Evidence of color coherence effects in W+jets events from p p collisions at s =1.8 TeV

7 October 1999

Ž .Physics Letters B 464 1999 145–155

Evidence of color coherence effects in Wq jets events(from pp collisions at s s1.8 TeV

DØ Collaboration

B. Abbott as, M. Abolins ap, V. Abramov r, B.S. Acharya k, I. Adam ar,D.L. Adams bb, M. Adams ab, S. Ahn aa, V. Akimov p, G.A. Alves b, N. Amos ao,

E.W. Anderson ah, M.M. Baarmand au, V.V. Babintsev r, L. Babukhadia t,A. Baden al, B. Baldin aa, S. Banerjee k, J. Bantly ay, E. Barberis u, P. Baringer ai,

J.F. Bartlett aa, A. Belyaev q, S.B. Beri i, I. Bertram s, V.A. Bezzubov r,P.C. Bhat aa, V. Bhatnagar i, M. Bhattacharjee au, G. Blazey ac, S. Blessing y,

P. Bloom v, A. Boehnlein aa, N.I. Bojko r, F. Borcherding aa, C. Boswell x,A. Brandt aa, R. Breedon v, G. Briskin ay, R. Brock ap, A. Bross aa, D. Buchholz ad,

V.S. Burtovoi r, J.M. Butler am, W. Carvalho c, D. Casey ap, Z. Casilum au,H. Castilla-Valdez n, D. Chakraborty au, K.M. Chan at, S.V. Chekulaev r, W. Chen au,

D.K. Cho at, S. Choi m, S. Chopra y, B.C. Choudhary x, J.H. Christenson aa,M. Chung ab, D. Claes aq, A.R. Clark u, W.G. Cobau al, J. Cochran x,

L. Coney af, W.E. Cooper aa, D. Coppage ai, C. Cretsinger at, D. Cullen-Vidal ay,M.A.C. Cummings ac, D. Cutts ay, O.I. Dahl u, K. Davis t, K. De az,

K. Del Signore ao, M. Demarteau aa, D. Denisov aa, S.P. Denisov r, H.T. Diehl aa,M. Diesburg aa, G. Di Loreto ap, P. Draper az, Y. Ducros h, L.V. Dudko q,S.R. Dugad k, A. Dyshkant r, D. Edmunds ap, J. Ellison x, V.D. Elvira au,

R. Engelmann au, S. Eno al, G. Eppley bb, P. Ermolov q, O.V. Eroshin r, J. Estrada at,H. Evans ar, V.N. Evdokimov r, T. Fahland w, M.K. Fatyga at, S. Feher aa, D. Fein t,T. Ferbel at, H.E. Fisk aa, Y. Fisyak av, E. Flattum aa, G.E. Forden t, M. Fortner ac,

K.C. Frame ap, S. Fuess aa, E. Gallas aa, A.N. Galyaev r, P. Gartung x, V. Gavrilov p,T.L. Geld ap, R.J. Genik II ap, K. Genser aa, C.E. Gerber aa, Y. Gershtein ay,

B. Gibbard av, G. Ginther at, B. Gobbi ad, B. Gomez e, G. Gomez al, P.I. Goncharov r,´ ´J.L. Gonzalez Solıs n, H. Gordon av, L.T. Goss ba, K. Gounder x, A. Goussiou au,´ ´

N. Graf av, P.D. Grannis au, D.R. Green aa, J.A. Green ah, H. Greenlee aa,S. Grinstein a, P. Grudberg u, S. Grunendahl aa, G. Guglielmo ax, J.A. Guida t,¨

J.M. Guida ay, A. Gupta k, S.N. Gurzhiev r, G. Gutierrez aa, P. Gutierrez ax,

0370-2693r99r$ - see front matter q 1999 Published by Elsevier Science B.V. All rights reserved.Ž .PII: S0370-2693 99 01015-1

( )B. Abbott et al.rPhysics Letters B 464 1999 145–155146

N.J. Hadley al, H. Haggerty aa, S. Hagopian y, V. Hagopian y, K.S. Hahn at,R.E. Hall w, P. Hanlet an, S. Hansen aa, J.M. Hauptman ah, C. Hays ar,

C. Hebert ai, D. Hedin ac, A.P. Heinson x, U. Heintz am, R. Hernandez-Montoya n,´T. Heuring y, R. Hirosky ab, J.D. Hobbs au, B. Hoeneisen f, J.S. Hoftun ay,

F. Hsieh ao, Tong Hu ae, A.S. Ito aa, J. Jaques af, S.A. Jerger ap,R. Jesik ae, T. Joffe-Minor ad, K. Johns t, M. Johnson aa,

A. Jonckheere aa, M. Jones z, H. Jostlein aa, S.Y. Jun ad, S. Kahn av, D. Karmanov q,¨D. Karmgard y, R. Kehoe af, S.K. Kim m, B. Klima aa, C. Klopfenstein v,

B. Knuteson u, W. Ko v, J.M. Kohli i, D. Koltick ag, A.V. Kostritskiy r, J. Kotcher av,A.V. Kotwal ar, A.V. Kozelov r, E.A. Kozlovsky r, J. Krane ah,

M.R. Krishnaswamy k, S. Krzywdzinski aa, M. Kubantsev aj, S. Kuleshov p,Y. Kulik au, S. Kunori al, F. Landry ap, G. Landsberg ay, A. Leflat q, J. Li az,

Q.Z. Li aa, J.G.R. Lima c, D. Lincoln aa, S.L. Linn y, J. Linnemann ap, R. Lipton aa,J.G. Lu d, A. Lucotte au, L. Lueking aa, A.K.A. Maciel ac, R.J. Madaras u,

R. Madden y, L. Magana-Mendoza n, V. Manankov q, S. Mani v, H.S. Mao d,˜R. Markeloff ac, T. Marshall ae, M.I. Martin aa, R.D. Martin ab, K.M. Mauritz ah,B. May ad, A.A. Mayorov r, R. McCarthy au, J. McDonald y, T. McKibben ab,

J. McKinley ap, T. McMahon aw, H.L. Melanson aa, M. Merkin q, K.W. Merritt aa,C. Miao ay, H. Miettinen bb, A. Mincer as, C.S. Mishra aa, N. Mokhov aa,

N.K. Mondal k, H.E. Montgomery aa, M. Mostafa a, H. da Motta b, F. Nang t,M. Narain am, V.S. Narasimham k, A. Narayanan t, H.A. Neal ao, J.P. Negret e,

P. Nemethy as, D. Norman ba, L. Oesch ao, V. Oguri c, N. Oshima aa, D. Owen ap,P. Padley bb, A. Para aa, N. Parashar an, Y.M. Park l, R. Partridge ay, N. Parua g,M. Paterno at, B. Pawlik o, J. Perkins az, M. Peters z, R. Piegaia a, H. Piekarz y,Y. Pischalnikov ag, B.G. Pope ap, H.B. Prosper y, S. Protopopescu av, J. Qian ao,

P.Z. Quintas aa, R. Raja aa, S. Rajagopalan av, O. Ramirez ab, N.W. Reay aj,S. Reucroft an, M. Rijssenbeek au, T. Rockwell ap, M. Roco aa, P. Rubinov ad,

R. Ruchti af, J. Rutherfoord t, A. Sanchez-Hernandez n, A. Santoro b, L. Sawyer ak,´ ´R.D. Schamberger au, H. Schellman ad, J. Sculli as, E. Shabalina q, C. Shaffer y,H.C. Shankar k, R.K. Shivpuri j, D. Shpakov au, M. Shupe t, R.A. Sidwell aj,

H. Singh x, J.B. Singh i, V. Sirotenko ac, P. Slattery at, E. Smith ax,R.P. Smith aa, R. Snihur ad, G.R. Snow aq, J. Snow aw, S. Snyder av, J. Solomon ab,

X.F. Song d, M. Sosebee az, N. Sotnikova q, M. Souza b, N.R. Stanton aj,G. Steinbruck ax, R.W. Stephens az, M.L. Stevenson u, F. Stichelbaut av,¨D. Stoker w, V. Stolin p, D.A. Stoyanova r, M. Strauss ax, K. Streets as,

M. Strovink u, A. Sznajder c, P. Tamburello al, J. Tarazi w, M. Tartaglia aa,T.L.T. Thomas ad, J. Thompson al, D. Toback al, T.G. Trippe u, P.M. Tuts ar,V. Vaniev r, N. Varelas ab, E.W. Varnes u, A.A. Volkov r, A.P. Vorobiev r,

( )B. Abbott et al.rPhysics Letters B 464 1999 145–155 147

H.D. Wahl y, J. Warchol af, G. Watts ay, M. Wayne af, H. Weerts ap, A. White az,J.T. White ba, J.A. Wightman ah, S. Willis ac, S.J. Wimpenny x,

J.V.D. Wirjawan ba, J. Womersley aa, D.R. Wood an, R. Yamada aa, P. Yamin av,T. Yasuda aa, P. Yepes bb, K. Yip aa, C. Yoshikawa z, S. Youssef y, J. Yu aa,

Y. Yu m, M. Zanabria e, Z. Zhou ah, Z.H. Zhu at, M. Zielinski at, D. Zieminska ae,A. Zieminski ae, V. Zutshi at, E.G. Zverev q, A. Zylberstejn h

a UniÕersidad de Buenos Aires, Buenos Aires, Argentinab LAFEX, Centro Brasileiro de Pesquisas Fısicas, Rio de Janeiro, Brazil´

c UniÕersidade do Estado do Rio de Janeiro, Rio de Janeiro, Brazild Institute of High Energy Physics, Beijing, People’s Republic of China

e UniÕersidad de los Andes, Bogota, Colombia´f UniÕersidad San Francisco de Quito, Quito, Ecuador

g Institut des Sciences Nucleaires, IN2P3-CNRS, UniÕersite de Grenoble 1, Grenoble, France´h DAPNIArSerÕice de Physique des Particules, CEA, Saclay, France

i Panjab UniÕersity, Chandigarh, Indiaj Delhi UniÕersity, Delhi, India

k Tata Institute of Fundamental Research, Mumbai, Indial Kyungsung UniÕersity, Pusan, South Korea

m Seoul National UniÕersity, Seoul, South Korean CINVESTAV, Mexico City, Mexico

o Institute of Nuclear Physics, Krakow, Poland´p Institute for Theoretical and Experimental Physics, Moscow, Russia

q Moscow State UniÕersity, Moscow, Russiar Institute for High Energy Physics, ProtÕino, Russia

s Lancaster UniÕersity, Lancaster, UKt UniÕersity of Arizona, Tucson, AZ 85721, USA

u Lawrence Berkeley National Laboratory and UniÕersity of California, Berkeley, CA 94720, USAv UniÕersity of California, DaÕis, CA 95616, USAw UniÕersity of California, IrÕine, CA 92697, USA

x UniÕersity of California, RiÕerside, CA 92521, USAy Florida State UniÕersity, Tallahassee, FL 32306, USA

z UniÕersity of Hawaii, Honolulu, HI 96822, USAaa Fermi National Accelerator Laboratory, BataÕia, IL 60510, USA

ab UniÕersity of Illinois at Chicago, Chicago, IL 60607, USAac Northern Illinois UniÕersity, DeKalb, IL 60115, USAad Northwestern UniÕersity, EÕanston, IL 60208, USA

ae Indiana UniÕersity, Bloomington, IN 47405, USAaf UniÕersity of Notre Dame, Notre Dame, IN 46556, USA

ag Purdue UniÕersity, West Lafayette, IN 47907, USAah Iowa State UniÕersity, Ames, IA 50011, USA

ai UniÕersity of Kansas, Lawrence, KS 66045, USAaj Kansas State UniÕersity, Manhattan, KS 66506, USAak Louisiana Tech UniÕersity, Ruston, LA 71272, USA

al UniÕersity of Maryland, College Park, MD 20742, USAam Boston UniÕersity, Boston, MA 02215, USA

an Northeastern UniÕersity, Boston, MA 02115, USAao UniÕersity of Michigan, Ann Arbor, MI 48109, USA

ap Michigan State UniÕersity, East Lansing, MI 48824, USAaq UniÕersity of Nebraska, Lincoln, NE 68588, USAar Columbia UniÕersity, New York, NY 10027, USAas New York UniÕersity, New York, NY 10003, USA

at UniÕersity of Rochester, Rochester, NY 14627, USAau State UniÕersity of New York, Stony Brook, NY 11794, USA

av BrookhaÕen National Laboratory, Upton, NY 11973, USA


aw Langston UniÕersity, Langston, OK 73050, USAax UniÕersity of Oklahoma, Norman, OK 73019, USA

ay Brown UniÕersity, ProÕidence, RI 02912, USAaz UniÕersity of Texas, Arlington, TX 76019, USA

ba Texas A&M UniÕersity, College Station, TX 77843, USAbb Rice UniÕersity, Houston, TX 77005, USA

Received 13 August 1999; accepted 16 August 1999Editor: L. Montanet

Abstract

We report the results of a study of color coherence effects in pp collisions based on data collected by the DØ detector'during the 1994–1995 run of the Fermilab Tevatron Collider, at a center of mass energy s s1.8 TeV. Initial-to-final state

color interference effects are observed by examining particle distribution patterns in events with a W boson and at least onejet. The data are compared to Monte Carlo simulations with different color coherence implementations and to an analyticmodified-leading-logarithm perturbative calculation based on the local parton-hadron duality hypothesis. q 1999 Publishedby Elsevier Science B.V. All rights reserved.

PACS: 13.87.ya; 12.38.Aw; 12.38.Qk; 13.85.y tKeywords: Experimental; pp interactions; QCD studies; Color coherence; Wq jets; Local parton-hadron duality

Color coherence phenomena in the final statehave been studied since the early 1980’s in eqey

w xannihilations 1–6 and are very well established.The study of coherence effects in hadron–hadroncollisions is considerably more subtle than those ineqey annihilations due to the presence of coloredconstituents in both the initial and final states. In thispaper we report the first results on initial-to-finalstate color interference effects in pp interactionsusing Wq jets events.

Coherence phenomena are an intrinsic property ofany gauge theory. In quantum chromodynamicsŽ .QCD , color coherence phenomena can be instruc-tively separated into two regions: intrajet and interjet

w xcoherence 7,8 . Intrajet coherence deals with coher-ent effects in partonic cascades, resulting on average

Ž .in the angular ordering AO of the sequential partonbranches, which give rise to the depletion of soft

Žparticle production the so called ‘‘hump-backed. w xplateau’’ inside jets 9–11 . Interjet coherence is

w x w xresponsible for the string 12 or drag 13 effect firstobserved in the final state products of eqey annihi-lations. It deals with the angular structure of softparticle flows when three or more energetic partonsare involved in the hard process. In this case, theoverall structure of particle angular distributions is

governed by the underlying color dynamics of thehard scattering processes at short distances.

Ž .Perturbative quantum chromodynamics pQCDcalculations have been used to describe the produc-tion of jet final states. However, descriptions of thecharacteristic particle structure of high energy hardcollisions still rely on phenomenological models toexplain how the partonic cascade evolves into final

Ž .state hadrons. Within Monte Carlo MC simulationsincorporating such models, the primary partons fromthe hard scatter evolve into jets of partons via gluonand quark emission according to pQCD. This processcontinues until a cut-off transverse-momentum scaleŽ .Q f1 GeVrc is reached. After this phase, non-0

perturbative processes take over, which ‘‘cluster’’the final partons into color singlet hadronic states viaa mechanism described by phenomenological frag-

w xmentation models, like the Lund ‘‘string’’ 14 or thew x‘‘cluster’’ 15 fragmentation models. These simula-

tions usually involve many a priori unknown param-eters that need to be tuned to the data.

A different and purely analytical approach givingquantitative predictions of hadronic spectra is basedon the concept of Local Parton Hadron DualityŽ . w xLPHD 9 . The key assumption of this hypothesis isthat the particle yield is described by a parton cas-


cade in which the conversion of partons into hadronsoccurs at a low virtuality scale, of the order of

Ž 2 .hadronic masses Q f200 MeVrc and indepen-0

dent of the scale of the primary hard process, andinvolves only low momentum transfers. It is assumedthat the results obtained for partons apply to hadronsas well in an inclusive and average sense. Within theLPHD approach, resummed pQCD calculations forthe parton cascade have been carried out in the

Ž .simplest case high energy limit in the DoubleŽ . w xLogarithmic Approximation DLA 10,16 , and in

the Modified Leading Logarithmic ApproximationŽ . w xMLLA 9,17,18 , which includes higher order terms

Ž .of relative order a e.g., finite energy corrections .( s

These higher order terms are essential for quantita-w xtive agreement with data at present energies 7,19 .

The AO approximation is an important conse-quence of color coherence. It results in the suppres-sion of soft gluon radiation in partonic cascades incertain regions of phase space. For the case ofoutgoing partons, AO requires that the emissionangles of soft gluons decrease monotonically as thepartonic cascade evolves away from the hard pro-cess. MC simulations including coherence effectsprobabilistically by means of AO are available for

Žboth initial and final state evolutions. Parton showerevent generators incorporate AO effects in the initialstate as the time reversed process of the outgoingpartonic cascade, i.e. the emission angles increasefor the incoming partons as the process develops

.from the initial hadrons to the hard subprocess. AOis an element of the DLA and MLLA analytic pQCDcalculations, which provides the probabilistic inter-pretation of soft-gluon cascades. In fact, beyond theMLLA a probabilistic picture of the parton cascadeevolution is not feasible due to 1rN 2-suppressedcŽ .where N is the number of colors soft interferencec

contributions that appear in the higher-order calcula-w xtions 8,17 .

w x w xBoth the CDF 20 and DØ 21 Collaborationshave measured spatial correlations between the softerthird jet and the second leading-E jet in pp ™T

3 jetsqX events to explore the initial-to-final statecoherence effects in pp interactions. The extractionof the color coherence signal in these measurementsrelies on comparisons of data distributions to MCsimulations with and without coherence effects. Inthe analysis described here, the coherence signal in

the data is extracted in a more direct way by compar-ing the soft particle angular distributions around thecolorless W boson and opposing jet in the sameevent.

The DØ detector is described in detail elsewherew x22 . This analysis uses the tracking system and theuraniumrliquid-argon sampling calorimeter. The DØcalorimeter has a transverse granularity of Dh=Df

s0.1=0.1 forming projective towers, where h isŽ w Ž .xthe pseudorapidity hsyln tan ur2 , u is the po-

.lar angle with respect to the proton beam , and f isthe azimuthal angle. It has hermetic coverage for< <h -4.1 with fractional transverse energy E resolu-T

tion of f80%r E GeV for jets and fractional( Ž .T

(energy resolution of f15%r E GeV for elec-Ž .trons.

w xThe data sample for this analysis 23 , represent-ing an integrated luminosity of 85 pby1, was col-lected during the 1994–1995 Tevatron Colliderrun. Events from W™eqn decays were collectedwith a trigger that required a minimum missing

Ž .transverse energy Eu of 15 GeV and an isolatedTŽ .electromagnetic EM cluster with transverse

energy E )20 GeV. The offline kinematic require-T

ments imposed on this sample were Eu )25 GeV,Te < < 2E ) 25 GeV, h - 1.1, and 40 GeVrcT e

Ž . 2 Ž .- M e, Eu - 110 GeVrc , where M e, EuT T T T

2(s 2 p e p n 1ycos Df rc is the W bo-Ž . Ž . Ž .T T

son transverse mass and Df is the azimuthal se-paration between the electron and neutrino. The

Ž .transverse momentum of the neutrino, p n , wasT

calculated using the calorimetric measurement of theEu in the event.T

Events were required to have one electron clusterpassing four quality criteria based on shower profile

Ž .and tracking information: i the ratio of the EMenergy to the total shower energy had to be greater

Ž .than 0.95, ii the position of the calorimeter energydeposition of the electron had to match with a track

Ž .found in the drift chambers, iii the lateral andlongitudinal shape of the energy cluster had to be

Ž .consistent with those of an electron, and iv theelectron had to be isolated from other energy de-posits in the calorimeter with isolation fraction fiso

-0.1. The isolation fraction is defined as f sisow Ž . Ž .x Ž . Ž .E 0.4 y E 0.2 rE 0.2 , where E REM EM coneŽ Ž .. Ž .E R is the total electromagnetic energyEM cone


2 2(within a cone of radius R s Dh q DfŽ . Ž .cone

centered around the electron.Jets in the events were reconstructed offline using

an iterative fixed-cone clustering algorithm with coneradius R s0.7. Spurious jets from isolated noisycone

calorimeter cells and accelerator losses were elimi-nated by loose cuts on the jet shape.

Events were required to have a measured vertexwith longitudinal position within 20 cm of the detec-tor center to preserve the projective geometry. Since

Žmultiple interactions more than one proton–antipro-.ton interaction in the same bunch crossing are ex-

pected to increase the global energy level in thecalorimeter affecting the color coherence signal, weretained only events with a single reconstructed ver-tex and additionally required the beam-beam ho-doscope timing information to be consistent with asingle interaction. Finally, events were eliminatedwhen there was significant pileup energy in thecalorimeter around the region where the TevatronMain Ring passes through the DØ detector.

We study color coherence in Wq jets events bycomparing the distributions of soft particles aroundthe W boson and opposing leading-E jet. Since theT

W boson is a colorless object, it should not con-tribute to the production of secondary particles,thereby providing a template against which the pat-tern around the jet may be compared. This compari-son reduces the sensitivity to global detector andunderlying event biases that may be present in thevicinity of both the W boson and the jet.

The W boson was reconstructed from its electronand neutrino decay products, resulting in a twofold

Žambiguity in the W boson rapidity y due to theW

corresponding ambiguity in the neutrino longitudinal.momentum p . MC studies have shown that theZ

< <smaller y is closer to the true W boson rapidityW

approximately 2r3 of the time, so this is the solutionchosen. This choice was also made in the MC Wboson reconstruction to retain consistency in thecomparison with the data.

Once the W boson direction has been determined,the opposing jet was identified by selectingthe leading-E jet in the azimuthal hemisphere oppo-T

site to the W boson. Annular regions are drawnŽ .around both the W boson and the tagged jet in h,f

space. The angular distributions of calorimeter tow-ers with E ) 250 MeV are measured in theseT

annular regions using the polar variables R2 2(s Dh q Df andŽ . Ž .

sign h PDfŽ .X Xy1b s tan ;X ž /DhX

where XsW or jet, Dh sh yy , Dh sW tower W jet

h yh , and Df sf yf , in a searchtower jet X tower XŽ .disk of 0.7-R-1.5 Fig. 1 . For events in which

the jet and the W boson annuli overlapped, allcalorimeter towers in the shared region were as-signed to the nearest object.

We define b s0 to point along the beamW Ž jet.Ž .direction nearest to the W boson jet . Calorimeter

towers which correspond to partially instrumentedŽcalorimeter regions regions between the central and

end-cap calorimeter cryostats with pseudorapidities< < .1.1- h -1.4 were not included in the b dis-tower

tributions for either the data or MC simulations.Calorimeter cells that belong to the electron clusterin a cone of R s0.3 from the centroid centercone

were eliminated. We study the interference effects in< < < <regions h -0.7 and y -0.7, requiring thejet W

tagged jet to have E )10 GeV and the W bosonT

p )10 GeVrc. After application of all selectionT

criteria 390 events remain.The measured angular distributions are compared

w xto the predictions of PYTHIA 5.7 24 parton showerevent generator with different levels of color coher-ence effects and to an analytic pQCD calculation of

w xKhoze and Stirling 25 based on MLLA and LPHD.The PYTHIA MC sample was processed through a

Fig. 1. Annular regions around the W boson and the tagged jet inŽ .h,f space.


w xfull GEANT-based detector simulation 26 . To bestmodel the calorimeter noise effects in the MC simu-lations, we overlaid noise contributions for eachcalorimeter cell from the data. The generated eventswere subsequently processed using the same criteriaemployed for analyzing the data.

PYTHIA incorporates initial and final state colorinterference effects by means of the AO approxima-tion of the parton cascades. After the perturbative

Ž .phase it employs the Lund string fragmentation SFŽ .model or an independent fragmentation IF model

as the phenomenological model to describe the non-perturbative hadronization process. The SF modelhas been supported by the observations of colorcoherence phenomena in eqey annihilations. In PY-THIA, the AO constraint can be turned off. When

both AO and SF are implemented, PYTHIA accountsfor color coherence effects at both the perturbativeand non-perturbative levels. Turning off AO removesthe perturbative contribution, and using IF eliminatesthe non-perturbative component.

We check whether the MC simulations describethe event characteristics in the data using distribu-tions of electron E , the event Eu , and the azimuthalT T

and rapidity separation of the W boson and thetagged jet. These are compared to the PYTHIAsimulations with full coherence effects in Fig. 2. Forall distributions, PYTHIA is in good agreement withthe data.

Ž .Fig. 3 a shows the measured angular distribu-tions of the number of towers above threshold aroundthe jet and around the W boson. The E of eachT

Ž . Ž . Ž . Ž . Ž .Fig. 2. Comparisons of data points with PYTHIA events with AO and SF histogram for a electron E , b event Eu , c azimuthalT TŽ .separation of the W boson and tagged jet, and d rapidity separation of the W boson and tagged jet.


Ž . Ž .Fig. 3. a Calorimeter tower multiplicity around the jet and W boson as a function of b. b Ratio of the tower multiplicity around the jet tothe tower multiplicity around the W boson as a function of b.

Žcalorimeter cell each calorimeter tower is con-structed from many calorimeter cells following theprojective geometry of the detector with respect to

.the detector center was corrected for offsets due tonoise, zero suppression, and energy pileup effectsw x23 . A prominent feature of both curves is a strongpeaking around pr2-b-3pr4. This is becausethe shapes of the b distributions are sensitive both toprocess dynamics and to phase space effects result-ing from our event selection criteria and thecalorimeter tower E threshold.T

Ž .Fig. 3 b shows the ratio of the tower multiplicityaround the jet to the tower multiplicity around the Wboson as a function of b. The errors include onlystatistical uncertainties, which are the dominantsource of uncertainty. The data show that the towermultiplicity around the jet is greater than that aroundthe W boson and the excess is enhanced in the event

Žplane i.e., the plane defined by the directions of the.W boson or jet and the beam axis: bs0, p and

Ž .minimized in the transverse plane bspr2 . Thisis consistent with the expectation from initial-to-finalstate color interference that there is an enhancementof soft particle production around the tagged jet inthe event plane relative to the transverse plane whencompared with the particle production around the Wboson. It is also in agreement with our published

w xmultijet analysis results 21 .The ratio of the tower multiplicity around the jet

to the tower multiplicity around the W boson isshown in Fig. 4 for the data and the MC predictions

as a function of b. All predictions have been nor-malized to the integral of the data b-distribution.PYTHIA with AO and SF is in good agreement withthe Wq jets data. PYTHIA with AO off and SFagrees less well, and PYTHIA with AO off and IFdoes not reproduce the data. The shape of the ana-lytic prediction based on MLLA and LPHD is con-

Ž .sistent with the data as shown in Fig. 4 d . We notethat the analytic calculation was performed assumingthat the W boson and the outgoing parton wereproduced centrally and does not include any underly-ing event or detector simulation effects.

To measure the color coherence signal, we con-struct the variable R as the jetrW boson towersig

Ž .multiplicity ratio of the event plane bs0,p to theŽ .transverse plane bspr2 . We define

R bs0,pŽ .R s ,sig R bspr2Ž .where

N 1stbin qN 7 thbinjet jet

R bs0,p s ,Ž . st th1 bin 7 binN qNW W

N 4thbinjet

R bspr2 s ,Ž . th4 binNW

and N i thbin is the number of towers above thresholdW Ž jet.th Ž .for the i bin of the W boson jet b distribution.

R is expected to be near unity in the absence ofsig

color coherence effects. In addition R is insensi-sig


Ž . Ž . Ž .Fig. 4. Comparison of the JetrW boson tower multiplicity ratio from data to a PYTHIA with AO on and SF, b AO off and SF, c AOŽ .off and IF, and to the d MLLAqLPHD predictions. The predictions have been normalized to the data. All uncertainties are statistical

only.

tive to the overall normalization of the individualdistributions, and MC studies have shown that it isrelatively insensitive to detector smearing effects.Fig. 5 compares the R variable for the data to thesig

various PYTHIA predictions and to the MLLAqLPHD calculation. Clearly the value of R for thesig

data deviates from unity in agreement with PYTHIAwith AO on and SF, and in disagreement with AOoff and SF or AO off and IF. These comparisonsimply that for the process under study, string frag-mentation alone cannot describe the effects seen inthe data. The AO approximation is an element ofparton-shower event generators that needs to be in-cluded if color coherence effects are to be modeledsuccessfully. Finally, the analytic prediction by

Khoze and Stirling is consistent with the data, thusgiving additional evidence supporting the validity ofthe LPHD hypothesis.

The dominant source of uncertainty on the data b

distributions is statistical due to the limited eventsample. The statistical error for R is 7%. Since wesig

report ratios of event distributions any possible un-certainty on quantities that affect the overall rate ofevents is minimized. Sources of systematic uncer-tainty arise from background contamination to Wqjets events, uncertainties in the calorimeter channel-to-channel offset correction, multiple pp interac-tions, and uncertainties associated with the calorime-ter tower E threshold. The primary background toT

Wq jets events is dijet production in which one of


the jets mimics the characteristics of an electron. TheEu in such events typically arises from shower fluc-T

tuations or calorimeter imperfections. For our selec-tion criteria, the estimated background level is about5% resulting in a 1–2% uncertainty on the coherencesignal. The effect on the R variable due to uncer-sig

tainties in the offset correction were found to be atthe 1% level. To evaluate possible effects on thesignal coming from residual multiple interaction con-tamination, we examined the dependence of R assig

a function of luminosity. No systematic dependenceof the signal variable was observed for our eventsample.

The dependence of the signal variable R on thesig

calorimeter tower E threshold was studied by vary-T

ing the threshold from 200 to 350 MeV. Fig. 6shows how the R variable varies as a function ofsig

the tower threshold for the data and for the threePYTHIA predictions. Although there seems to be adependence of R with the tower E threshold forsig T

both the data and the simulations, for all towerthresholds examined the PYTHIA predictions withAO on and with SF resemble the data best.

Fig. 5. R for DØ data, PYTHIA with various coherence imple-sig

mentations, and a MLLAqLPHD QCD calculation. The errorsare statistical only and the shaded band shows the statisticaluncertainty on the R variable for the data.sig

Fig. 6. R as a function of the tower E threshold for the datasig TŽ .solid circles and for the various PYTHIA predictions. All uncer-tainties are statistical only.

In summary, color coherence effects in pp inter-actions have been observed and studied by the DØCollaboration. We have presented the first results oncolor coherence effects in Wq jets events. The datashow an enhancement of soft particle radiation aroundthe jet in the event plane with respect to the trans-verse plane, consistent with color coherence asimplemented in the PYTHIA parton shower eventgenerator, which includes the angular ordering ap-proximation and string fragmentation. In addition,the relative amount of enhancement is consistentwith an analytic perturbative QCD calculation basedon modified leading logarithmic approximation andlocal parton-hadron duality.

Acknowledgements

We appreciate fruitful discussions with V. Khozeand J. Stirling. We also thank T. Sjostrand for¨helping us with the color coherence implementationsin PYTHIA. We thank the Fermilab and collaborat-ing institution staffs for contributions to this work,and acknowledge support from the Department of

Ž .Energy and National Science Foundation USA ,


Ž .Commissariat a L’Energie Atomique France , Min-`istry for Science and Technology and Ministry for

Ž . Ž .Atomic Energy Russia , CAPES and CNPq Brazil ,Departments of Atomic Energy and Science and

Ž . Ž .Education India , Colciencias Colombia , CONA-Ž .CyT Mexico , Ministry of Education and KOSEF

Ž . Ž .Korea , and CONICET and UBACyT Argentina .

References

w x1 JADE Collaboration, W. Bartel et al., Phys. Lett. B 101Ž . Ž .1981 129; Z. Phys. C 21 1983 37; Phys. Lett. B 134Ž . Ž .1984 275; Phys. Lett. B 157 1985 340.

w x2 TPCr2g Collaboration, H. Aihara et al., Phys. Rev. Lett. 54Ž . Ž .1985 270; Z. Phys. C 28 1985 31; Phys. Rev. Lett. 57Ž .1986 945.

w x Ž .3 TASSO Collaboration, M. Althoff, Z. Phys. C 29 1985 29.w x4 MARK II Collaboration, P.D. Sheldon, Phys. Rev. Lett. 57

Ž .1986 1398.w x5 OPAL Collaboration, M.Z. Akrawy et al., Phys. Lett. B 247

Ž . Ž .1990 617; Phys. Lett. B 261 1991 334; P.D. Acton et al.,Ž . Ž .Phys. Lett. B 287 1992 401; Z. Phys. C 58 1993 207.

w x Ž .6 L3 Collaboration, M. Acciarri, Phys. Lett. B 353 1995 145.w x7 For a recent discussion of the phenomenological status and

comparisons to data, see V.A. Khoze, W. Ochs, Int. J. Mod.Ž .Phys. A 12 1997 2949.

w x8 Yu. L. Dokshitzer, V.A. Khoze, A.H. Mueller, S.I. Troyan,Basics of Perturbative QCD, Editions Frontieres, Gif-sur-`Yvette, 1991.

w x9 Ya. I. Azimov, Yu. L. Dokshitzer, V.A. Khoze, S.I. Troyan,Ž . Ž .Z. Phys. C 27 1985 65 and C 31 1986 213.

w x10 Yu. L. Dokshitzer, V.S. Fadin, V.A. Khoze, Phys. Lett. BŽ . Ž .115 1982 242; Z. Phys. C 15 1982 325.

w x11 A. Bassetto, M. Ciafaloni, G. Marchesini, A.H. Mueller,Ž .Nucl. Phys. B 207 1982 242.

w x12 B. Andersson, G. Gustafson, T. Sjostrand, Phys. Lett. B 94¨Ž .1980 211.

w x13 Ya. I. Azimov, Yu. L. Dokshitzer, V.A. Khoze, S.I. Troyan,Ž .Phys. Lett. B 165 1985 147; Sov. Journ. Nucl. Phys. 43

Ž .1986 95.w x14 B. Andersson, G. Gustafson, G. Ingelman, T. Sjostrand,¨

Ž .Phys. Rep. 97 1983 31.w x Ž .15 G. Marchesini, Comp. Phys. Commun. 67 1992 465.w x16 A. Bassetto, M. Ciafaloni, G. Marchesini, Phys. Rep. C 100

Ž .1983 201.w x17 Yu. L. Dokshitzer, S.I. Troyan, in Proceedings of the 19th

Winter School of the LNPI, Vol. 1, p. 144, Leningradpreprint LNPI-922, 1984.

w x Ž .18 A.H. Mueller, Nucl. Phys. B 213 1983 85; Erratum quotedŽ .ibid. B 241 1984 141.

w x19 N. Varelas, in Proceedings of the XVIII Physics in CollisionConference, Frascati, Italy, edited by S. Bianco, A. Cal-caterra, P. de Simone, F.L. Fabbri, INFN Laboratory, Fras-cati, 1998, p. 107, hep-exr9809019.

w x Ž .20 CDF Collaboration, F. Abe, Phys. Rev. D 50 1994 5562.w x Ž .21 DØ Collaboration, B. Abbott et al., Phys. Lett. B 414 1997

419.w x22 DØ Collaboration, S. Abachi et al., Nucl. Instrum. Meth. A

Ž .338 1994 185.w x23 J. Jaques, Ph.D. Dissertation, University of Notre Dame,

Ž .1996, unpublished , http:rrwww-d0.fnal.govrresultsrpub-lications_talksrthesisrthesis.html.

w x Ž .24 T. Sjostrand, Comp. Phys. Commun. 82 1994 74.¨w x Ž .25 V.A. Khoze, J. Stirling, Z. Phys. C 76 1997 59.w x Ž26 R. Brun et al., GEANT 3.14, CERN, DDrEEr84-1, unpub-

.lished .

Evidence of color coherence effects in W+jets events from p p collisions at s =1.8 TeV

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