(z) µm2. The total material budget of the ITS ison average 7.7% of radiation length for tracks crossing the ITS perpendicularly to the detector surfaces(η = 0) [13, 14]. These features enable measurement of the track impact parameter (i.e. the distance ofclosest approach of the track to the primary interaction vertex) in the bending plane (rφ ) with a resolutionbetter than 75µm for transverse momentapt > 1 GeV/c (see section 3.1).

The ITS modules were aligned using survey information, cosmic-ray tracks, and pp data, with themethods described in [14]. For the residual misalignment along therφ coordinate, a r.m.s. of about8 µm for SPD and 15µm for SSD modules was estimated [14,15]. For SDD, with the current calibrationlevel, the space point resolution alongrφ is ≈ 60 µm for those modules (about 60% of the total) thatdo not suffer from significant drift field non-uniformities.To account for the fact that a correction forthese effects was not applied in the reconstruction of 2010 data, a systematic uncertainty of 300µmwas assumed for SDD points. Alongz, the estimated precision of the alignment is 50µm for SPD andSDD and a few hundredµm for SSD. These values are added in quadrature to the uncertainty on thereconstructed ITS hits in the track reconstruction algorithm. In the detector simulation, to account inan effective way for the residual misalignment, the ITS modules are randomly displaced with respect totheir ideal positions according to the estimated precisionof the alignment.

At larger radii (85< r < 247 cm), the cylindrical Time Projection Chamber (TPC) [16]provides trackreconstruction with up to 159 three-dimensional space points per track, as well as particle identificationvia the measurement of the specific energy deposit dE/dx. The TPC has an active length of 500 cmalong thez direction and its 90 m3 gas volume is filled with a mixture composed of Ne (85.7%), CO2

(9.5%), and N2 (4.8%). Its position resolution is 1100–1250µm along thez axis (corresponding to thedrift direction) and 800–1100µm alongrφ . Using cosmic-ray muons and data taken in pp collisions, therelative dE/dx resolution was measured to be about 5.5% for tracks that cross the entire detector [16].

The charged particle identification capability of the TPC issupplemented by the Time-Of-Flight detector(TOF), that is based on Multi-gap Resistive Plate Chambers (MRPCs) in a cylindrical configuration atradius 370–399 cm from the beam axis, with readout consisting of 152,928 sensitive pads of dimension2.5× 3.5 cm2. The TOF resolution on the particle arrival time is at present better than 100 ps. Thestart time of the collision (event time zero) is measured by the T0 detector, an array of Cherenkovcounters located at+350 cm and−70 cm along the beam-line, or, for the events in which the T0 signalis not present, estimated using the particle arrival times at the TOF. The particle identification is basedon the difference between the measured time-of-flight and its expected value, computed for each masshypothesis from the track momentum and length. The overall resolution on this difference is about160 ps. In this analysis, the time-of-flight measurement provides kaon/pion separation up to a momentumof about 1.5 GeV/c. Results from the TOF commissioning with cosmic-ray particles are reported in [17].

The data sample used for the analysis presented here consists of 314 million minimum-bias (MB)events, corresponding to an integrated luminosityLint = 5 nb−1, collected during the 2010 LHC runwith pp collisions at

√s = 7 TeV. The minimum-bias trigger was based on the SPD and VZERO

detectors. The VZERO detector consists of two arrays of 32 scintillators each, placed around the beam


s = 7 TeV 5

vacuum tube on either side of the interaction region. The twoarrays cover the pseudorapidity ranges−3.7 < η < −1.7 and 2.8 < η < 5.1. Minimum-bias collisions were triggered by requiring at leastone hit in either of the VZERO counters or in the SPD (|η | < 2), in coincidence with the arrival ofproton bunches from both directions. This trigger was estimated to be sensitive to about 87% of the ppinelastic cross section [18]. It was verified on Monte Carlo simulations based on the PYTHIA 6.4.21event generator [19] (with Perugia-0 tuning [20]) that the minimum-bias trigger is 100% efficient for Dmesons withpt > 1 GeV/c and|y| < 0.5. Contamination from beam-induced background was rejectedoffline using the timing information from the VZERO and the correlation between the number of hits andtrack segments (tracklets) in the SPD detector. The instantaneous luminosity in the ALICE experimentwas limited to 0.6–1.2×1029 cm−2s−1 by displacing the beams in the transverse plane by 3.8 times ther.m.s of their transverse profile. In this way, the interaction probability per bunch crossing was kept in therange 0.04–0.08, with probability of collision pile-up below 4% per triggered event. The luminous regionwas measured with high precision from the distribution of the interaction vertices reconstructed from thecharged particles tracked in the ALICE central barrel detectors, yieldingσ luminous

x ≈ σ luminousy ≈ 35–

50 µm in the transverse plane andσ luminousz ≈ 4–6 cm along the beam direction (the quoted ranges are

due to the variations of the beam conditions during the data taking). Only events with interaction vertexin the range|z|< 10 cm were used for the analysis.

3 Reconstruction of D meson decays

The measurement of charm production was performed by reconstructing three decay modes for Dmesons, D0 → K−π+ (with branching ratio, BR, of 3.89±0.05%), D+ → K−π+π+ (BR of 9.4±0.4%),and D∗(2010)+ → D0π+ (strong decay with BR of 67.7±0.5%) with D0 → K−π+, together with theircharge conjugates [21]. The D0 and D+ mesons have mean proper decay lengthscτ ≈ 123 and 312µm,respectively [21]. Their decay secondary vertices are therefore typically displaced by a few hundredµmfrom the primary vertex of the pp interaction. The analysis strategy for the extraction of the D0 and D+

signals from the large combinatorial background due to uncorrelated tracks is based on the reconstructionand selection of secondary vertex topologies that have significant separation from the primary vertex.The identification of the charged kaons in the TPC and TOF detectors provides additional backgroundrejection in the low-momentum region. A particle identification strategy that has high efficiency for theD meson signal (see section 4.1) was adopted. Finally, an invariant mass analysis was used to extract thesignal yield. In the D∗+ →D0π+ case, the decay vertex can not be resolved from the primary vertex. Theanalysis exploits topological selections on the D0, together with the sharp peak in the difference betweenthe invariant mass of the three final state hadrons and that ofthe two D0 decay prongs. Given that themass difference∆m = mD∗+ −mD0 ≈ 145.4 MeV/c2 [21] is only slightly larger than the charged pionmass, for lowpt D∗+ mesons the produced pion has low momentum and is referred to here as a ‘softpion’.

3.1 Track and vertex reconstruction

The procedure for track reconstruction in the central barrel detectors [13,22] starts with a first determina-tion of the primary vertex position, performed by correlating hit pairs (tracklets) in the two layers of theSilicon Pixel Detector. The same algorithm is used to searchfor vertices from pile-up collisions startingfrom the tracklets that do not point to the first found vertex.An event is rejected from the analyzed datasample if a second interaction vertex is found, it has at least 3 associated tracklets, and it is separatedfrom the first one by more than 8 mm. The remaining undetected pile-up is negligible for the analysisdescribed in this paper. Track seeds are built using this first estimate of the interaction vertex positiontogether with pairs of reconstructed space points in adjacent readout pad rows of the TPC. Tracks arethen projected inward in the radial direction using a Kalmanfilter algorithm, incorporating space pointsin the TPC and then hits in the six layers of the ITS (referred to here as TPC+ITS tracks). The ITS hitsnot associated to TPC+ITS tracks by this procedure are then used to search for ITS-only tracks, most


(GeV/c)t

p-110 1 10

m)

µ r

esol

utio

n (

φ r 0d

0

50

100

150

200

250

300

DataSim., residual misalignment

= 7 TeVspp,

Figure 1: Track impact parameter (d0) resolution in the transverse plane (rφ direction) as a function ofpt

comparing data and simulation. This resolution includes the uncertainty in the primary vertex position, whichis reconstructed excluding the track being probed.

importantly to recover pions with 80< pt < 200 MeV/c that have very low reconstruction efficiencyin the TPC, for geometrical reasons. ITS-only tracks are found using a hit grouping algorithm thatprojects angular windows radially outward from the primaryvertex. The tracks found in TPC+ITS arethen propagated outward in order to associate signals in thelarge-radius detectors that perform particleidentification. Finally, all tracks are re-propagated withthe Kalman filter in the inward direction. Therelativept resolution at the primary vertex for this procedure is about1% (6%) at 1 GeV/c for TPC+ITS(ITS-only) tracks. The last step of event reconstruction isthe re-determination of the primary vertexposition from the accepted tracks [23]. The primary vertex coordinates and covariance matrix are ob-tained via an analyticχ2 minimization method applied to the tracks approximated by straight lines afterpropagation to their common point of closest approach. The algorithm is then repeated excluding thetracks with distance to the primary vertex, normalized to its estimated uncertainty, larger than 3, whichare incompatible with being produced by primary particles.The vertex fit is constrained in the transverseplane using the information on the position and spreadσ luminous

x,y of the luminous region. The latter isdetermined from the distribution of primary vertices averaged over the run and is tabulated as a functionof time during the full data-taking period. The position resolution of the primary vertex reconstructedfrom tracks depends on the particle multiplicity. It was measured to beσz (µm) ≈ 430/N0.7

tracklets in thelongitudinal direction andσx,y (µm) ≈ min(σ luminous

x,y ,600/N0.9tracklets) in the transverse coordinates by fit-

ting its dependence on the number of tracklets in the SPD (Ntracklets), which corresponds to about twicethe multiplicity of primary charged particles per unit of rapidity. Thus, for the average luminous regionspreadσ luminous

x,y ≈ 40 µm, the transverse position of the vertex has a resolution that ranges from 40µmin low-multiplicity events (i.e. below 10 charged particles per unit of rapidity) to about 10µm in eventswith a multiplicity of about 40.

Secondary vertices of D0 and D+ meson candidates were reconstructed using tracks having|η | < 0.8,pt > 0.4 GeV/c, at least 70 associated space points (out of a maximum of 159)and χ2/ndf < 2 inthe TPC, and at least one hit in either of the two layers of the SPD. For tracks passing this selection,the average number of hits in the six ITS layers is 4.5–4.7, depending on the data taking period. Thisquantity is influenced by the fraction of inactive channels and its distribution is well reproduced in MonteCarlo simulations. For the D∗+ soft pion, all TPC+ITS and ITS-only tracks with at least 4 hits in the ITS,including at least one in the SPD, andpt > 80 MeV/c were considered. Figure 1 shows the transversemomentum dependence of the transverse (rφ ) impact parameter (d0) resolution achieved with the present


s = 7 TeV 7

ITS alignment precision for tracks that satisfy the TPC and ITS selection criteria, for data and MonteCarlo simulations. The simulations utilize GEANT3 [24] andincorporate a detailed description of thedetector material, geometry and response. Proton–proton collisions were simulated using the PYTHIA6.4.21 event generator [19] with Perugia-0 tuning [20]. Theimpact parameter resolution was estimatedby fitting the inclusive distribution ofd0 with respect to the event primary vertex, in intervals ofpt.The fit function is the sum of a Gaussian, that accounts for thecomponent due to prompt particlesproduced at the primary vertex, and two exponential functions, that account for secondary particles,mainly from weak decays of strange hadrons. The widthσ of the Gaussian provides an estimate ofthed0 resolution, which includes the resolution of the track parameters and the primary vertex position.In order to obtain an unbiased estimate ofd0, the primary vertex is recalculated excluding the trackbeing probed. The figure shows that thed0 resolution measured in data, with values of 75(20) µmat pt = 1(15) GeV/c, is reproduced within about 10% by the Monte Carlo simulation incorporatingthe residual ITS misalignment described in section 2. The effect of the difference between data andsimulation on the results of the D meson analysis is discussed in section 4.2.

3.2 D meson selection

D0, D+, and D∗+ candidates were filtered by applying kinematical and topological cuts, and particleidentification criteria. A fiducial acceptance cut|yD|< yfid(pt) was applied, withyfid smoothly increasingfrom 0.5 to 0.8 in 0< pt < 5 GeV/c andyfid = 0.8 above 5 GeV/c. For D0 and D+ decays, the secondaryvertex was reconstructed with the same algorithm used to compute the primary vertex from tracks. Theresolution on the position of D0 and D+ decay vertices was estimated by Monte Carlo simulations to beof the order of 100µm with little pt-dependence forpt > 1 GeV/c [23]. For the D0 and D+ selection,the primary vertex was recalculated for each D candidate, excluding the decay tracks.

The cut variables for the three mesons are described in the following. The actual cut values arept

dependent and were tuned to optimize the statistical significance of the signal, resulting in a selectionefficiency that increases with increasingpt. The cut values applied for D mesons at lowpt are reportedfor reference in the next paragraphs.

For D0 mesons, the two decay candidate tracks were further selected with pt > 0.7 GeV/c (pt >

0.4 GeV/c for 1 < pD0

t < 2 GeV/c) andrφ impact parameter significance|d0|/σd0 > 0.5. Secondaryvertices were required to have a minimum displacement of 100µm from the primary vertex and amaximum distance of closest approach between the two tracksof 300µm. The cut|cosθ∗|< 0.8, whereθ∗ is the angle between the kaon momentum in the D0 rest frame and the boost direction, was appliedto reduce the contamination of background candidates that do not represent real two-body decays andtypically have large values of|cosθ∗|. Well-displaced D0 → K−π+ topologies are characterized by largeand opposite-sign values of the decay trackrφ impact parameters (dπ

0 anddK0 ) and good pointing of the

reconstructed D0 momentum to the primary vertex, i.e. a small value of the pointing angleθpointing

between the momentum and flight line. Due to the strong correlation of these two features in the signal,the requirementdπ

0 × dK0 < −(120 µm)2 and cosθpointing > 0.8 was found to be effective in increasing

the signal-to-background ratio.

The D+ selection is based on a similar strategy to that for D0 mesons. A looser cut on thept of decaytracks, 0.4 GeV/c, was applied, due to the lower average momentum of the products of a three-bodydecay. The candidate triplets were selected on the basis of the sum of the distances of the decay tracksto the reconstructed decay vertex, the decay length, the cosine of the pointing angle, and the sum of thesquares of therφ impact parameters of the three tracks. Typical cut values for low-pt candidates are:decay length larger than 800µm, cosθpointing > 0.95, and sum of the squares of the three decay tracksimpact parametersΣd2

0 > (750 µm)2. The topological selection cuts are, in general, tighter than forthe D0 case due to the largercτ of the D+ meson, resulting in a better separation between primary andsecondary vertices, and to the higher combinatorial background in the three-particle final state.


)2) (GeV/cπInvariant Mass (K1.75 1.8 1.85 1.9 1.95 2 2.05

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233±)=1531σ3±S(

-1= 5 nbint

= 7 TeV, Lspp,

)2) (GeV/cπInvariant Mass (K1.7 1.75 1.8 1.85 1.9 1.95 2 2.05 2.1

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12<p

Figure 2: Invariant mass distributions for D0 (top) and D+ (middle) candidates, and mass difference distributionfor D∗+ candidates (bottom), for threept intervals. The curves show the fit functions as described in the text. Thevalues of mean (µ) and width (σ ) of the signal peak are reported (for D0 and D+ they are expressed in GeV/c2).

The D∗+ candidates were filtered by applying kinematical selections on the final decay products and cutson the topology of the D0 decay. The single track minimum transverse momentum was setto 0.4 GeV/cfor the D0 decay tracks and 80 MeV/c for the soft pion track. The variables used to select the topologyof the D0 decay are the same as for the D0 analysis described above. However, a selection with higherefficiency could be applied in this case, because the background in the region around∆m ≈ 145 MeV/c2,which is close to the phase space boundary, is much lower thanthat around the D0 mass. In particular,for D∗+ candidates withpt > 6 GeV/c, the topological cuts could be opened so as to select about 90%of the signal passing single track cuts.

The particle identification selection used the specific energy deposit and the time-of-flight from theTPC and TOF detectors, respectively. In order to assign the kaon or pion mass to the decay tracks,compatibility cuts were applied to the difference between the measured and expected signals. For bothdE/dx and time-of-flight, a 3σ compatibility cut was used. Tracks without a TOF signal wereidentifiedusing only the TPC information, and tracks with incompatible TOF and TPC indications were treated as


s = 7 TeV 9

Table 1: Measured raw yields for D0, D+, and D∗+ mesons, and their anti-particles, in a minimum-bias pp samplecorresponding to 5 nb−1 at

√s = 7 TeV, in transverse momentum intervals. The systematic uncertainty estimation

is described in section 4.2.

pt interval N ±stat.±syst.

(GeV/c) D0+D0

D++D− D∗++D∗−

1–2 1531±233±340 122±23± 30 78±16± 82–3 1978±168±190 390±57± 97 244±26±103–4 1950±129± 75 405±40±101 363±29±114–5 1184± 78± 40 516±38± 46 478±36±145–6 623± 50± 25 361±31± 33 374±28±186–7 339± 32± 13 294±30± 15 279±22±147–8 199± 25± 14 213±27± 11 170±19± 98–12 427± 38± 30 434±30± 22 408±28±18

12–16 139± 27± 14 89±20± 9 115±12±1016–24 – 52±14± 5 41± 6± 8

non-identified, but still used in the analysis and considered to be compatible with both a pion and a kaon.Two-prong candidates were accepted (as D0, D

0, or both) or rejected, according to the compatibility with

the K∓π± final state. For D0 candidates used in D∗+ reconstruction, compatibility with the appropriatefinal state, consistent with the soft pion charge, was required. Particle identification was not applied tothe soft pion tracks. In the case of the D+ → K−π+π+ decay, the particle with the opposite charge signwith respect to the D meson is a kaon. Hence, the triplets wererejected if the opposite sign track wasnot compatible with the kaon hypothesis, or at least one of the two same sign tracks was not compatiblewith the pion hypothesis. For all three D meson species, a comparison of the invariant mass distributionsobtained without and with particle identification shows that this selection reduces the combinatorialbackground by a factor 2–3 in the lowpt region, while preserving close to 100% of the D meson signal.

The raw signal yields were extracted in the intervals ofpt listed in Table 1, by a fit to the invariant massdistributions (or mass difference for the D∗+), as shown in Fig. 2 for three selectedpt intervals. For D0

and D+ mesons, the fitting function consists of a Gaussian describing the signal and an exponential termfor the background. In the D0 case, the contribution of signal candidates with wrong massassignmentto the final state hadrons is also present in the invariant mass spectrum. It was verified on Monte Carlosimulations that this does not bias the extracted signal yield, because the invariant mass distributionof these candidates is wide enough to be accounted for by the background function. Moreover, atlow pt, the particle identification selection strongly suppresses this contribution. For D∗+ mesons, thefitting function consists of a Gaussian describing the signal and the terma

√∆m−mπ ·eb(∆m−mπ) for the

background [7]. For all three D meson species, the mean of theGaussian is compatible with the PDGvalue [21] within errors, and its width is well reproduced inthe simulation. The extracted D meson rawyields are reported in Table 1.

4 D meson cross sections

4.1 Corrections

The production cross sections of prompt charmed mesons werecalculated as (e.g. for D+):

dσD+

dpt

∣

∣

∣

∣

∣

|y|<0.5

=12

1∆y∆pt

fprompt(pt) ·ND± raw(pt)∣

∣

∣

|y|<yfid

(Acc× ε)prompt(pt) ·BR·Lint. (1)

ND± raw(pt) is the measured inclusive raw yield, obtained from the invariant mass analysis in eachpt

interval (of width∆pt); fprompt is the prompt fraction of the raw yield;(Acc× ε)prompt is the acceptance


(GeV/c)t

p

5 10 15

Effi

cien

cy×

Acc

epta

nce

-310

-210

-110

1 = 7 TeVspp, +π- K→0D

(GeV/c)t

p

5 10 15 20

-3

-2

-1

1+π+π- K→+D

(GeV/c)t

p

5 10 15 20

-3

-2

-1

1

Prompt D

Prompt D, no PID

D from B decay

+π0 D→*+D

Figure 3: Acceptance× efficiency for D0, D+, and D∗+ mesons, as a function ofpt (see text for details).

times efficiency of prompt mesons, whereε accounts for vertex reconstruction, track reconstructionandselection, and for D meson candidate selection with the secondary vertex and particle identification cutsdescribed in section 3.∆y (= 2yfid) is the width of the rapidity coverage and BR is the decay branchingratio [21]. The factor 1/2 accounts for the fact that the measured yields include particles and anti-particles while the cross sections are given for particles only. The integrated luminosity was computedasLint = Npp,MB/σpp,MB , whereNpp,MB andσpp,MB are the number and the cross section, respectively,of pp collisions passing the minimum-bias trigger condition defined in section 2. Theσpp,MB value,62.3 mb, was derived from a measurement using a van der Meer scan [25] of the cross section ofcollisions that give signals in both sides of the VZERO scintillator detector (σVZERO-AND) [18]. Thenormalization factor,σpp,VZERO-AND/σpp,MB ≈ 0.87, was found to be stable within 1% in the data sample.The uncertainty onσpp,MB is determined by the systematic uncertainty of 4% onσVZERO-AND , which isdue to the uncertainties on the beam intensities [26] and on the analysis procedure related to the van derMeer scan of the signal.

The rapidity acceptance correction, using the factor 2yfid, with yfid varying from 0.5 at lowpt to 0.8at high pt, assumes that the rapidity distribution of D mesons is uniform in the range|y| < yfid. Thisassumption was checked using the PYTHIA 6.4.21 event generator [19] with Perugia-0 tuning [20] andthe FONLL pQCD calculation [1,27], both of which generate a Dmeson yield that is uniform within 1%in the range|y|< 0.8. The(Acc×ε) correction was determined using Monte Carlo simulations based onthe GEANT3 transport code [24]. The luminous region distribution and the conditions of all the ALICEdetectors in terms of active channels, gain, noise level, and alignment, and their evolution with timeduring the 2010 LHC run, were included in the simulations. Proton–proton collisions were simulatedusing the PYTHIA 6.4.21 event generator [19] with Perugia-0tuning [20]. Only events containingD mesons were transported through the apparatus and reconstructed, and the efficiency was extractedseparately for prompt D mesons and D mesons from B meson decays. Figure 3 shows, as a functionof transverse momentum, the acceptance times efficiency(Acc× ε) for D0, D+, and D∗+ mesons with|y|< yfid. At low pt, the efficiencies are of order 1% or less, while for largept the efficiencies increase andflatten at about 10–20% for D0 and D+, and 30% for D∗+. The efficiencies without particle identificationselection, shown for comparison, are the same as those with particle identification, indicating that thisselection is essentially fully efficient for the signal. Theefficiencies for D0 and D+ mesons from B mesondecays, also shown for comparison, are larger by about a factor of two. This behaviour is due to the factthat feed-down D mesons decay further from the primary vertex, because of the large B meson lifetime(cτ ≈ 500 µm [21]). For D∗+ mesons, the efficiency for the prompt and feed-down components are thesame in thept range above 4 GeV/c, where no strong cuts on the separation of the D0 decay vertex from


s = 7 TeV 11

the primary vertex are applied.

The fraction fprompt of D mesons coming from c quark hadronization, i.e. the correction factor thataccounts for the feed-down from B meson decays, was evaluated using the B production cross sectionfrom the FONLL pQCD calculation [1, 27], which describes well beauty production at Tevatron [3] andat the LHC [5,6], and the B→ D decay kinematics from the EvtGen package [28]. The computed crosssection for the feed-down component for each of the three D meson species was used, together with theMonte Carlo acceptance times efficiency(Acc× ε)feed-downfor D mesons from B decays (see Fig. 3), tocompute the expected feed-down contribution in the measured raw yields:

fprompt= 1− (ND± from B raw/ND± raw) (2)

with:

ND± from B raw∣

∣

∣

|y|<yfid

= 2dσD+ from B

FONLL

dpt

∣

∣

∣

∣

∣

|y|<0.5

·∆y∆pt · (Acc× ε)feed-down·BR·Lint . (3)

The symbol of thept-dependence(pt) is omitted in the formulas, for brevity. The resulting promptfraction fprompt is shown in Fig. 4 (left-hand panel) by the solid horizontal lines, for the case of D0

mesons. The prompt fraction ranges between 80% and 90%, depending on thept interval, these valuesbeing determined also by the different efficiencies for prompt and feed-down D mesons. In order toestimate the systematic uncertainty, the perturbative uncertainty on the FONLL beauty production crosssection was considered, as well as an alternative way of using the FONLL calculation. The formercontribution was obtained by varying the b quark mass and thefactorization and renormalization scalesas suggested in, e.g., [29]. The alternative method consisted of computing the prompt fraction using theFONLL cross sections for prompt and feed-down D mesons (withB → D via EvtGen [28] for the latter)and their respective Monte Carlo efficiencies:

fprompt=

1+(Acc× ε)feed-down

(Acc× ε)prompt

dσD+ from BFONLL

dpt

∣

∣

∣

∣

|y|<0.5

dσD+FONLLdpt

∣

∣

∣

∣

|y|<0.5

−1

. (4)

The resulting prompt fraction is shown by the dashed horizontal lines in Fig. 4 (left-hand panel). The fullenvelope of the uncertainty bands from the two methods, which is shown by the boxes in the figure, wastaken as a systematic uncertainty. The uncertainty relatedto the B decay kinematics was disregarded,after verifying that the difference resulting from using the PYTHIA [19] decayer instead of EvtGen [28]is negligible with respect to the FONLL B meson cross sectionuncertainty.

The prompt fraction of D0 mesons in the reconstructed yield was also estimated with a data-drivenmethod based on the measured impact parameter distributionof D0 meson candidates in eachpt interval,as done previously by the CDF Collaboration [7]. This methodexploits the different shapes of the distri-butions of the impact parameter to the primary vertex of prompt and feed-down (displaced) D mesons.The impact parameter distribution of D0 mesons was obtained from the one measured for candidateswith invariant mass in the range|m−MD0| < 2σ , after subtracting the background contribution esti-mated from the candidates in the side-bands (in the range 4.5σ < |m−MD0| < 4.5σ + 100 MeV/c2).The prompt fraction was estimated by fitting the resulting impact parameter distribution with a two-component function. The first component is a detector resolution term, modelled by a Gaussian andan exponential term, describing the impact parameter of prompt D mesons. The second component ac-counts for the reconstructed impact parameter distribution of D from B decay, which is modelled by aconvolution of the same detector resolution term with a double-exponential function describing the trueimpact parameter of secondary D mesons. The fit parameters are the width of the Gaussian and the frac-tion of prompt D mesons, that is, the relative weight of the prompt and secondary D meson components.


(GeV/c)t

p0 2 4 6 8 10 12 14 16

prom

ptP

rom

pt fr

actio

n of

raw

yie

ld, f

0

0.2

0.4

0.6

0.8

1

1.2

1.4

FONLL-based methods

D impact parameter fit

+π- K→0D-1=5.0 nb

int = 7 TeV, Lspp,

(with total uncertainty)

m)µ D impact parameter (-800 -600 -400 -200 0 200 400 600 800

Ent

ries

(bac

kgro

und

subt

ract

ed)

-110

1

10

210

310

m)µ D impact parameter (-400 -200 0 200 400

0

100

200

300+π- K→0D-1=5 nb

int = 7 TeV, Lspp,

<3 GeV/c t

2<p

/ndf = 1.282χ

0.04± = 0.88 promptf

mµ 2 ± = 60 promptσpromptfrom Bsum

Figure 4: Left: prompt fractionfprompt of the D0 raw yield as a function ofpt, for the two FONLL-based methods(solid: central value, from Eq. (2); dashed: alternative method, from Eq. (4)) and for the impact parameter fitmethod (circles); the boxes show the envelope of the uncertainty bands of the two FONLL-based methods; the errorbars show the total uncertainty from the impact parameter fit, including the statistical and systematic contributions.Right: an example of D0 meson impact parameter distribution in the transverse plane, for 2< pt < 3 GeV/c;the distribution is background-subtracted and fitted with the two-component function for prompt and feed-downcontributions, as described in the text; the inset, with linear scale, shows also the negative entries, resulting fromthe background subtraction.

An example of such a fit is shown in Fig. 4 (right-hand panel) for the D0 mesons in the transverse mo-mentum interval 2< pt < 3 GeV/c. The prompt fraction of D0 mesons, measured with this method for2< pt < 12 GeV/c, is shown by the circles in Fig. 4 (left-hand panel) and is found to be in general agree-ment with the FONLL-based estimations. Because of the largebackground forpt < 2 GeV/c and thepoor statistics available forpt > 12 GeV/c, this method was used only as a check of the FONLL-basedprompt fraction estimation, for thept intervals with large signal yield.

4.2 Systematic uncertainties

Several sources of systematic uncertainty were considered, including those affecting the signal extractionfrom the invariant mass spectra and all the correction factors applied to obtain thept-differential crosssections. A summary of the estimated relative systematic uncertainties is given in Table 2, for the lowestand highestpt interval (see Table 1) for each meson species.

The systematic uncertainty on the yield extraction from theinvariant mass spectrum in a givenpt intervalwas determined by repeating the fit in a different mass range,and also varying the function to describethe background. A polynomial, instead of an exponential, was used for D0 and D+ mesons, while apower law convoluted with an exponential and a polynomial was considered for D∗+ mesons, instead ofthe function defined in section 3.2. A method based on bin counting (after subtraction of the backgroundestimated from a fit in the mass side bands) was also used. The uncertainty was defined as the maximumvariation in the extracted yields from these different methods.

The systematic uncertainty related to the tracking efficiency includes the effects arising from track findingin the TPC, from track propagation from the TPC to the ITS, andfrom track quality selection. It wasestimated from the comparison of data and simulation and from the variation of the track selection. Theresulting uncertainty is 8% for the two-body decay of D0 mesons and 12% for the three-body decay of


s = 7 TeV 13

D+ mesons. For the D∗+ case, a slightly larger systematic uncertainty of 13% was assigned in the lowpt region (below 3 GeV/c), because the soft pion often haspt < 150 MeV/c and is reconstructed only inthe ITS. The tracking efficiency in this case has a significantuncertainty arising from the description ofhadronic interactions in the simulation of the detector response.

A systematic effect can arise due to residual discrepancy between data and simulation for the variablesused to select the signal D meson candidates. The distributions of these variables were compared forcandidates passing loose topological cuts, i.e. essentially for background candidates, and found tobe well described in the simulation. The systematic effectsdue to residual differences between dataand simulation were quantified by repeating the analysis with different sets of cuts. In particular, thecut values were changed in order to vary the signal by at least20% belowpt = 8 GeV/c. From thecorresponding variation of the corrected spectra, a systematic uncertainty of about 10% was estimatedfor each D meson species. As a further cross-check, the secondary vertices in the simulation werereconstructed also after a track-by-track scaling by a factor 1.08 of the impact parameter residualswith respect to their true value. This scaling is aimed at reproducing the impact parameter resolutionobserved in the data (see Fig. 1) and accounts for possible residual detector misalignment effects notfully described in the simulation. The resulting variationof the efficiency was found to decrease from4% at pt = 1–2 GeV/c to less than 1% forpt > 5 GeV/c. This effect was not included explicitly inthe uncertainty estimation, since it is to some extent accounted for in the cut variation study and itsmagnitude is much smaller than the 10% uncertainty assignedto the cut efficiency corrections.

The systematic uncertainty induced by a different efficiency for particle identification in data andsimulation was evaluated by repeating the analysis either without applying this selection, or with a tighterselection (2σ compatibility instead of 3σ ). The variation of the corrected yields obtained without PIDand with these two selections (standard and tighter) was assigned as a systematic uncertainty. Typicalvalues are 3–5%, with the exception of 15% for D+ in 1–2 GeV/c.

The accuracy of the description of the evolution of the experimental conditions with time was verifiedby analyzing separately sub-samples of data collected during different periods and with different orien-tations of the magnetic field. The results were found to be compatible within statistical uncertainties forall three meson species. Furthermore, thept-differential yields for each D meson measured separatelyfor particles and anti-particles were found to be in agreement within statistical uncertainties.

The effect of the shape of the simulated D mesons spectrum within our pt intervals was estimated fromthe relative difference in the Monte Carlo efficiencies obtained with thept shapes from PYTHIA [19]with Perugia-0 tune [20] and from the FONLL pQCD calculation[1, 30]. These two models predict asignificantly different slope at highpt (dN/dpt ∝ p−4.8

t for FONLL and∝ p−2.5t for PYTHIA Perugia-

0), which however results in a systematic effect on the D meson selection efficiency of only 3% for1< pt < 2 GeV/c, and less than 1% at higherpt.

Table 2: Summary of relative systematic uncertainties for the lowest and highestpt interval for each meson species.

D0 D+ D∗+

Low pt High pt Low pt High pt Low pt High pt

Raw yield extraction 20% 10% 25% 10% 10% 20%Tracking efficiency 8% 12% 13% 12%Cut efficiency 10% 10% 10% 10% 22% 10%PID efficiency 5% 3% 15% 5% 4% 3%MC pt shape 3% 1% 3% 1% 3% 1%Feed-down from B + 5

−45%+ 8−10%

+ 3−40%

+ 8−10%

+ 4−45%

+3−7%

Branching ratio 1.3% 4.2% 1.5%Normalization 4%


The systematic uncertainty from the subtraction of feed-down D mesons from B decays was estimatedas described in the previous section. It ranges between+ 5

−45% at low pt (1–2 GeV/c) and+ 8−10% at highpt

(> 12 GeV/c).

Finally, the systematic uncertainty on the branching ratios [21] and the 4% uncertainty on the minimum-bias pp cross section were taken into account.

4.3 Results

The pt-differential inclusive cross sections are shown in Fig. 5 for prompt D0, D+, and D∗+ mesons.The error bars represent the statistical uncertainties, while the systematic uncertainties are shown asboxes around the data points. The numerical values for the differential cross sections are reported inTable 3, together with their statistical and systematic uncertainties, as well as the average transversemomentum〈pt〉 of D mesons in eachpt interval. The value of〈pt〉 was obtained from the mesonpt

distribution in the considered interval, after subtracting the background contribution from the side bandsin the invariant mass distribution. The resulting〈pt〉 values for the three D meson species are compatiblewithin uncertainties in allpt bins and their average is reported in the table.

The measured D meson inclusive differential production cross sections are compared to two theoreticalpredictions, namely FONLL [1, 30] and GM-VFNS [9, 31]. Both calculations use CTEQ6.6 partondistribution functions (PDF) [32] and vary the factorization and renormalization scales,µF and µR,independently in the ranges 0.5 < µF/mt < 2, 0.5 < µR/mt < 2, with the constraint 0.5 < µF/µR < 2,wheremt =

√

p2t +m2

c. The charm quark mass is varied in FONLL within 1.3 < mc < 1.7 GeV/c2.Both calculations are compatible with the measurements, within the uncertainties. The central value ofthe GM-VFNS predictions lies systematically above the data, while that of the FONLL predictions liesbelow the data. For FONLL, this feature was observed also at

√s = 0.2 TeV (pp) [10,11] and 1.96 TeV

(pp) [7]. With a reach down topt = 1 GeV/c, this measurement probes the gluon distribution in thex range of a few 10−4. Within the current uncertainties of the experimental measurement and of thetheoretical predictions, it is not possible to draw conclusions about saturation effects.

The pt-integrated visible cross sections,σ vis(pt > 1 GeV/c, |y| < 0.5), for the three mesons wereextrapolated down topt = 0 to estimate their production cross sections per unit of rapidity dσ/dy atmid-rapidity. The extrapolation factor was computed from the FONLL calculation [1, 27] as the ratio(dσFONLL/dy)/σ vis

FONLL and it amounts to 1.25+0.29−0.09 for D0 and D+, and 1.21+0.29

−0.08 for D∗+, for thecentral values of the calculation parameters. Its uncertainty was obtained as a quadratic sum of the

Table 3: Production cross section in|y|<0.5 for prompt D0, D+, and D∗+ mesons in pp collisions at√

s= 7 TeV, intransverse momentum intervals. The normalization systematic uncertainty of 4% is not included in the systematicuncertainties reported in the table.

pt interval 〈pt〉 dσ/dpt||y|<0.5 ±stat.±syst. (µb/GeV/c)(GeV/c) (GeV/c) D0 D+ D∗+

1–2 1.5±0.3 179±30+48−97 115±23+38

−60 99±22+28−54

2–3 2.5±0.2 114±11+20−33 37.2±6.0+12.4

−14.3 51.6±5.9+ 8.7−13.1

3–4 3.5±0.1 59.2±4.3+ 8.5−12.5 19.8±2.2+5.9

−6.4 27.9±2.3+4.6−5.2

4–5 4.5±0.1 28.9±2.1+4.1−5.7 11.36±0.94+2.17

−2.60 10.97±0.87+1.81−1.88

5–6 5.5±0.1 12.4±1.1+1.8−2.2 4.66±0.46+0.90

−0.99 5.68±0.45+0.97−0.99

6–7 6.5±0.1 6.31±0.69+0.93−1.07 2.72±0.32+0.49

−0.54 3.25±0.27+0.55−0.56

7–8 7.4±0.1 3.04±0.46+0.49−0.52 1.48±0.22+0.27

−0.28 1.73±0.21+0.29−0.30

8–12 9.4±0.3 1.22±0.13+0.19−0.21 0.567±0.056+0.101

−0.113 0.674±0.050+0.113−0.116

12–16 13.8±0.9 0.213±0.049+0.037−0.038 0.084±0.026+0.019

−0.019 0.160±0.016+0.030−0.031

16–24 17.0+2.0−1.0 – 0.020±0.006+0.004

−0.004 0.027±0.004+0.007−0.007


s = 7 TeV 15

GeV/c t

p0 5 10 15 20 25

b/G

eV/c

)µ

(

|y|<

0.5

| t /

dpσd

-210

-110

1

10

210

310ALICE

-1 = 5 nbint

= 7 TeV, Ls, pp 0D

4% global norm. unc. (not shown)±

stat. unc.

syst. unc.

FONLL

GM-VFNS

(GeV/c)t

p0 5 10 15 20 25

FO

NLL

Dat

a

00.5

11.5

22.5

33.5

(GeV/c) t

p0 5 10 15 20 25

GM

-VF

NS

Dat

a

00.5

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22.5

33.5

GeV/c t

p0 5 10 15 20 25

b/G

eV/c

)µ

(

|y|<

0.5

| t /

dpσd

-210

-110

1

10

210

310ALICE

-1 = 5 nbint

= 7 TeV, Ls, pp +D


stat. unc.

syst. unc.

FONLL

GM-VFNS

(GeV/c)t

p0 5 10 15 20 25

FO

NLL

Dat

a

00.5

11.5

22.5

33.5

(GeV/c) t

p0 5 10 15 20 25

GM

-VF

NS

Dat

a

00.5

11.5

22.5

33.5

GeV/c t

p0 5 10 15 20 25

b/G

eV/c

)µ

(

|y|<

0.5

| t /

dpσd

-210

-110

1

10

210

310ALICE

-1 = 5 nbint

= 7 TeV, Ls, pp *+D


stat. unc.

syst. unc.

FONLL

GM-VFNS

(GeV/c)t

p0 5 10 15 20 25

FO

NLL

Dat

a

00.5

11.5

22.5

33.5

(GeV/c) t

p0 5 10 15 20 25

GM

-VF

NS

Dat

a

00.5

11.5

22.5

33.5

Figure 5: (colour online) pt-differential inclusive cross section for prompt D0, D+, and D∗+ mesons in ppcollisions at

√s = 7 TeV compared with FONLL [1, 30] and GM-VFNS [9, 31] theoretical predictions. The

symbols are positioned horizontally at the centre of eachpt interval. The normalization uncertainty of 4% is notshown.

uncertainties from charm mass and perturbative scales, varied within the aforementioned ranges1, andfrom the CTEQ6.6 PDF sets [32]. The cross sections for the three mesons are:

dσD0/dy = 511±41(stat.)+ 69

−173(syst.)±20(lumi.)+119− 37(extr.) µb,

dσD+/dy = 245±29(stat.)+52

−90(syst.)±10(lumi.)+56−18(extr.) µb,

dσD∗+/dy = 244±27(stat.)+36

−80(syst.)±10(lumi.)+57−16(extr.) µb.

1The +0.29 (i.e.+23%) uncertainty is mainly determined by the caseµF = 0.5mt, for which the PDFs are used in the regionQ ≈ 0.5mc that is not constrained by experimental data [32]. If this case is not considered, the uncertainty becomes +13% onthe high side.


5 Summary

We have presented the measurement by the ALICE Collaboration of the inclusive differential productioncross sections of prompt D mesons at central rapidity, in pp collisions at

√s = 7 TeV within 1< pt <

24 GeV/c. D mesons were reconstructed for the decay channels D0 → K−π+, D+ → K−π+π+, andD∗+ → D0π+, and their charge conjugates. Thept-differential cross sections are reproduced withinuncertainties by theoretical predictions based on perturbative QCD, FONLL [30] and GM-VFNS [31].

References

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[12] S. Alekhinet al., arXiv:hep-ph/0601013 (2006), chapter IV.

[13] K. Aamodtet al. [ALICE Coll.], JINST 3 (2008) S08002.

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[18] M. Gagliardi et al. [ALICE Coll.], arXiv:1109.5369 [hep-ex]; ALICE Coll., Measurement ofinelastic, single and double diffractive cross sections inproton–proton collisions at the LHC withALICE, article in preparation.

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s = 7 TeV 17

[30] M. Cacciari, S. Frixione, N. Houdeau, M.L. Mangano, P. Nason, and G. Ridolfi, CERN-PH-TH/2011-227.

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Acknowledgements

The ALICE collaboration would like to thank all its engineers and technicians for their invaluable con-tributions to the construction of the experiment and the CERN accelerator teams for the outstandingperformance of the LHC complex. The ALICE Collaboration would like to thank M. Cacciari andH. Spiesberger for providing the pQCD predictions that are compared to these data. The ALICE collabo-ration acknowledges the following funding agencies for their support in building and running the ALICEdetector: Department of Science and Technology, South Africa; Calouste Gulbenkian Foundation fromLisbon and Swiss Fonds Kidagan, Armenia; Conselho Nacionalde Desenvolvimento Cientıfico e Tec-nologico (CNPq), Financiadora de Estudos e Projetos (FINEP), Fundacao de Amparo a Pesquisa doEstado de Sao Paulo (FAPESP); National Natural Science Foundation of China (NSFC), the ChineseMinistry of Education (CMOE) and the Ministry of Science andTechnology of China (MSTC); Ministryof Education and Youth of the Czech Republic; Danish NaturalScience Research Council, the CarlsbergFoundation and the Danish National Research Foundation; The European Research Council under theEuropean Community’s Seventh Framework Programme; Helsinki Institute of Physics and the Academyof Finland; French CNRS-IN2P3, the ‘Region Pays de Loire’, ‘Region Alsace’, ‘Region Auvergne’ andCEA, France; German BMBF and the Helmholtz Association; General Secretariat for Research andTechnology, Ministry of Development, Greece; Hungarian OTKA and National Office for Research andTechnology (NKTH); Department of Atomic Energy and Department of Science and Technology of theGovernment of India; Istituto Nazionale di Fisica Nucleare(INFN) of Italy; MEXT Grant-in-Aid forSpecially Promoted Research, Japan; Joint Institute for Nuclear Research, Dubna; National ResearchFoundation of Korea (NRF); CONACYT, DGAPA, Mexico, ALFA-EC and the HELEN Program (High-Energy physics Latin-American–European Network); Stichting voor Fundamenteel Onderzoek der Ma-terie (FOM) and the Nederlandse Organisatie voor Wetenschappelijk Onderzoek (NWO), Netherlands;Research Council of Norway (NFR); Polish Ministry of Science and Higher Education; National Author-ity for Scientific Research - NASR (Autoritatea Nationalapentru Cercetare Stiintifica - ANCS); FederalAgency of Science of the Ministry of Education and Science ofRussian Federation, International Sci-ence and Technology Center, Russian Academy of Sciences, Russian Federal Agency of Atomic Energy,Russian Federal Agency for Science and Innovations and CERN-INTAS; Ministry of Education of Slo-vakia; CIEMAT, EELA, Ministerio de Educacion y Ciencia of Spain, Xunta de Galicia (Consellerıa deEducacion), CEADEN, Cubaenergıa, Cuba, and IAEA (International Atomic Energy Agency); SwedishReseach Council (VR) and Knut & Alice Wallenberg Foundation(KAW); Ukraine Ministry of Educationand Science; United Kingdom Science and Technology Facilities Council (STFC); The United States De-partment of Energy, the United States National Science Foundation, the State of Texas, and the State ofOhio.


A The ALICE Collaboration

B. Abelev69 , A. Abrahantes Quintana6 , D. Adamova74 , A.M. Adare121 , M.M. Aggarwal78 ,G. Aglieri Rinella30 , A.G. Agocs60 , A. Agostinelli19 , S. Aguilar Salazar56 , Z. Ahammed117 , N. Ahmad14 ,A. Ahmad Masoodi14 , S.U. Ahn64 ,37, A. Akindinov46 , D. Aleksandrov89 , B. Alessandro95 ,R. Alfaro Molina56 , A. Alici 96 ,30 ,9, A. Alkin2 , E. Almaraz Avina56 , T. Alt36 , V. Altini 28 ,30, S. Altinpinar15 ,I. Altsybeev118 , C. Andrei71 , A. Andronic86 , V. Anguelov83 , C. Anson16 , T. Anticic87 , F. Antinori100 ,P. Antonioli96 , L. Aphecetche102 , H. Appelshauser52 , N. Arbor65 , S. Arcelli19 , A. Arend52 , N. Armesto13 ,R. Arnaldi95 , T. Aronsson121 , I.C. Arsene86 , M. Arslandok52 , A. Asryan118 , A. Augustinus30 , R. Averbeck86 ,T.C. Awes75 , J.Aysto38 , M.D. Azmi14 , M. Bach36 , A. Badala97 , Y.W. Baek64 ,37, R. Bailhache52 , R. Bala95 ,R. Baldini Ferroli9 , A. Baldisseri12 , A. Baldit64 , F. Baltasar Dos Santos Pedrosa30 , J. Ban47 , R.C. Baral48 ,R. Barbera24 , F. Barile28 , G.G. Barnafoldi60 , L.S. Barnby91 , V. Barret64 , J. Bartke105 , M. Basile19 ,N. Bastid64 , B. Bathen54 , G. Batigne102 , B. Batyunya59 , C. Baumann52 , I.G. Bearden72 , H. Beck52 ,I. Belikov58 , F. Bellini19 , R. Bellwied111 , E. Belmont-Moreno56 , S. Beole26 , I. Berceanu71 , A. Bercuci71 ,Y. Berdnikov76 , D. Berenyi60 , C. Bergmann54 , L. Betev30 , A. Bhasin81 , A.K. Bhati78 , N. Bianchi66 ,L. Bianchi26 , C. Bianchin22 , J. Bielcık34 , J. Bielcıkova74 , A. Bilandzic73 , F. Blanco111 , F. Blanco7 , D. Blau89 ,C. Blume52 , M. Boccioli30 , N. Bock16 , A. Bogdanov70 , H. Bøggild72 , M. Bogolyubsky43 , L. Boldizsar60 ,M. Bombara35 , J. Book52 , H. Borel12 , A. Borissov120 , C. Bortolin22 ,,ii, S. Bose90 , F. Bossu30 ,26, M. Botje73 ,S. Bottger51 , B. Boyer42 , P. Braun-Munzinger86 , M. Bregant102 , T. Breitner51 , M. Broz33 , R. Brun30 ,E. Bruna121 ,26 ,95, G.E. Bruno28 , D. Budnikov88 , H. Buesching52 , S. Bufalino26 ,95, K. Bugaiev2 , O. Busch83 ,Z. Buthelezi80 , D. Caffarri22 , X. Cai40 , H. Caines121 , E. Calvo Villar92 , P. Camerini20 , V. Canoa Roman8 ,1 ,G. Cara Romeo96 , W. Carena30 , F. Carena30 , N. Carlin Filho108 , F. Carminati30 , C.A. Carrillo Montoya30 ,A. Casanova Dıaz66 , M. Caselle30 , J. Castillo Castellanos12 , J.F. Castillo Hernandez86 , E.A.R. Casula21 ,V. Catanescu71 , C. Cavicchioli30 , J. Cepila34 , P. Cerello95 , B. Chang38 ,124, S. Chapeland30 , J.L. Charvet12 ,S. Chattopadhyay90 , S. Chattopadhyay117, M. Cherney77 , C. Cheshkov30 ,110, B. Cheynis110 , E. Chiavassa95 ,V. Chibante Barroso30 , D.D. Chinellato109 , P. Chochula30 , M. Chojnacki45 , P. Christakoglou73 ,45,C.H. Christensen72 , P. Christiansen29 , T. Chujo115 , S.U. Chung85 , C. Cicalo93 , L. Cifarelli19 ,30, F. Cindolo96 ,J. Cleymans80 , F. Coccetti9 , J.-P. Coffin58 , F. Colamaria28 , D. Colella28 , G. Conesa Balbastre65 ,Z. Conesa del Valle30 ,58, P. Constantin83 , G. Contin20 , J.G. Contreras8 , T.M. Cormier120 ,Y. Corrales Morales26 , P. Cortese27 , I. Cortes Maldonado1 , M.R. Cosentino68 ,109, F. Costa30 , M.E. Cotallo7 ,E. Crescio8 , P. Crochet64 , E. Cruz Alaniz56 , E. Cuautle55 , L. Cunqueiro66 , A. Dainese100 , H.H. Dalsgaard72 ,A. Danu50 , D. Das90 , I. Das90 , K. Das90 , S. Dash95 , A. Dash48 ,109, S. De117 , A. De Azevedo Moregula66 ,G.O.V. de Barros108 , A. De Caro25 ,9 , G. de Cataldo94 , J. de Cuveland36 , A. De Falco21 , D. De Gruttola25 ,H. Delagrange102 , E. Del Castillo Sanchez30 , A. Deloff101 , V. Demanov88 , N. De Marco95 , E. Denes60 ,S. De Pasquale25 , A. Deppman108, G. D Erasmo28 , R. de Rooij45 , D. Di Bari28 , T. Dietel54 , C. Di Giglio28 ,S. Di Liberto99 , A. Di Mauro30 , P. Di Nezza66 , R. Divia30 , Ø. Djuvsland15 , A. Dobrin120 ,29,T. Dobrowolski101, I. Domınguez55 , B. Donigus86 , O. Dordic18 , O. Driga102 , A.K. Dubey117 , L. Ducroux110,P. Dupieux64 , M.R. Dutta Majumdar117 , A.K. Dutta Majumdar90 , D. Elia94 , D. Emschermann54 , H. Engel51 ,H.A. Erdal32 , B. Espagnon42 , M. Estienne102 , S. Esumi115 , D. Evans91 , G. Eyyubova18 , D. Fabris22 ,100,J. Faivre65 , D. Falchieri19 , A. Fantoni66 , M. Fasel86 , R. Fearick80 , A. Fedunov59 , D. Fehlker15 ,L. Feldkamp54 , D. Felea50 , G. Feofilov118 , A. Fernandez Tellez1 , A. Ferretti26 , R. Ferretti27 , J. Figiel105 ,M.A.S. Figueredo108, S. Filchagin88 , R. Fini94 , D. Finogeev44 , F.M. Fionda28 , E.M. Fiore28 , M. Floris30 ,S. Foertsch80 , P. Foka86 , S. Fokin89 , E. Fragiacomo98 , M. Fragkiadakis79 , U. Frankenfeld86 , U. Fuchs30 ,C. Furget65 , M. Fusco Girard25 , J.J. Gaardhøje72 , M. Gagliardi26 , A. Gago92 , M. Gallio26 ,D.R. Gangadharan16 , P. Ganoti75 , C. Garabatos86 , E. Garcia-Solis10 , I. Garishvili69 , J. Gerhard36 ,M. Germain102 , C. Geuna12 , A. Gheata30 , M. Gheata30 , B. Ghidini28 , P. Ghosh117 , P. Gianotti66 ,M.R. Girard119 , P. Giubellino30 ,26, E. Gladysz-Dziadus105 , P. Glassel83 , R. Gomez107 , E.G. Ferreiro13 ,L.H. Gonzalez-Trueba56 , P. Gonzalez-Zamora7 , S. Gorbunov36 , A. Goswami82 , S. Gotovac103, V. Grabski56 ,L.K. Graczykowski119, R. Grajcarek83 , A. Grelli45 , A. Grigoras30 , C. Grigoras30 , V. Grigoriev70 ,S. Grigoryan59 , A. Grigoryan122, B. Grinyov2 , N. Grion98 , P. Gros29 , J.F. Grosse-Oetringhaus30 ,J.-Y. Grossiord110 , R. Grosso30 , F. Guber44 , R. Guernane65 , C. Guerra Gutierrez92 , B. Guerzoni19 , M.Guilbaud110, K. Gulbrandsen72 , T. Gunji114 , A. Gupta81 , R. Gupta81 , H. Gutbrod86 , Ø. Haaland15 ,C. Hadjidakis42 , M. Haiduc50 , H. Hamagaki114, G. Hamar60 , B.H. Han17 , L.D. Hanratty91 , A. Hansen72 ,Z. Harmanova35 , J.W. Harris121 , M. Hartig52 , D. Hasegan50 , D. Hatzifotiadou96 , A. Hayrapetyan30 ,122,M. Heide54 , H. Helstrup32 , A. Herghelegiu71 , G. Herrera Corral8 , N. Herrmann83 , K.F. Hetland32 , B. Hicks121 ,P.T. Hille121 , B. Hippolyte58 , T. Horaguchi115, Y. Hori114 , P. Hristov30 , I. Hrivnacova42 , M. Huang15 ,S. Huber86 , T.J. Humanic16 , D.S. Hwang17 , R. Ichou64 , R. Ilkaev88 , I. Ilkiv 101 , M. Inaba115 , E. Incani21 ,


s = 7 TeV 19

P.G. Innocenti30 , G.M. Innocenti26 , M. Ippolitov89 , M. Irfan14 , C. Ivan86 , A. Ivanov118, M. Ivanov86 ,V. Ivanov76 , O. Ivanytskyi2 , A. Jachołkowski30 , P. M. Jacobs68 , L. Jancurova59 , S. Jangal58 , M.A. Janik119 ,R. Janik33 , P.H.S.Y. Jayarathna111 , S. Jena41 , R.T. Jimenez Bustamante55 , L. Jirden30 , P.G. Jones91 , H. Jung37 ,W. Jung37 , A. Jusko91 , A.B. Kaidalov46 , V. Kakoyan122 , S. Kalcher36 , P. Kalinak47 , M. Kalisky54 ,T. Kalliokoski38 , A. Kalweit53 , K. Kanaki15 , J.H. Kang124 , V. Kaplin70 , A. Karasu Uysal30 ,123,O. Karavichev44 , T. Karavicheva44 , E. Karpechev44 , A. Kazantsev89 , U. Kebschull62 ,51, R. Keidel125 ,M.M. Khan14 , S.A. Khan117 , P. Khan90 , A. Khanzadeev76 , Y. Kharlov43 , B. Kileng32 , S. Kim17 , D.W. Kim37 ,J.H. Kim17 , J.S. Kim37 , M. Kim124 , S.H. Kim37 , T. Kim124 , B. Kim124 , D.J. Kim38 , S. Kirsch36 ,30, I. Kisel36 ,S. Kiselev46 , A. Kisiel30 ,119, J.L. Klay4 , J. Klein83 , C. Klein-Bosing54 , M. Kliemant52 , A. Kluge30 ,M.L. Knichel86 , K. Koch83 , M.K. Kohler86 , A. Kolojvari118 , V. Kondratiev118, N. Kondratyeva70 ,A. Konevskikh44 , C. Kottachchi Kankanamge Don120 , R. Kour91 , M. Kowalski105 , S. Kox65 ,G. Koyithatta Meethaleveedu41 , J. Kral38 , I. Kralik47 , F. Kramer52 , I. Kraus86 , T. Krawutschke83 ,31,M. Kretz36 , M. Krivda91 ,47, F. Krizek38 , M. Krus34 , E. Kryshen76 , M. Krzewicki73 , Y. Kucheriaev89 ,C. Kuhn58 , P.G. Kuijer73 , P. Kurashvili101 , A.B. Kurepin44 , A. Kurepin44 , A. Kuryakin88 , V. Kushpil74 ,S. Kushpil74 , H. Kvaerno18 , M.J. Kweon83 , Y. Kwon124 , P. Ladron de Guevara55 , I. Lakomov118, R. Langoy15 ,C. Lara51 , A. Lardeux102, P. La Rocca24 , D.T. Larsen15 , C. Lazzeroni91 , R. Lea20 , Y. Le Bornec42 , S.C. Lee37 ,K.S. Lee37 , F. Lefevre102, J. Lehnert52 , L. Leistam30 , M. Lenhardt102, V. Lenti94 , H. Leon56 ,I. Leon Monzon107, H. Leon Vargas52 , P. Levai60 , X. Li11 , J. Lien15 , R. Lietava91 , S. Lindal18 ,V. Lindenstruth36 , C. Lippmann86 ,30, M.A. Lisa16 , L. Liu15 , P.I. Loenne15 , V.R. Loggins120 , V. Loginov70 ,S. Lohn30 , D. Lohner83 , C. Loizides68 , K.K. Loo38 , X. Lopez64 , E. Lopez Torres6 , G. Løvhøiden18 ,X.-G. Lu83 , P. Luettig52 , M. Lunardon22 , J. Luo40 , G. Luparello45 , L. Luquin102 , C. Luzzi30 , R. Ma121 ,K. Ma40 , D.M. Madagodahettige-Don111, A. Maevskaya44 , M. Mager53 ,30, D.P. Mahapatra48 , A. Maire58 ,M. Malaev76 , I. Maldonado Cervantes55 , L. Malinina59 ,,iii, D. Mal’Kevich46 , P. Malzacher86 , A. Mamonov88 ,L. Manceau95 , L. Mangotra81 , V. Manko89 , F. Manso64 , V. Manzari94 , Y. Mao65 ,40, M. Marchisone64 ,26,J. Mares49 , G.V. Margagliotti20 ,98, A. Margotti96 , A. Marın86 , C. Markert106 , I. Martashvili113 ,P. Martinengo30 , M.I. Martınez1 , A. Martınez Davalos56 , G. Martınez Garcıa102 , Y. Martynov2 , A. Mas102 ,S. Masciocchi86 , M. Masera26 , A. Masoni93 , L. Massacrier110 , M. Mastromarco94 , A. Mastroserio28 ,30,Z.L. Matthews91 , A. Matyja105 ,102, D. Mayani55 , C. Mayer105 , M.A. Mazzoni99 , F. Meddi23 ,A. Menchaca-Rocha56 , J. Mercado Perez83 , M. Meres33 , Y. Miake115 , A. Michalon58 , J. Midori39 ,L. Milano26 , J. Milosevic18 ,,iv, A. Mischke45 , A.N. Mishra82 , D. Miskowiec86 ,30, C. Mitu50 , J. Mlynarz120 ,A.K. Mohanty30 , B. Mohanty117, L. Molnar30 , L. Montano Zetina8 , M. Monteno95 , E. Montes7 , T. Moon124 ,M. Morando22 , D.A. Moreira De Godoy108 , S. Moretto22 , A. Morsch30 , V. Muccifora66 , E. Mudnic103 ,S. Muhuri117 , H. Muller30 , M.G. Munhoz108, L. Musa30 , A. Musso95 , B.K. Nandi41 , R. Nania96 , E. Nappi94 ,C. Nattrass113 , N.P. Naumov88 , S. Navin91 , T.K. Nayak117 , S. Nazarenko88 , G. Nazarov88 , A. Nedosekin46 ,M. Nicassio28 , B.S. Nielsen72 , T. Niida115 , S. Nikolaev89 , V. Nikolic87 , V. Nikulin76 , S. Nikulin89 ,B.S. Nilsen77 , M.S. Nilsson18 , F. Noferini96 ,9 , P. Nomokonov59 , G. Nooren45 , N. Novitzky38 , A. Nyanin89 ,A. Nyatha41 , C. Nygaard72 , J. Nystrand15 , H. Obayashi39 , A. Ochirov118 , H. Oeschler53 ,30, S.K. Oh37 ,J. Oleniacz119 , C. Oppedisano95 , A. Ortiz Velasquez55 , G. Ortona30 ,26, A. Oskarsson29 , P. Ostrowski119 ,I. Otterlund29 , J. Otwinowski86 , K. Oyama83 , K. Ozawa114 , Y. Pachmayer83 , M. Pachr34 , F. Padilla26 ,P. Pagano25 , G. Paic55 , F. Painke36 , C. Pajares13 , S. Pal12 , S.K. Pal117 , A. Palaha91 , A. Palmeri97 ,V. Papikyan122, G.S. Pappalardo97 , W.J. Park86 , A. Passfeld54 , B. Pastircak47 , D.I. Patalakha43 , V. Paticchio94 ,A. Pavlinov120, T. Pawlak119 , T. Peitzmann45 , M. Perales10 , E. Pereira De Oliveira Filho108 , D. Peresunko89 ,C.E. Perez Lara73 , E. Perez Lezama55 , D. Perini30 , D. Perrino28 , W. Peryt119 , A. Pesci96 , V. Peskov30 ,55,Y. Pestov3 , V. Petracek34 , M. Petran34 , M. Petris71 , P. Petrov91 , M. Petrovici71 , C. Petta24 , S. Piano98 ,A. Piccotti95 , M. Pikna33 , P. Pillot102 , O. Pinazza30 , L. Pinsky111 , N. Pitz52 , F. Piuz30 , D.B. Piyarathna111 ,M. Płoskon68 , J. Pluta119 , T. Pocheptsov59 ,18, S. Pochybova60 , P.L.M. Podesta-Lerma107,M.G. Poghosyan30 ,26, K. Polak49 , B. Polichtchouk43 , A. Pop71 , S. Porteboeuf-Houssais64 , V. Pospısil34 ,B. Potukuchi81 , S.K. Prasad120 , R. Preghenella96 ,9 , F. Prino95 , C.A. Pruneau120 , I. Pshenichnov44 ,G. Puddu21 , A. Pulvirenti24 ,30, V. Punin88 , M. Putis35 , J. Putschke120 ,121, E. Quercigh30 , H. Qvigstad18 ,A. Rachevski98 , A. Rademakers30 , S. Radomski83 , T.S. Raiha38 , J. Rak38 , A. Rakotozafindrabe12 ,L. Ramello27 , A. Ramırez Reyes8 , S. Raniwala82 , R. Raniwala82 , S.S. Rasanen38 , B.T. Rascanu52 ,D. Rathee78 , K.F. Read113 , J.S. Real65 , K. Redlich101 ,57, P. Reichelt52 , M. Reicher45 , R. Renfordt52 ,A.R. Reolon66 , A. Reshetin44 , F. Rettig36 , J.-P. Revol30 , K. Reygers83 , H. Ricaud53 , L. Riccati95 , R.A. Ricci67 ,M. Richter18 , P. Riedler30 , W. Riegler30 , F. Riggi24 ,97, M. Rodrıguez Cahuantzi1 , D. Rohr36 , D. Rohrich15 ,R. Romita86 , F. Ronchetti66 , P. Rosnet64 , S. Rossegger30 , A. Rossi22 , F. Roukoutakis79 , C. Roy58 , P. Roy90 ,A.J. Rubio Montero7 , R. Rui20 , E. Ryabinkin89 , A. Rybicki105 , S. Sadovsky43 , K. Safarık30 , P.K. Sahu48 ,


J. Saini117 , H. Sakaguchi39 , S. Sakai68 , D. Sakata115 , C.A. Salgado13 , S. Sambyal81 , V. Samsonov76 ,X. Sanchez Castro55 , L. Sandor47 , A. Sandoval56 , M. Sano115 , S. Sano114 , R. Santo54 , R. Santoro94 ,30,J. Sarkamo38 , E. Scapparone96 , F. Scarlassara22 , R.P. Scharenberg84 , C. Schiaua71 , R. Schicker83 ,H.R. Schmidt86 ,116, C. Schmidt86 , S. Schreiner30 , S. Schuchmann52 , J. Schukraft30 , Y. Schutz30 ,102,K. Schwarz86 , K. Schweda86 ,83, G. Scioli19 , E. Scomparin95 , R. Scott113 , P.A. Scott91 , G. Segato22 ,I. Selyuzhenkov86 , S. Senyukov27 ,58, J. Seo85 , S. Serci21 , E. Serradilla7 ,56 , A. Sevcenco50 , I. Sgura94 ,G. Shabratova59 , R. Shahoyan30 , N. Sharma78 , S. Sharma81 , K. Shigaki39 , M. Shimomura115, K. Shtejer6 ,Y. Sibiriak89 , M. Siciliano26 , E. Sicking30 , S. Siddhanta93 , T. Siemiarczuk101, D. Silvermyr75 ,G. Simonetti28 ,30, R. Singaraju117, R. Singh81 , S. Singha117 , T. Sinha90 , B.C. Sinha117 , B. Sitar33 , M. Sitta27 ,T.B. Skaali18 , K. Skjerdal15 , R. Smakal34 , N. Smirnov121 , R. Snellings45 , C. Søgaard72 , R. Soltz69 , H. Son17 ,J. Song85 , M. Song124 , C. Soos30 , F. Soramel22 , M. Spyropoulou-Stassinaki79 , B.K. Srivastava84 , J. Stachel83 ,I. Stan50 , I. Stan50 , G. Stefanek101 , G. Stefanini30 , T. Steinbeck36 , M. Steinpreis16 , E. Stenlund29 , G. Steyn80 ,D. Stocco102 , M. Stolpovskiy43 , P. Strmen33 , A.A.P. Suaide108 , M.A. Subieta Vasquez26 , T. Sugitate39 ,C. Suire42 , M. Sukhorukov88 , R. Sultanov46 , M. Sumbera74 , T. Susa87 , A. Szanto de Toledo108 , I. Szarka33 ,A. Szostak15 , C. Tagridis79 , J. Takahashi109 , J.D. Tapia Takaki42 , A. Tauro30 , G. Tejeda Munoz1 , A. Telesca30 ,C. Terrevoli28 , J. Thader86 , J.H. Thomas86 , D. Thomas45 , R. Tieulent110 , A.R. Timmins111 , D. Tlusty34 ,A. Toia36 ,30, H. Torii39 ,114, L. Toscano95 , F. Tosello95 , T. Traczyk119 , D. Truesdale16 , W.H. Trzaska38 ,T. Tsuji114 , A. Tumkin88 , R. Turrisi100 , T.S. Tveter18 , J. Ulery52 , K. Ullaland15 , J. Ulrich62 ,51, A. Uras110 ,J. Urban35 , G.M. Urciuoli99 , G.L. Usai21 , M. Vajzer34 ,74, M. Vala59 ,47, L. Valencia Palomo42 , S. Vallero83 ,N. van der Kolk73 , P. Vande Vyvre30 , M. van Leeuwen45 , L. Vannucci67 , A. Vargas1 , R. Varma41 ,M. Vasileiou79 , A. Vasiliev89 , V. Vechernin118, M. Veldhoen45 , M. Venaruzzo20 , E. Vercellin26 , S. Vergara1 ,D.C. Vernekohl54 , R. Vernet5 , M. Verweij45 , L. Vickovic103 , G. Viesti22 , O. Vikhlyantsev88 , Z. Vilakazi80 ,O. Villalobos Baillie91 , A. Vinogradov89 , L. Vinogradov118, Y. Vinogradov88 , T. Virgili 25 , Y.P. Viyogi117 ,A. Vodopyanov59 , K. Voloshin46 , S. Voloshin120 , G. Volpe28 ,30, B. von Haller30 , D. Vranic86 , G. Øvrebekk15 ,J. Vrlakova35 , B. Vulpescu64 , A. Vyushin88 , V. Wagner34 , B. Wagner15 , R. Wan58 ,40, Y. Wang40 , D. Wang40 ,Y. Wang83 , M. Wang40 , K. Watanabe115 , J.P. Wessels30 ,54, U. Westerhoff54 , J. Wiechula83 ,116, J. Wikne18 ,M. Wilde54 , G. Wilk101 , A. Wilk54 , M.C.S. Williams96 , B. Windelband83 , L. Xaplanteris Karampatsos106 ,H. Yang12 , S. Yano39 , S. Yasnopolskiy89 , J. Yi85 , Z. Yin40 , H. Yokoyama115, I.-K. Yoo85 , J. Yoon124 ,W. Yu52 , X. Yuan40 , I. Yushmanov89 , C. Zach34 , C. Zampolli96 ,30, S. Zaporozhets59 , A. Zarochentsev118,P. Zavada49 , N. Zaviyalov88 , H. Zbroszczyk119, P. Zelnicek30 ,51, I. Zgura50 , M. Zhalov76 , X. Zhang64 ,40,F. Zhou40 , D. Zhou40 , Y. Zhou45 , X. Zhu40 , A. Zichichi19 ,9 , A. Zimmermann83 , G. Zinovjev2 ,Y. Zoccarato110, M. Zynovyev2

Affiliation notesi Deceasedii Also at: Dipartimento di Fisica dell’Universita, Udine, Italyiii Also at: M.V.Lomonosov Moscow State University, D.V.Skobeltsyn Institute of Nuclear Physics, Moscow,

Russiaiv Also at: ”Vinca” Institute of Nuclear Sciences, Belgrade,Serbia

Collaboration Institutes1 Benemerita Universidad Autonoma de Puebla, Puebla, Mexico2 Bogolyubov Institute for Theoretical Physics, Kiev, Ukraine3 Budker Institute for Nuclear Physics, Novosibirsk, Russia4 California Polytechnic State University, San Luis Obispo,California, United States5 Centre de Calcul de l’IN2P3, Villeurbanne, France6 Centro de Aplicaciones Tecnologicas y Desarrollo Nuclear(CEADEN), Havana, Cuba7 Centro de Investigaciones Energeticas Medioambientalesy Tecnologicas (CIEMAT), Madrid, Spain8 Centro de Investigacion y de Estudios Avanzados (CINVESTAV), Mexico City and Merida, Mexico9 Centro Fermi – Centro Studi e Ricerche e Museo Storico della Fisica “Enrico Fermi”, Rome, Italy

10 Chicago State University, Chicago, United States11 China Institute of Atomic Energy, Beijing, China12 Commissariat a l’Energie Atomique, IRFU, Saclay, France13 Departamento de Fısica de Partıculas and IGFAE, Universidad de Santiago de Compostela, Santiago de

Compostela, Spain


s = 7 TeV 21

14 Department of Physics Aligarh Muslim University, Aligarh,India15 Department of Physics and Technology, University of Bergen, Bergen, Norway16 Department of Physics, Ohio State University, Columbus, Ohio, United States17 Department of Physics, Sejong University, Seoul, South Korea18 Department of Physics, University of Oslo, Oslo, Norway19 Dipartimento di Fisica dell’Universita and Sezione INFN,Bologna, Italy20 Dipartimento di Fisica dell’Universita and Sezione INFN,Trieste, Italy21 Dipartimento di Fisica dell’Universita and Sezione INFN,Cagliari, Italy22 Dipartimento di Fisica dell’Universita and Sezione INFN,Padova, Italy23 Dipartimento di Fisica dell’Universita ‘La Sapienza’ andSezione INFN, Rome, Italy24 Dipartimento di Fisica e Astronomia dell’Universita and Sezione INFN, Catania, Italy25 Dipartimento di Fisica ‘E.R. Caianiello’ dell’Universit`a and Gruppo Collegato INFN, Salerno, Italy26 Dipartimento di Fisica Sperimentale dell’Universita andSezione INFN, Turin, Italy27 Dipartimento di Scienze e Tecnologie Avanzate dell’Universita del Piemonte Orientale and Gruppo

Collegato INFN, Alessandria, Italy28 Dipartimento Interateneo di Fisica ‘M. Merlin’ and SezioneINFN, Bari, Italy29 Division of Experimental High Energy Physics, University of Lund, Lund, Sweden30 European Organization for Nuclear Research (CERN), Geneva, Switzerland31 Fachhochschule Koln, Koln, Germany32 Faculty of Engineering, Bergen University College, Bergen, Norway33 Faculty of Mathematics, Physics and Informatics, ComeniusUniversity, Bratislava, Slovakia34 Faculty of Nuclear Sciences and Physical Engineering, Czech Technical University in Prague, Prague,

Czech Republic35 Faculty of Science, P.J.Safarik University, Kosice, Slovakia36 Frankfurt Institute for Advanced Studies, Johann WolfgangGoethe-Universitat Frankfurt, Frankfurt,

Germany37 Gangneung-Wonju National University, Gangneung, South Korea38 Helsinki Institute of Physics (HIP) and University of Jyvaskyla, Jyvaskyla, Finland39 Hiroshima University, Hiroshima, Japan40 Hua-Zhong Normal University, Wuhan, China41 Indian Institute of Technology, Mumbai, India42 Institut de Physique Nucleaire d’Orsay (IPNO), Universite Paris-Sud, CNRS-IN2P3, Orsay, France43 Institute for High Energy Physics, Protvino, Russia44 Institute for Nuclear Research, Academy of Sciences, Moscow, Russia45 Nikhef, National Institute for Subatomic Physics and Institute for Subatomic Physics of Utrecht University,

Utrecht, Netherlands46 Institute for Theoretical and Experimental Physics, Moscow, Russia47 Institute of Experimental Physics, Slovak Academy of Sciences, Kosice, Slovakia48 Institute of Physics, Bhubaneswar, India49 Institute of Physics, Academy of Sciences of the Czech Republic, Prague, Czech Republic50 Institute of Space Sciences (ISS), Bucharest, Romania51 Institut fur Informatik, Johann Wolfgang Goethe-Universitat Frankfurt, Frankfurt, Germany52 Institut fur Kernphysik, Johann Wolfgang Goethe-Universitat Frankfurt, Frankfurt, Germany53 Institut fur Kernphysik, Technische Universitat Darmstadt, Darmstadt, Germany54 Institut fur Kernphysik, Westfalische Wilhelms-Universitat Munster, Munster, Germany55 Instituto de Ciencias Nucleares, Universidad Nacional Autonoma de Mexico, Mexico City, Mexico56 Instituto de Fısica, Universidad Nacional Autonoma de M´exico, Mexico City, Mexico57 Institut of Theoretical Physics, University of Wroclaw58 Institut Pluridisciplinaire Hubert Curien (IPHC), Universite de Strasbourg, CNRS-IN2P3, Strasbourg,

France59 Joint Institute for Nuclear Research (JINR), Dubna, Russia60 KFKI Research Institute for Particle and Nuclear Physics, Hungarian Academy of Sciences, Budapest,

Hungary61 Kharkiv Institute of Physics and Technology (KIPT), National Academy of Sciences of Ukraine (NASU),

Kharkov, Ukraine62 Kirchhoff-Institut fur Physik, Ruprecht-Karls-Universitat Heidelberg, Heidelberg, Germany


63 Korea Institute of Science and Technology Information64 Laboratoire de Physique Corpusculaire (LPC), Clermont Universite, Universite Blaise Pascal,

CNRS–IN2P3, Clermont-Ferrand, France65 Laboratoire de Physique Subatomique et de Cosmologie (LPSC), Universite Joseph Fourier, CNRS-IN2P3,

Institut Polytechnique de Grenoble, Grenoble, France66 Laboratori Nazionali di Frascati, INFN, Frascati, Italy67 Laboratori Nazionali di Legnaro, INFN, Legnaro, Italy68 Lawrence Berkeley National Laboratory, Berkeley, California, United States69 Lawrence Livermore National Laboratory, Livermore, California, United States70 Moscow Engineering Physics Institute, Moscow, Russia71 National Institute for Physics and Nuclear Engineering, Bucharest, Romania72 Niels Bohr Institute, University of Copenhagen, Copenhagen, Denmark73 Nikhef, National Institute for Subatomic Physics, Amsterdam, Netherlands74 Nuclear Physics Institute, Academy of Sciences of the CzechRepublic,Rez u Prahy, Czech Republic75 Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States76 Petersburg Nuclear Physics Institute, Gatchina, Russia77 Physics Department, Creighton University, Omaha, Nebraska, United States78 Physics Department, Panjab University, Chandigarh, India79 Physics Department, University of Athens, Athens, Greece80 Physics Department, University of Cape Town, iThemba LABS,Cape Town, South Africa81 Physics Department, University of Jammu, Jammu, India82 Physics Department, University of Rajasthan, Jaipur, India83 Physikalisches Institut, Ruprecht-Karls-Universitat Heidelberg, Heidelberg, Germany84 Purdue University, West Lafayette, Indiana, United States85 Pusan National University, Pusan, South Korea86 Research Division and ExtreMe Matter Institute EMMI, GSI Helmholtzzentrum fur

Schwerionenforschung, Darmstadt, Germany87 Rudjer Boskovic Institute, Zagreb, Croatia88 Russian Federal Nuclear Center (VNIIEF), Sarov, Russia89 Russian Research Centre Kurchatov Institute, Moscow, Russia90 Saha Institute of Nuclear Physics, Kolkata, India91 School of Physics and Astronomy, University of Birmingham,Birmingham, United Kingdom92 Seccion Fısica, Departamento de Ciencias, Pontificia Universidad Catolica del Peru, Lima, Peru93 Sezione INFN, Cagliari, Italy94 Sezione INFN, Bari, Italy95 Sezione INFN, Turin, Italy96 Sezione INFN, Bologna, Italy97 Sezione INFN, Catania, Italy98 Sezione INFN, Trieste, Italy99 Sezione INFN, Rome, Italy

100 Sezione INFN, Padova, Italy101 Soltan Institute for Nuclear Studies, Warsaw, Poland102 SUBATECH, Ecole des Mines de Nantes, Universite de Nantes,CNRS-IN2P3, Nantes, France103 Technical University of Split FESB, Split, Croatia104 test institute105 The Henryk Niewodniczanski Institute of Nuclear Physics, Polish Academy of Sciences, Cracow, Poland106 The University of Texas at Austin, Physics Department, Austin, TX, United States107 Universidad Autonoma de Sinaloa, Culiacan, Mexico108 Universidade de Sao Paulo (USP), Sao Paulo, Brazil109 Universidade Estadual de Campinas (UNICAMP), Campinas, Brazil110 Universite de Lyon, Universite Lyon 1, CNRS/IN2P3, IPN-Lyon, Villeurbanne, France111 University of Houston, Houston, Texas, United States112 University of Technology and Austrian Academy of Sciences,Vienna, Austria113 University of Tennessee, Knoxville, Tennessee, United States114 University of Tokyo, Tokyo, Japan115 University of Tsukuba, Tsukuba, Japan


s = 7 TeV 23

116 Eberhard Karls Universitat Tubingen, Tubingen, Germany117 Variable Energy Cyclotron Centre, Kolkata, India118 V. Fock Institute for Physics, St. Petersburg State University, St. Petersburg, Russia119 Warsaw University of Technology, Warsaw, Poland120 Wayne State University, Detroit, Michigan, United States121 Yale University, New Haven, Connecticut, United States122 Yerevan Physics Institute, Yerevan, Armenia123 Yildiz Technical University, Istanbul, Turkey124 Yonsei University, Seoul, South Korea125 Zentrum fur Technologietransfer und Telekommunikation (ZTT), Fachhochschule Worms, Worms,

Germany

Measurement of charm production at central rapidity in proton-proton collisions at $\sqrt{s}$ = 7...

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