Particle production in pp and Pb-Pb collisions with the ALICE ...

Particle production in pp and Pb-Pb collisions with the ALICE experiment at the LHC

F. Bellini* for the ALICE Collaboration *Università di Bologna, I.N.F.N Sez. di Bologna – Italy

Rencontres de Moriond - QCD and High Energy Interactions La Thuile, 14th March 2012

Outline

The ALICE detector at LHC

Results in pp collisions

Results in Pb-Pb collisions

Medium evolution and (selected) “soft” probes

Summary and conclusions

14/03/12 F. Bellini, Rencontres de Moriond -‐ QCD 2

Iden=fied primary par=cle spectra

Test of hydrodynamic model

Blast-‐wave fit to spectra

Mul=-‐strange baryon spectra

Par=cle ra=os tes=ng thermal model

Strange baryon/meson ra=o

Strangeness enhancement

Iden=fied primary par=cle spectra and ra=os

Resonances spectra

Mul=-‐baryon spectra

Comparison with Monte Carlo (MC) models

pp collisions PbPb collisions

ITS

TPC

TOF

The ALICE detector at LHC


Inner Tracking System – ITS: Primary vertex, PID via dE/dx Time Projec9on Chamber – TPC: Global tracking, PID via dE/dx Time Of Flight system – TOF: PID via =me-‐of-‐flight measurement

•  Unique PID capabili=es: dE/dx, =me-‐of-‐flight, Cherenkov, transi=on radia=on, calorimetry, decay topology (V0, cascades)

•  Central region with the smallest material budget at the LHC

pp collisions at √s = 0.9 and 7 TeV


Transverse momentum spectra (pT) of identified π, K and p, resonances and multi-strange

baryons, as well as particle ratios measured in pp collisions:

o  Provide valuable reference spectra for the understanding of heavy ion data

o  Allow to constrain the phenomenology of QCD-inspired models (Pythia, Phojet …)

Primary π/K/p spectra


-1 |y|<

0.5

(G

eV

/c)

tdN

/dp

INE

L1/N

-310

-210

-110

= 7 TeVspp at 2+stat2syserror=

Preliminary

Normalization Error 8.3%

p

Phojet

Pythia D6T

Pythia Perugia0

Pythia Perugia 2011

(GeV/c)t

p0 0.5 1 1.5 2 2.5 3

MC

/data

0

0.5

1

1.5

2

-1 |y|

<0

.5 (

Ge

V/c

)t

dN

/dp

INE

L1

/N

-310

-210

-110

1


Preliminary


-K

Phojet

Pythia D6T

Pythia Perugia0

Pythia Perugia 2011

(GeV/c)t

p0 0.5 1 1.5 2 2.5 3

MC

/data

0

0.5

1

1.5

2

K-‐ p

Best agreement with MC models for Pythia -‐ Perugia 2011 for Kaons at all pT and protons for pT>0.7 GeV/c, while pions are overes=mated at high pT.

π-‐

Iden=fied primary π/K/p spectra by combined analysis in ITS, TPC and TOF. See backup slides for posi=ve par=cles.

<pT> and ratios


)2M (GeV/c0 0.2 0.4 0.6 0.8 1 1.2

(G

eV

/c)

! t p"

0

0.2

0.4

0.6

0.8

1

1.2

1.4 = 7 TeVsALICE, pp,

= 900 GeVsALICE, pp,

= 900 GeVsp, pUA5,

= 200 GeVsSTAR/PHENIX, pp,

# K p

ALICE preliminary = 7 TeVspp at

2+stat2syserror =

(GeV)s10 210 310

!K

/

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.16

0.18

ALICE ppSTAR ppNA49 ppPHENIX pp

pE735 ppUA5 p


2+stat2syserror =

(GeV)s10 210 310

!p

/

0

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

-!/pALICE pp

+!ALICE pp p/

-!/pPHENIX pp

+!PHENIX pp p/


2+stat2syserror = p/π

K/π

As collision energy increases, spectra get harder.

Mean pT increases linearly with hadron mass. K/π and p/π ra=os saturate between √s=0.9 and 7 TeV. p/π+ and p/π-‐ converge to the same value.

Resonances and comparison with models


-1 (

Ge

V/c

)|y

|<0.8

|d

yT

dp

N2 d

evt

sN

1

-410

-310

-210 = 7 TeVspp INEL,

) / 2+

*!+ -

*!ALICE (

PHOJETPYTHIA D6T (109)

PYTHIA ATLAS-CSC (306)

PYTHIA Perugia 2011 (350)

ALICE Preliminary2 + syst2statuncert. =

) / 2+

*!+ -

*!(

(GeV/c)T

p0 0.5 1 1.5 2 2.5 3 3.5 4

0 0.5 1 1.5 2 2.5 3 3.5 4M

OD

EL / D

ATA

0

0.5

1

Models do not provide good descrip=on of resonance spectra yet.

Analysis exploits PID in TPC and TOF.

Yields extracted by fitting the spectra with a

Tsallis-Levy function.

(GeV/c)T

p0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5

-1 (

GeV

/c)

dy

Tdp

N2 d

evt

sN

1

-410

-310

-210

= 7 TeVs, pp !

= 0.9 TeVs, pp !

ALICE Preliminary

Φ(1020)

Multi-strange baryons and comparison with models


-1 (

Ge

V/c

)|y

| <

0.5

dy

|t

N/d

p2

) d

inel.

evt

sN

!1

/(2 -6

10

-510

-410

-310

-"

-#

+"

+#

ALICE data 2010

= 7 TeVspp

2+syst2statUncert.:

ALICE Preliminary

vy FunctionseFitted with L

Normalization uncert.

(GeV/c)t

p0 1 2 3 4 5 6 7 8 9

Data

/ P

yth

ia P

20

11

2

4

Identification via “cascade” topology in ITS

+TPC, PID via dE/dx in TPC

Yields extracted by fitting the spectra with a

Tsallis-Levy function.

ALICE and CMS agree on Ξ . Difference due to different normaliza=on choice (INEL vs. NSD) Pythia – Perugia 2011 underes=mates the absolute values of yields, ⟨pT⟩and gain when going from √s = 0.2 to 7 TeV.

STAR, PRC 75, 064901 (2007) CMS, JHEP 05, 064 (2011)

dN/dy |(y~0) ~ s0.22 ,⟨pT⟩ ~ s0.25

0!

|y|

dN

/dy |

-410

-310

-210

pp collisions

, ALICE Inel±"

, CMS NSD±"

, STAR NSD±"

, PYTHIA Perugia 2011±"

, ALICE Inel±#

, STAR NSD±#

, PYTHIA Perugia 2011±#

(GeV) s210

310 410

> (

Ge

V/c

)t

<p

0.4

0.6

0.8

1

1.2

1.4

RHIC200 GeV

LHC900 GeV

LHC7 TeV

ALICE Preliminary

Ξ Ω

Summary - pp collisions


•  π/K/p, resonances and mul=-‐strange baryon spectra have been measured with ALICE down to low pT

•  Comparison with Monte Carlo models show that Pythia -‐ Perugia 2011 tune

provides the best agreement for –  Kaons and protons yields

but s=ll it is not sufficient to describe –  Charged pions spectra at high pT (overes=mate) –  Resonances spectra (underes=mate) –  Mul=-‐strange baryons yields (underes=mate)

References for Pb-‐Pb collision available More effort needed to tune properly MC models

Pb-Pb collisions at √sNN = 2.76 TeV


Medium evolution and “soft” probes

Hadronic states provide many useful observables to characterize medium produced

in ultra-relativistic heavy ion collisions


Kine=c freeze-‐out – Stop elas=c interac=ons

Resonances re-‐scaqering and regenera=on

Chemical freeze-‐out -‐ Stop inelas=c interac=ons -‐ Fix par=cle ra=os

Thermaliza=on (hydrodynamics) -‐ Flow

Quark-‐Gluon Plasma forma=on (τ ~ 1 fm/c aser collision)

Pre-‐equilibrium state

Parton hard scaqering/Jets -‐  pQCD regime -‐  Hard probes

QGP expansion (life=me @ LHC ~ fm/c) -‐ Hadroniza=on

0-‐5% most central

80-‐90% peripheral

Centrality

Primary π/K/p spectra from RHIC to LHC


!/dch

dN210 310

(G

eV

/c)

"T

p#

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2 (ALICE)-$ (ALICE)

-K

(ALICE)p (STAR Au-Au 200 GeV)-$ (STAR Au-Au 200 GeV)

-K

(STAR Au-Au 200 GeV)pALICE Preliminary

= 2.76 TeVNNsPb-Pb

2syst% + 2

stat%

(GeV/c)T

p0 0.5 1 1.5 2 2.5 3

-1d

y) (

Ge

V/c

)T

N/(

dp

2 d

-110

1

10

210

310

0-5% most centralALICE Preliminary

= 2.76 TeVNNsALICE, Pb-Pb,

= 200 GeVNNsSTAR, Au-Au, = 200 GeVNNsPHENIX, Au-Au,

S0

K -!

-K p

Nega9ve par9cles

STAR, PRC 79, 034909 (2009) PHENIX, PRC 69, 03409 (2004)

Harder spectra and flaqer protons than at RHIC <pT> increases with dNch /dη <pT> higher than at RHIC for similar dNch /dη

Indica=ons for stronger radial flow at LHC

π/K/p spectra testing hydrodynamic models


(GeV/c)T

p0 0.5 1 1.5 2 2.5 3

-1d

y) (

Ge

V/c

)T

N/(

dp

2 d

-110

1

10

210

310


Curves: Hydro (arXiv:1105.3226)

= 2.76 TeVNNsALICE, Pb-Pb, S0

K +!

+K p

Posi9ve par9cles

(GeV/c)T

p0 0.5 1 1.5 2 2.5 3

-1dy

) (G

eV/c

)T

N/(d

p2 d

-110

1

10

210

310


Curves: Hydro + UrQMD (VISHNU) = 2.76 TeVNNsALICE, Pb-Pb, S

0K + +K p

Posi9ve par9cles

Hydro → UrQMD: decoupling temperature tuned to STAR p/π Hydro+UrQMD describes proton yield beUer than pure hydro.

Model: U.Heinz et al, arXiv:1105.3226 Model: U.Heinz et al, arXiv: 1108.5323

Yields and shape of π and K are in good agreement with hydro predic=ons.

Blast-wave fit to π/K/p spectra


> (c)<0 0.2 0.4 0.6

(GeV

)foT

0

0.05

0.1

0.15

0.2, K, pAu-Au, pp, 0.2 TeV,

, K, pPb-Pb, 2.76 TeV,

Preliminary

Global Blast-‐wave fit to extract (kine=c) freeze-‐out temperature (Tfo) and average radial flow ‹β›

Tfo parameter of the model depends on π fit range. Resonances effect to be inves=gated

Schnedermann et al., Phys. Rev. C 48, 2462 (1993) STAR, PRC 79, 034909 (2009)

‹β› ≃0.66 c ~10% higher than at RHIC

Tfo seems slightly lower than at RHIC

Multi-strange baryons spectra


(GeV/c)T

p0 1 2 3 4 5 6 7 8 9

-1 (

Ge

V/c

)|y

|<0.5

|T

dN

/dp

evt

s1

/N

-610

-510

-410

-310

-210

-110

1

10 = 2.76 TeVNNsPb-Pb at

2 + sys2staterror =

+!

0-20%

20-40%

40-60%

60-90%

Preliminary

(GeV/c)T

p0 1 2 3 4 5 6 7 8

-1 (

Ge

V/c

)|y

|<0.5

|T

dN

/dp

evt

s1

/N

-610

-510

-410

-310

-210

-110

1 = 2.76 TeVNNsPb-Pb at

2 + sys2staterror =

+!

0-20%

20-40%

40-60%

60-90%

Preliminary

An=-‐par=cle/par=cle ra=o ~ 1 also for mul=-‐strange baryons, as expected.

(GeV/c)T

p0 1 2 3 4 5 6 7 8 9 10

-1 (G

eV/c

)|y

|<0.

5 | T

dN

/dp

evts

1/N

-610

-510

-410

-310

-210

-110

1 = 2.76 TeVNNsPb-Pb at 0-90% centrality

2 + sys2staterror =

-

+

-

+

Preliminary

Spectra for mul=-‐strange par=cles measured in: •  0-‐90 % centrality •  4 centrality bins for Ξ and Ω (both charges) Iden=fica=on via “cascade” decay topology

Particle ratios testing the thermal model


Model main assump=on: grand canonical descrip=on (also for Kaons and mul=-‐strange par=cles)

A.Andronic et al, Phys.LeE.B 673, 142 (2009)

Par=cle ra=os are (partly) in agreement with thermal model predic=ons

Protons difficult to fit in a thermal model with T = 160-‐170 MeV Ad hoc model with T = 148 MeV describes beUer p but fails for mul9-‐strange

At LHC an=-‐par=cle/par=cle ra=os at mid-‐rapidity are compa=ble with 1, as expected. à μB ~ 0

Λ/K0s from pp to Pb-Pb, from RHIC to LHC


(GeV/c)T

p0 1 2 3 4 5 6 7 8

0 S /

K!

0

0.5

1

1.5

2

2.5

3 = 2.76 TeV, |y|<0.75NNsPb-Pb at

data points include stat. errorsestimated syst. error ~10 %

!/! "STAR: Au-Au 200 GeV

ALICE:

with 10% feed-down correction

0-5% centrality60-80% centrality 0-5% centrality60-80% centrality

Preliminary

(GeV/c)T

p0 1 2 3 4 5 6 7 8

S0 /

K!

0

0.5

1

1.5

2

2.5

0-5 % centrality20-40 % centrality40-60 % centrality60-80 % centrality80-90 % centrality

= 7 TeV)s(pp at = 0.9 TeV)s(pp at

data points include stat. errorsestimated syst. error ~10 %

Preliminary

= 2.76 TeV, |y|<0.75NNsPb-Pb at

Baryon/meson vs. pT increases from pp to a value above 1 for central Pb-‐Pb collisions Magnitude decreases from central to peripheral events, where it reaches pp ra=o. Radial flow (pT) shis when compared with STAR. Λ/K0s Magnitude increases with both centrality and beam energy from RHIC to LHC

Nuclear modifica9on factor defined as <Ncoll> = total number of binary nucleon-‐nucleon collisions (from a Glauber model)

Nuclear modification factor of Λ and K0s


Suppression of strange baryon and meson measured in Pb-‐Pb collisions with respect to pp (here for most central collisions): Λ: baryon enhancement at intermediate pT and suppression at high pT K0s: meson suppression up to high pT For central collisions and high pT(> 8 GeV/c), the RAA of K0s and Λ is compa=ble with the one of charged par=cles

No strong flavor dependence of RAA is observed

(GeV/c)T

p0 5 10 15 20

AA

R

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6 = 2.76 TeV, 0 - 5 %sALICE, Pb-Pb,

, | y | < 0.750K

, | y | < 0.750!

| < 0.8"unidentified charged particles, |

, | y | < 0.8-# + +#

ALICE Preliminary



>part<N1 10 210

> r

ela

tive

to

pp

part

Yie

ld / <

N

1

10

Preliminary

= 2.76 TeVNN

sPb-Pb at

-!

>part<N1 10 210

> r

ela

tive

to

pp

part

Yie

ld / <

N1

10

+"+

-"

+!

Enhancement increases following hierarchy of strangeness content (valence quarks) of baryons.

Strangeness enhancement with respect to pp collisions defined as:



ALICE results compared with lower energy data (SPS, RHIC): •  enhancement decreases as energy increases •  same trend observed from RHIC to SPS Details of centrality dependence and energy dependence are work in progress..

Enhancement increases following hierarchy of strangeness content (valence quarks) of baryons.

Strangeness enhancement with respect to pp collisions defined as:

>part<N1 10 210

> r

ela

tive

to

pp

/Be

part

Yie

ld / <

N

1

10

Preliminary

= 2.76 TeVNN

sPb-Pb at

-!

"

NA57 (17.2 GeV):

STAR (200 GeV):

>part<N1 10 210

> r

ela

tive

to

pp

/Be

part

Yie

ld / <

N1

10

+#+

-#

+!

"

NA57 (17.2 GeV):

STAR (200 GeV):

Summary and conclusions - Pb-Pb collisions


•  π/K/p and mul=-‐strange baryon spectra have been measured for different centrality and compared to measurements at lower energies

•  π/K/p, spectra mass and centrality dependence indicate a stronger radial flow at the LHC than at RHIC

•  An9-‐par9cle/par9cle ra9o ~ 1, as expected à μB~0 at LHC •  Ra9os of Ξ, Ω, K to π compa9ble with thermal model expecta9ons •  p/π difficult to understand in a thermal model prediction with T=160-170 MeV

•  Strange baryon/meson ra9o vs. pT increases from pp to a value above unity for central Pb-‐Pb

•  Suppression of strange par9cle (RAA) produc=on shows no obvious flavor dependence

•  Strangeness enhancement observed

ALICE provides many observables to characterize the medium evolu=on and proper=es at the LHC These were only selected highlights of the “sos” side of the story…

some more in the other presenta=ons at this conference !!!


Backup

Identified particles pT-spectra analysis - details


Events with primary vertex reliably measured by the SPD Primary tracks are selected. ITS: Primary vertex reconstruction (SPD) + Standalone tracking

PID via measurement of dE/dx in silicon dE/dx resolution: ~10-15% in PbPb TPC:

Global tracking PID via measurement of DE/dx in gas dE/dx resolution: ~5 % in Pb-Pb

TOF:

PID via gaussian unfolding of TOF signal

Global* time resolution: 80 ps in Pb-Pb

*Includes resolution on event time zero determination

p (GeV/c)0.2 0.3 1 2 3 4 5 6 7 10 20

TPC

sig

nal (

arb.

uni

ts)

50

100

150

200

e

K p d

ALICE Performance = 2.76 TeV min. biasNNsPb-Pb, 18/05/2011

π/K/p spectra in pp collisions at √s = 0.9 TeV


pp at √s = 0.9 TeV Total pt-‐dependent error (syst. + stat.) Normaliza=on error 3.6%

pp at √s = 0.9 TeV Total pt-‐dependent error (syst. + stat.) Normaliza=on error 3.6%

Eur. Phys. J. C (2011) 71:1655

Different PID techniques exploited

Combined analysis in ITS, TPC and TOF

Min. pT = 0.1 (π), 0.2 (K) and 0.3 (p) GeV/c

Spectra have been fitted with Tsallis-Levy function in order to extract mean pT , particle yields and compute particle ratios.

π/K/p spectra in pp collisions at √s = 7 TeV


Spectra have been fitted with Tsallis-Levy function in order to extract mean pT , particle yields and compute particle ratios.

(GeV/c)t

p0 0.5 1 1.5 2 2.5

-1 |

y|<

0.5

(G

eV

/c)

t d

N/d

pIN

EL

1/N

-310

-210

-110

1

10ALICE preliminary

= 7 TeVspp at

2+stat2syserror =

Levy fit

-!

-K

p

Normaliztion Error 8.3%

Nega9ve Total pt-‐dependent error (syst. + stat.)

Different PID techniques exploited

Combined analysis in ITS, TPC and TOF

Min. pT = 0.1 (π), 0.2 (K) and 0.3 (p) GeV/c

(GeV/c)t

p0 0.5 1 1.5 2 2.5

-1 |y

|<0.

5 (G

eV/c

)t

dN

/dp

INEL

1/N

-310

-210

-110

1

10ALICE preliminary

= 7 TeVspp at 2+stat2syserror =

Levy fit

+

+K

p

Normaliztion Error 8.3%

Posi9ve Total pt-‐dependent error (syst + stat.) shown

Measured spectra in pp@7TeV vs. models

K+ p -1 |y|

<0

.5 (

Ge

V/c

)t

dN

/dp

INE

L1

/N

-310

-210

-110

1


Preliminary


+K

Phojet

Pythia D6T

Pythia Perugia0

Pythia Perugia 2011

(GeV/c)t

p0 0.5 1 1.5 2 2.5 3

MC

/data

0

0.5

1

1.5

2

-1 |y|

<0

.5 (

Ge

V/c

)t

dN

/dp

INE

L1

/N

-310

-210

-110


Preliminary


p

Phojet

Pythia D6T

Pythia Perugia0

Pythia Perugia 2011

(GeV/c)t

p0 0.5 1 1.5 2 2.5 3

MC

/data

0

0.5

1

1.5

2

K-‐ p -1 |y|<

0.5

(G

eV

/c)

tdN

/dp

INE

L1/N

-310

-210

-110


Preliminary


p

Phojet

Pythia D6T

Pythia Perugia0

Pythia Perugia 2011

(GeV/c)t

p0 0.5 1 1.5 2 2.5 3

MC

/data

0

0.5

1

1.5

2

-1 |y|

<0

.5 (

Ge

V/c

)t

dN

/dp

INE

L1

/N

-310

-210

-110

1


Preliminary


-K

Phojet

Pythia D6T

Pythia Perugia0

Pythia Perugia 2011

(GeV/c)t

p0 0.5 1 1.5 2 2.5 3

MC

/data

0

0.5

1

1.5

2


π/K/p spectra in Pb-Pb collisions at √sNN = 2.76 TeV


(GeV/c)T

p0 0.5 1 1.5 2 2.5 3

(c/G

eV

)T

dyd

pN2

d

-110

1

10

210

310

0-5%

5-10%

10-20%

20-30%

30-40%

40-50%

50-60%

60-70%

-!

|y| < 0.5

2syst" + 2

stat"

(GeV/c)T

p0 0.5 1 1.5 2 2.5 3

(c

/Ge

V)

Td

yd

pN2

d

-210

-110

1

10

210

0-5%

5-10%

10-20%

20-30%

30-40%

40-50%

50-60%

60-70%

p

|y| < 0.5

2syst! + 2

stat!

(GeV/c)T

p0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 2.2

(c/G

eV

)T

dyd

pN2

d

-110

1

10

210

0-5%

5-10%

10-20%

20-30%

30-40%

40-50%

50-60%

60-70%

-K

|y| < 0.5

2syst! + 2

stat!

(GeV/c)T

p0 0.5 1 1.5 2 2.5 3

(c/G

eV

)T

dyd

pN2

d

-110

1

10

210

310

0-5%

5-10%

10-20%

20-30%

30-40%

40-50%

50-60%

60-70%

+!

|y| < 0.5

2syst" + 2

stat"

(GeV/c)T

p0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 2.2

(c

/Ge

V)

Td

yd

pN2

d

-110

1

10

210

0-5%

5-10%

10-20%

20-30%

30-40%

40-50%

50-60%

60-70%

+K

|y| < 0.5

2syst! + 2

stat!

(GeV/c)T

p0 0.5 1 1.5 2 2.5 3

(c/G

eV

)T

dyd

pN2

d

-210

-110

1

10

210

0-5%

5-10%

10-20%

20-30%

30-40%

40-50%

50-60%

60-70%

p

|y| < 0.5

2syst! + 2

stat!

Blast-‐wave fit to extract mean pT and integrated par=cle yields

(GeV/c)T

p0 0.5 1 1.5 2 2.5 3

-1dy)

(G

eV

/c)

TN

/(dp

2 d

-110

1

10

210

310




STAR (TOF, feed-down corr.)

S0

K +!

+K p

(GeV/c)T

p0 0.5 1 1.5 2 2.5 3

-1d

y) (

Ge

V/c

)T

N/(

dp

2 d-110

1

10

210

310




S0

K -!

-K p


Measured spectra in Pb-Pb at LHC vs RHIC

Particle ratios in Pb-Pb collisions


At LHC an=-‐par=cle/par=cle ra=os at mid-‐rapidity are compa=ble with 1, as expected for μB ~ 0. K/π and p/π ra=os at LHC and RHIC exhibit similar centrality dependence. K/π slightly increases with dNch/dη from pp to very central events

+!/-!

+/K

-K /pp

0.5

0.6

0.7

0.8

0.9

1

1.1

1.2

1.3

= 2.76 TeV (ALICE Pb-Pb)NN

s

= 200 GeV (STAR Au-Au)NN

s

= 200 GeV (PHENIX Au-Au)NN

s

ALICE Preliminary

0-5% centrality

!/dch

dN210 310

-"/-

K

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.16

0.18

0.2

= 2.76 TeVNNsALICE Pb-Pb

= 200 GeVNNsSTAR Au-Au ALICE Preliminary = 2.76 TeVNNsPb-Pb

2syst# + 2

stat#

= 7 TeVspp,

!/dch

dN210 310

-"/

p

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.16

0.18

0.2 = 2.76 TeV (feed-down corrected)NNsALICE Pb-Pb

= 200 GeV (not feed-down corrected)NNsSTAR Au-Au

= 200 GeV (feed-down corrected)NNsPHENIX Au-Au

= 200 GeV (feed-down corrected, stat. errors only)NNsBRAHMS Au-Au

ALICE Preliminary

= 2.76 TeVNNsPb-Pb

2syst# + 2

stat#

= 7 TeVspp,

STAR, not feed-‐down subtracted

STAR, PRC 79, 034909 (2009) PHENIX, PRC 69, 03409 (2004) BRAHMS, PRC 72, 014908 (2005)

Feed-down correction for p for pp analysis

Feed-down correction needed to eliminate contributions from weak decay of strange particle (Λ,Ξ,…) to the primary spectra:

•  for protons ~40% •  for pions ~15%

Distance of closest approach (DCA) in the bending plane used to correct. Correction estimated from a fit using Monte Carlo templates for distributions of •  primaries •  secondaries from weak decay of strange

particles •  secondaries from interaction with the

material.


Global tracks, TPC PID (0.45 < pT < 0.50 GeV/c)

•  ITS vertexing + tracking •  TPC tracking + PID (dE/dx) •  Topological reconstruction of decays:

–  Selection of Λ based on cuts on impact

parameter and invariant mass –  TPC PID for all decays products

•  Signal extraction: –  Polynomial+gaussian fit for mean and σ –  Bin counting in (±3σ) signal region –  Integral of background fit function in signal

region –  Signal = bin counting - integral

Multi-strange baryons analysis ALICE Physics Performance Report vol. II

HIJING Pb-‐Pb event at √sNN = 5.5 TeV

π-‐ Λ π-‐

p

Ξ-‐


Centrality in Pb-Pb collisions


0-‐5% most central

80-‐90% peripheral

The collision centrality is determined using the total mul=plicity detected in the forward VZERO detectors and reported as a frac=on of the total hadronic cross sec=on.

ALICE, PRL 106, 032301 (2011)

Nuclear modification factor of Λ and K0s


(GeV/c)T

p0 5 10 15 20

AA

R

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6 = 2.76 TeV, 0 - 5 %sALICE, Pb-Pb,

, | y | < 0.750K

, | y | < 0.750!


ALICE Preliminary

(GeV/c)T

p0 5 10 15 20

AAR

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6 = 2.76 TeV, 60 - 80 %sALICE, Pb-Pb,

, | y | < 0.750K, | y | < 0.750

| < 0.8unidentified charged particles, |

ALICE Preliminary

Suppression of strange baryon and meson measured in central (les) and peripheral (right) Pb-‐Pb collisions with respect to pp: Λ: baryon enhancement at intermediate pT , suppression at high pT K0s: meson suppression up to high pT

(GeV/c)T

p0 5 10 15 20

AA

R

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6 = 2.76 TeV, 60 - 80 %sALICE, Pb-Pb,

, | y | < 0.750K

, | y | < 0.750!


, | y | < 0.8-# + +#

ALICE Preliminary

Particle production in pp and Pb-Pb collisions with the ALICE ...

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