Azimuthal correlations of hadrons with high transverse momenta serve as a
measure to study the energy loss and the fragmentation pattern of jets emerging
from hard parton-parton interactions in heavy ion collisions. Preliminary
results from the CERES experiment on two- and three-particle correlations in
central Pb-Au collisions are presented. A strongly non-Gaussian shape on the
away-side of the two-particle correlation function is observed, indicating
significant interactions of the emerging partons with the medium. Mechanisms
like deflection of the initial partons or the evolution of a mach cone in the
medium can lead to similar modifications of the jet structure on the away-side.
An analysis based on three-particle correlations is presented which helps to
shed light on the origin of the observed away-side pattern.Comment: 4 pages, 2 figures, contribution to the Quark Matter conference 200

Mateusz Ploskon

Ploskon M (CERES Collaboration)

Sjöstrand T

Stefan Kniege

the CERES Collaboration

Ulery J Wang F

English

arXiv

Crossref

Two- and three-particle azimuthal correlations of high-
                    
                      p
                      t

Azimuthal correlations of hadrons with high transverse momenta serve as a measure to study the energy loss and the fragmentation pattern of jets emerging from hard parton-parton interactions in heavy ion collisions. Preliminary results from the CERES experiment on two- and three-particle correlations in central Pb-Au collisions are presented. A strongly non-Gaussian shape on the away-side of the two-particle correlation function is observed, indicating significant interactions of the emerging partons with the medium. Mechanisms like deflection of the initial partons or the evolution of a mach cone in the medium can lead to similar modifications of the jet structure on the away-side. An analysis based on three-particle correlations is presented which helps to shed light on the origin of the observed away-side pattern.Azimuthal correlations of hadrons with high transverse momenta serve as a measure to study the energy loss and the fragmentation pattern of jets emerging from hard parton-parton interactions in heavy ion collisions. Preliminary results from the CERES experiment on two- and three-particle correlations in central Pb-Au collisions are presented. A strongly non-Gaussian shape on the away-side of the two-particle correlation function is observed, indicating significant interactions of the emerging partons with the medium. Mechanisms like deflection of the initial partons or the evolution of a mach cone in the medium can lead to similar modifications of the jet structure on the away-side. An analysis based on three-particle correlations is presented which helps to shed light on the origin of the observed away-side pattern.Azimuthal correlations of hadrons with high transverse momenta serve as a measure to study the energy loss and the fragmentation pattern of jets emerging from hard parton-parton interactions in heavy ion collisions. Preliminary results from the CERES experiment on two- and three-particle correlations in central Pb-Au collisions are presented. A strongly non-Gaussian shape on the away-side of the two-particle correlation function is observed, indicating significant interactions of the emerging partons with the medium. Mechanisms like deflection of the initial partons or the evolution of a mach cone in the medium can lead to similar modifications of the jet structure on the away-side. An analysis based on three-particle correlations is presented which helps to shed light on the origin of the observed away-side pattern.Azimuthal correlations of hadrons with high transverse momenta serve as a measure to study the energy loss and the fragmentation pattern of jets emerging from hard parton-parton interactions in heavy ion collisions. Preliminary results from the CERES experiment on two- and three-particle correlations in central Pb-Au collisions are presented. A strongly non-Gaussian shape on the away-side of the two-particle correlation function is observed, indicating significant interactions of the emerging partons with the medium. Mechanisms like deflection of the initial partons or the evolution of a mach cone in the medium can lead to similar modifications of the jet structure on the away-side. An analysis based on three-particle correlations is presented which helps to shed light on the origin of the observed away-side pattern.Azimuthal correlations of hadrons with high transverse momenta serve as a measure to study the energy loss and the fragmentation pattern of jets emerging from hard parton-parton interactions in heavy ion collisions. Preliminary results from the CERES experiment on two- and three-particle correlations in central Pb-Au collisions are presented. A strongly non-Gaussian shape on the away-side of the two-particle correlation function is observed, indicating significant interactions of the emerging partons with the medium. Mechanisms like deflection of the initial partons or the evolution of a mach cone in the medium can lead to similar modifications of the jet structure on the away-side. An analysis based on three-particle correlations is presented which helps to shed light on the origin of the observed away-side pattern

Kniege, Stefan

Ploskon, Mateusz

CERN Document Server

arXiv:nucl-ex/0703008v1   5 Mar 2007Two- and three-particle azimuthal correlations ofhigh-pt charged hadrons in Pb-Au collisions at158A GeV/cStefan Kniege and Mateusz Ploskon for the CERES collaborationInstitut fu¨r Kernphysik, Max-von-Laue-Str. 1, 60438 Frankfurt am Main, GermanyE-mail: kniege@ikf.uni-frankfurt.de, ploskon@ikf.uni-frankfurt.deAbstract. Azimuthal correlations of hadrons with high transverse momenta serve asa measure to study the energy loss and the fragmentation pattern of jets emerging fromhard parton-parton interactions in heavy ion collisions. Preliminary results from theCERES experiment on two- and three-particle correlations in central Pb-Au collisionsare presented. A strongly non-Gaussian shape on the away-side of the two-particlecorrelation function is observed, indicating significant interactions of the emergingpartons with the medium. Mechanisms like deflection of the initial partons or theevolution of a mach cone in the medium can lead to similar modifications of thejet structure on the away-side. An analysis based on three-particle correlations ispresented which helps to shed light on the origin of the observed away-side pattern.1. IntroductionThe suppression of high-pt charged hadrons in A-A relative to p-p collisions observedat RHIC [1, 2] was assumed to arise from the interaction of partons emerging fromhard collisions within a dense colored medium [3]. In such a picture hard collisionsclose to the surface of the reaction zone could lead to a jet fragmenting into thevacuum while the second jet would have to traverse the medium suffering energyloss and hence redistributing part of its energy to the medium. Measuring azimuthalangular correlations between a high-pt trigger particle and associated particles revealeddifferences in the shape of the correlation function close to the trigger particle (near-side)and around ∆φ = pi (away-side). Depending on the momentum of the associated andtrigger particles a strong suppression of the yield on the away-side of the correlationfunction was observed [4, 5, 6, 7] as compared to p-p data. In addition, the away-side exhibited a double peak structure which was proposed to arise from Mach coneshock waves [8], induced gluon radiation [9] or Cherenkov-like radiation in the medium[10]. Those scenarios can not be distinguished from an event-by-event deflectionof the jets in the medium based on two-particle correlations only. Three-particlecorrelation measurements, i.e. measurements of correlations among two associatedparticles in an event with respect to a trigger particle can help to disentagle theTwo- and three-particle correlations of high-pt charged hadrons at 158A GeV/c 2different proposed scenarios. The two-particle analysis follows previous measurements ofnon-triggered azimuthal correlations performed by the CERES collaboration [5] whichalready indicated modifications on the away-side of the di-jet correlation function in Pb-Au collisions at SPS energy. The present analysis is based on 30 million Pb-Au eventsat 158A GeV/c recorded with the upgraded CERES spectrometer including a TimeProjection Chamber covering the full range in azimuthal angle, and 2.1 < η < 2.7.2. Two-particle correlationsThe (jet-) associated particles are investigated by measuring the distribution S(∆φ) ofthe difference in the azimuthal angle ∆φ of a trigger particle with 2.5 < pt < 4.0 GeV/cand all associated particles with 1.0 < pt < 2.5 GeV/c from the same event. To accountfor acceptance effects, the signal distribution is divided by a background distributionB(∆φ) where trigger and associated particles are taken from different events. Thecorrelation function C2(∆φ) =nbackgrnsig· S(∆φ)B(∆φ), normalized to the ratio of background tosignal pairs (nbackgr/nsig), is assumed to be composed of two components: The jet-likecorrelations, and the correlations due to the elliptic flow. In this two-source approachthe measured correlation function can be written in the form:C2(∆φ) = C2,jet(∆φ) + b · (1 + 2 < vT2 vA2 > cos(2∆φ)).The elliptic flow coefficients for the trigger (vT2 ) and associated (vA2 ) range are determinedby a reaction plane analysis method [5, 11]. Assuming Zero Yield At Minimum (ZYAM)[7] for the jet-like correlation, C2,jet, the elliptic flow contribution is determined andsubtracted from the correlation function. After normalisation we obtain the conditionalyield as the number of jet-associated particles per trigger NT : Jˆ2(∆φ) =1NTdNTAd∆φ=C2,jet(∆φ)∫(C2(∆φ′)d(∆φ′))NTANT. The upper row of Figure 1 shows the correlation functions C2(∆φ)and the estimated flow contributions for different charge combinations of trigger andassociated particles. The lower row shows the extracted conditional yield corrected forefficiency which was estimated to be 85% for momenta above 1 GeV/c. As reportedearlier [5, 12] a non-Gaussian shape is observed on the away-side, indicating significantmedium modifications in central Pb-Au collisions at SPS energy. The observed shapeand relative magnitude of near- and away-side depend on the trigger and associatedparticle charges. For a given trigger charge, the associated yield on the near sideis larger for unlike-sign combinations as compared to like-sign. This is qualitativelyexplained by local charge conservation in the fragmentation process. The trend isdifferent on the away-side: For both trigger charges, the yield is significantly larger forpositive associated particles compared to negative associates. PYTHIA [13] calculationsat√sNN = 17.3 GeV indicate isospin-dependent effects when comparing positive andnegative associated yields in pp, pn and nn collisions. This situation arises due to thedominant contribution of large-x (valence quark) scattering to the di-jet yield aroundmid-rapidity at SPS energy. However, a full account of the observed charge asymmetrycould not be achieved based on PYTHIA calculations. It should be noted thatPYTHIA shows little or no charge asymmetries at RHIC and LHC due to the increasingTwo- and three-particle correlations of high-pt charged hadrons at 158A GeV/c 3 (rad)φ∆0 1 2 3 4 5 6φ∆dTAdN TN100.010.020.030.040.05 (rad)φ∆1 2 3 4 5 6 (rad)φ∆1 2 3 4 5 6 (rad)φ∆1 2 3 4 5 6)φ∆(2C0.980.9911.011.021.03+ / +preliminary+ / -preliminary- / +preliminary- / -preliminaryFigure 1: Two-particle correlation function (upper row) and conditional yield (lower row) for differentcarge combinations ( trigger charge/associated charge ) in the centrality range σ/σgeom= (0-5)%. Theflow contribution is indicated by the black lines in the upper row. The dashed and dotted curves showthe systematic uncertainties in the flow contribution (σ<vA2> ≈ 15% , σ<vT2> ≈ 25%) . The effect of theflow uncertainty on the conditional yield is shown by the blue and red line (lower row) (color online).contribution from gluons. For all charge combinations, a significant broadening of theaway-side peak compared to the expectation in pp is observed, perhaps even indicatingthe occurance of a slight dip at ∆φ = pi in the case of unlike-sign combinations.3. Three-particle correlationsTo analyze the shape on the away-side the differences in the azimuthal angle for twoassociated particles with respect to a trigger particle (∆φ1,∆φ2) are acquired to obtainthe three-particle signal distribution J3(∆φ1,∆φ2). The same pt-intervals for trigger andassociated particles are used as for the two-particle analysis. Dividing the signal by anormalized background distribution where all three particles are taken from differentevents the three particle correlation function C3(∆φ1,∆φ2) is obtained as depictedin Figure 2(a). The signal distribution is composed of the genuine three particle jetyield Jˆ3(∆φ1,∆φ2) and several background components which are subtracted from thesignal as described in detail in [14]: Jˆ3(∆φ1,∆φ2) = J3(∆φ1,∆φ2) − a · Jˆ2 ⊗ B2 −ba2(Bmb3 + Bmb,flow3 ). The hard-soft component Jˆ2 ⊗ B2 corresponds to the case whenone associated particle is jet-like correlated to the trigger while the second one is fromthe background. It is constructed by folding the 2-particle conditional yield Jˆ2 withthe flow modulated background. The third term is denoted as soft-soft background andcorresponds to the case when the trigger is a non-jet particle. Bmb3 is constructed bymixing two associate particles from one event with a trigger particle from another eventto account for all correlations among the associated particles which are not correlated tothe trigger. The term Bmb,flow3 accounts for all three particles being flow-like correlated.The mixing is based on events without trigger condition labled mb (minimum bias). TheTwo- and three-particle correlations of high-pt charged hadrons at 158A GeV/c 4paramter a accounts for the different multiplicities in triggered and non-triggered events.Subtracting the hard-soft background, one is left with the correlations where all particles∆φ2 (rad)∆φ1 (rad)01234561 2 3 4 5 61.011.00.990.980.97C3(∆φ1,∆φ2 )a)-0.025-0.02-0.015-0.01-0.00500.005∆φ2 (rad)∆φ1(rad)01234561 2 3 4 5 6a.u.b)preliminarypreliminaryFigure 2: Three-particle correlation function a) and jet-like three-particle yield after backgroundsubtraction b) for all charge combinations in central Pb-Au collisions at 158A GeV/c.are background particles or jet-like correlated. Deploying the ZYAM method for thegenuine jet-like correlations the soft-soft background is adjusted with the parameterb and subtracted to obtain the jet-correlated yield of the associated particles (Figure2(b)). Clear off-diagonal components are observed indicating a cone-like emission of theassociated particles in high-pt triggered events.4. Conclusion and OutlookWe presented two- and three-particle correlations for high-pt charged hadrons at 158AGeV/c indicating that already at SPS energy medium effects on high-pt particles areobserved. The magnitude and shape of the two-particle jet yield depends on the chargecombination of trigger and associated particles. In addition, a non-Gaussian shape onthe away-side is observed which is reflected in off-diagonal structures in the three-articlejet yield. Further investigations are needed to substantiate these findings.References[1] C. Adler et al. [STAR Collab.], Phys. Rev. Lett. 89, 202301 (2002).[2] K. Adcox et al. [PHENIX Collab.], Phys. Rev. Lett. 88, 022301 (2002).[3] X.-N. Wang and M. Gyulassy, Phys. Rev. D 44, 3501 (1991).[4] C. Adler et al. [STAR Collab.] Phys. Rev. Lett. 90, 082302 (2003).[5] G. Agakichiev et al. [CERES Collab.], Phys. Rev. Lett. 92 032301 (2004).[6] J. Adams et al. [STAR Collab.], Phys. Rev. Lett. 95, 152301 (2005).[7] S. S. Adler et al. [PHENIX Collab.], Phys. Rev. Lett. 97, 052301 (2006).[8] H. Sto¨cker, Nucl. Phys. A 750, 121 (2005),J. Casalderrey-Solana, E. V. Shuryak and D. Teaney, Nucl. Phys. A 774, 577-580 (2006).[9] I. Vitev, Phys. Lett. B 630, 78 (2005).[10] V. Koch, A. Majumder and X.-N. Wang, Phys. Rev. Lett. 96, 172302 (2006).[11] J. Milosevic et al. [CERES Collab.], Nucl. Phys. A 774, 503-506 (2006).[12] M. Ploskon et al. [CERES Collab.], nucl-ex/0511043 (2005), Nucl. Phys. A 783, 527-530 (2007).[13] T. Sjo¨strand et al., J. High Energy Physics 05 , 026 (2006).[14] J. Ulery and Fuquiang Wang, nucl-ex/0609016 (2006).

Two- and three-particle azimuthal correlations of high-$p_{t}$ charged hadrons in Pb - Au collisions at 158A GeV/c

Two- and three-particle azimuthal correlations of high-pt charged
  hadrons in Pb-Au collisions at 158A GeV/c

Two- and three-particle azimuthal correlations of high-pt charged hadrons in Pb-Au collisions at 158A GeV/c

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