Measurement of cumulants of conserved charge multiplicity distributions in Au+Au collisions from the STAR experiment
NNuclear Physics A 00 (2020) 1–4
NuclearPhysics A / locate / procedia XXVIIIth International Conference on Ultrarelativistic Nucleus-Nucleus Collisions(Quark Matter 2019)
Measurement of cumulants of conserved charge multiplicitydistributions in Au + Au collisions from the STAR experiment
Ashish Pandav for the STAR Collaboration
School of Physical Sciences, National Institute of Science Education and Research, HBNI, Jatni-752050, INDIA
Abstract
We report the collision-centrality dependence of cumulants of event-by-event net-proton, net-charge and net-kaon distri-butions in Au + Au collisions for center-of-mass energy √ s NN = C n , n ≤
4) of the net-particle distributions. The cumulant ratios C / C , C / C and C / C exhibit a weak collision-centrality dependence. The C / C of net-proton and net-charge distributionsfor most central gold nuclei collisions at √ s NN = ff ects of acceptance and baryon number conservation, the measurementsare compared to expectations from the UrQMD and HIJING models calculated within the STAR detector acceptance. Keywords:
QCD phase diagram, QCD critical point, conserved charge fluctuations, cumulants
1. Introduction
One of the major goals of heavy-ion collision experiments is to explore the Quantum Chromodynamics(QCD) phase diagram and search for the QCD critical point. Lattice QCD calculations have shown thatfor vanishing baryonic chemical potential ( µ B ), the nature of phase transition between quark-gluon plasma(QGP) and hadronic matter is a smooth crossover [1] whereas QCD-based models suggest this phase tran-sition to be of first order for finite µ B [2]. The cumulants of conserved quantities in strong interactions areproposed to be sensitive observables for the search of the QCD critical point and the phase transition be-tween quark-gluon plasma (QGP) and hadronic matter [3]. The cumulants and their ratios are related to thecorrelation length of the hot and dense medium formed in the heavy ion collisions and the thermodynamicsusceptibilities that are calculable via various QCD-based models and lattice QCD [4, 5].Cumulants quantify the traits of a distribution, for example, the first- and second-order cumulant ( C and C ) are the mean and variance of a distribution whereas the third- and fourth-order cumulants ( C and C ) reflect the skewness and kurtosis of a distribution, respectively. Cumulants and their ratios for event-by-event distributions of net-charge, net-kaon and net-proton in collision of gold nuclei were measured by theSTAR detector in the phase I of Beam Energy Scan (BES) program at the Relativistic Heavy Ion Collider(RHIC) [6–10]. Non-monotonic dependence on beam energy is observed for the cumulant ratios C / C a r X i v : . [ nu c l - e x ] M a r / Nuclear Physics A 00 (2020) 1–4 and C / C of net-proton distribution in the most central (0-5%) collisions, which may hint at existence of apossible critical point. The sixth-order cumulants ( C ) could also provide insights into the nature of phasetransition. Negative C / C of net-baryon and net-charge distributions are predicted from a QCD-basedmodel for crossover phase transitions, if the chemical freeze-out is close to the chiral phase transition [11].
2. Analysis techniques
About ∼
550 million events are analysed for obtaining cumulants of net-particle distributions in Au + Aucollisions at √ s NN = p T range 0.4 – 2.0 GeV / c, and charged pions and kaons within p T range 0.2 – 1.6 GeV / c. The rapidity coverage | y | < | η | < ff ect [13]. In order tosuppress the volume fluctuation e ff ects, centrality bin width correction is applied to the measurement ofthe cumulants [14]. Cumulants are correction for finite e ffi ciency and acceptance e ff ects of the detectorwith the assumption that the distribution of the detector response is binomial [15, 16]. For estimation ofstatistical uncertainties of cumulants and their ratios, delta theorem method and a resampling method calledthe bootstrap are used [17, 18]. Systematic uncertainties of the C n ’s are estimated varying tracking e ffi ciency,track selection and particle identification criteria.
3. Results
Cumulants up to the 4 th order of the event-by-event net-proton, net-charge and net-kaon distributions forAu + Au collisions at √ s NN = < Npart > ) are presented in Fig. 1. The statistical and systematic uncertainties onthe measurements are shown by red bars and black brackets, respectively. Cumulants of the net-partcledistributions increase from peripheral to central collisions. Cumulants of the net-charge distribution havethe largest statistical uncertainties for a given centrality which can be attributed to the larger width of net-charge distributions among all net-particle distributions for a given centrality. QM2019 - Ashish Pandav |y| < 0.5 < 2.0 GeV/c T (a) C C u m u l an t s (b) C STAR Preliminary = 54.4 GeV NN sAu+Au: (c) C N e t - P r o t on Average Number
Average Number of (d) C of Participant Nucleons Participant Nucleons p a r | < 0.5 η | ) < 1.6 GeV/c - , K + , K - π , + π ( T T (a) C C u m u l an t s (b) C STAR Preliminary = 54.4 GeV NN sAu+Au: (c) C N e t - C ha r ge Average Number (d) C of Participant Nucleons p a r < " > < " > |y| < 0.5 < 1.6 GeV/c T (a) C C u m u l an t s (b) C STAR Preliminary = 54.4 GeV NN sAu+Au: (c) C N e t - K aon (d) C < " > Fig. 1. Cumulants ( C n , n ≤
4) of net-proton, net-charge, net-kaon distributions as a function of average number of participant nucleonsfor Au + Au collisions at √ s NN = Figure 2 shows collision-centrality dependence of cumulant ratios C / C , C / C and C / C ( σ / M , S σ and κσ respectively) of net-particle distributions constructed from the measured cumulant values. While C / C decreases with collision centrality, the cumulant ratios C / C and C / C exhibit a weak dependenceon collision centrality for net-proton, net-charge and net-kaon distributions. The expectations from the Nuclear Physics A 00 (2020) 1–4 UrQMD and HIJING models are also compared to the measurements [19, 20]. The model expectations areinconsistent with the measurements and only qualitatively reproduce the measured centrality dependenceof the cumulant ratios. The Skellam baseline for C / C , which is the expected value of C / C under theassumption that protons and antiprotons follow Poisson distribution independently, fails to describe themeasured values. centrality / M σ STAR Preliminary < 1.6 GeV/c, |y| < 0.5 T NN sAu+Au centrality σ S σ κ Net-kaonUrQMDHIJING
Skellam Expectation < " > centrality / M σ STAR Preliminary | < 0.5 η | ) < 1.6 GeV/c - , K + , K - π , + π ( T T NN sAu+Au centrality σ S σ κ Net-chargeUrQMDHIJING
Skellam Expectation < " > centrality / M σ STAR Preliminary < 2.0 GeV/c, |y| < 0.5 T NN sAu+Au centrality σ S σ κ Net-protonUrQMDHIJING
Skellam Expectation < " > Fig. 2. Cumulant ratios C / C , C / C and C / C of net-proton, net-charge and net-kaon distributions as a function of average numberof participant nucleons for Au + Au collisions at √ s NN = / C of net-particle distributions Beam-energy dependence of the cumulant ratio C / C of net-particle distributions for peripheral (70-80%) and most central (0-5%) collisions with inclusion of the results from the current measurement (openand solid red markers respectively) are shown in the Fig. 3. Non-monotonic beam energy dependence of C / C is observed for net-proton distribution while C / C of net-charge and net-kaon distributions showmonotonic variation as a function of beam energy. The new C / C measurements of net-particle distribu-tions at √ s NN = NN sColliding energy 01234 σ κ T ExpectationSkellam & HRG NN sColliding energy 15 − − − σ κ | < 0.5 η | ) < 1.6 GeV/c - , K + , K - π , + π ( T T ExpectationSkellam & HRG NN sColliding energy 2 − − σ κ T ExpectationSkellam & HRG Fig. 3. Beam-energy dependence of C / C for net-proton, net-charge and net-kaon distributions for 0-5% and 70-80% most centralcollisions for Au + Au collisions with inclusion of the results from the C / C measurement at √ s NN = The collision-centrality dependence of the ratio of the sixth- to second-order cumulants ( C / C ) of net-proton and net-charge distribution for Au + Au collisions are shown in Fig 4. The C / C of net-protondistributions for central collisions (0-40%) at √ s NN = C / C measured at √ s NN =
200 GeV is negative for the same collision centrality [21]. A negative C / C is predicted for the crossoverphase transition between hadronic matter and quark-gluon plasma in QCD-based calculations [11]. TheUrQMD model expectations for Au + Au collisions at √ s NN =
200 GeV and √ s NN = / Nuclear Physics A 00 (2020) 1–4 to be positive and consistent with the Skellam baseline across all collision centralities. The C / C of net-charge distribution for Au + Au collisions at √ s NN = QM2019 - AshishPandav 〉 part N 〈 − / C C STAR Preliminary (GeV/c) < 2.0 T Net-proton, |y| < 0.5, 0.4< p = 54.4 GeV NN s = 200 GeV NN s = 54.4 GeV NN sUrQMD, = 200 GeV NN sUrQMD, = 0 MeV) B µ LQCD, PRD.95.054504, (T = 160 MeV, = 0 MeV) B µ LQCD, JHEP.10.205, (T = 160 MeV, < " > − C C HIJING = 0 / C C = 54.4 GeV NN sAu+Au: STAR PreliminaryNet-charge < " > Fig. 4. (Left plot) The C / C of net-proton distribution for Au + Au collisions at √ s NN = + Au collisions at √ s NN =
200 and 54.4 GeV,respectively. The yellow and turquoise band are the Lattice QCD predictions. (Right plot) The C / C of net-charge distribution forAu + Au collisions at √ s NN =
4. Summary
We presented the collision-centrality dependence of cumulants and cumulant ratios of net-particle dis-tributions from high statistics Au + Au collisions at √ s NN = C n , n ≤
4) of net-particle distributions increase with average number of participant nucleons whereas the cumulant ratios C / C , C / C and C / C exhibit a weak collision-centrality dependence. The C / C ( κσ ) measurement ofnet-particle distributions at √ s NN = C / C from the BES-I program. Furthurmore, the C / C of net-proton distribution for Au + Au collisions at √ s NN =
200 GeV for most central collisions (0-40%) is negative, and this could be an experimental evidence ofcrossover phase transition. The new measurements at √ s NN =
5. Acknowledgments
We acknowledge the financial support by Department of Atomic Energy, Govt. of India.
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