Measurement of the Sixth-Order Cumulant of Net-Proton Distributions in Au+Au Collisions from the STAR Experiment
aa r X i v : . [ nu c l - e x ] F e b Nuclear Physics A 00 (2020) 1–4
NuclearPhysics A / locate / procedia XXVIIIth International Conference on Ultrarelativistic Nucleus-Nucleus Collisions(Quark Matter 2019)
Measurement of the Sixth-Order Cumulant of Net-ProtonDistributions in Au + Au Collisions from the STAR Experiment
Toshihiro Nonaka for the STAR Collaboration
Central China Normal University, Wuhan 430079, ChinaUniversity of Tsukuba, Tsukuba, Ibaraki 305, Japan
Abstract
Higher-order cumulants of the net-baryon multiplicity distributions are predicted to be sensitive to the properties of thenuclear matter created in high-energy nuclear collisions. In this talk, we present the collision centrality and acceptance(rapidity and transverse momentum) dependence of the ratio of the 6 th - to the 2 nd -order cumulant ratio ( C / C ) of net-proton in Au + Au collisions at √ s NN = . Keywords:
1. Introduction
One of the main goals of heavy-ion collision experiments is to study the phase diagram of QuantumChromodynamics (QCD). Higher-order cumulants of conserved charges are predicted to be sensitive to theQCD phase structure, especially the possible critical end point. The STAR experiment has investigatedthe QCD phase structure at finite µ B by measuring the cumulants up to the 4 th order ( C n , n <
4) and theirratios in the STAR experiment at RHIC [1–4]. Recently, the non-monotonic beam-energy dependence isobserved for C / C of net-proton multiplicity distributions [5], which could be an experimental signatureof the critical point. At µ B = th - to2 nd -order cumulant ratio, C / C , of the net-charge and net-baryon multiplicity distributions are predicted tobe negative if the freeze-out temperature is close to the pseudocritical temperature [7].In this proceedings, we present the centrality dependence of C / C of net-proton multiplicity distribu-tions at √ s NN = . C / C is also discussed. / Nuclear Physics A 00 (2020) 1–4
2. Analysis methods
The data are collected by STAR using the Time Projection Chamber (TPC) and Time-Of-Flight (TOF)detector. Collision events occurred within 30 cm in the beam direction and 2 cm in the radial directionare selected. Protons are identified by TPC and TOF at rapidity window − . < y < . . < p T (GeV / c) < .
0. The average PID e ffi ciency is about 50% with a weakcentrality dependence [5]. Centrality is determined in | η | < . ff ects [8, 9]. The centrality bin widthaveraging (CBWC) is done in order to suppress the e ff ect of the initial volume fluctuations [10]. Cumulantsare calculated for each multiplicity bin, which are averaged in 10% step of the centrality bin. In order toreduce the statistical uncertainties in central collisions, results in 0-10, 10-20, 20-30 and 30-40% centralityintervals are averaged using the inverse of the error squared as a weight.E ffi ciency and acceptance corrections are done assuming that the detector responses do not dependstrongly on the event multiplicity [11–15]. Within the aforementioned acceptance, the e ffi ciency for an-tiprotons is found to be lower than that of protons by ∼
5% independent of centrality. Unlike the case forparticle yield analysis, the e ffi ciency correction of the higher-order cumulants is expressed by the convo-lutions of lower-order cumulants, which means that the correction is a ff ected not only by the value of thesingle-particle e ffi ciency but also strongly a ff ected by the shape of the distributions.
3. Centrality dependence
Figure 1 shows the centrality dependence of C / C of net-proton multiplicity distributions in Au + Aucollisions at √ s NN = . √ s NN =
200 GeV. The C / C value at √ s NN = . = C / C of net-proton multiplicitydistributions for all the centralities at √ s NN = . T =
160 MeV ( µ B = √ s NN =
200 GeV, we findthat those results are consistent within large uncertainties.
4. Acceptance dependence
When we compare the experimental results with LQCD calculations, we need to consider the e ff ects ofthe experimental acceptance. With the narrow acceptance the net-baryon multiplicity distributions shouldreach the Skellam distribution, which exhibits C / C =
1. By increasing the acceptance, the physics signalsincrease, which may lead to deviations from unity. The C / C is expected to reach 0 with the full 4 π acceptance, where the net-baryon number is no longer fluctuate. Figure 2 shows the acceptance dependenceof C / C of net-proton multiplicity distributions in Au + Au collisions √ s NN =
200 GeV for each centralitybin, where the rapidity window ( | y | < α , α = .
1, 0.2, 0.3, 0.4 and 0.5) and p T acceptance (0 . < p T < β , β = .
8, 1.1, 1.4, 1.7 and 2.0 GeV / c) is enlarged from left to right along the x-axis. For both the rapidityand p T acceptance dependence, the C / C values are close to the Skellam baseline when the acceptance issmall, but the values decrease for larger acceptance. The negative sign is observed for large acceptance incentral collisions. It is found that the values are saturated for p T acceptance dependence at β > .
4. Thisis because the mean p T of protons is at around p T ≈ . / c, hence the number of protons is saturatedaround β > . / c.
5. Summary
We report the first measurements of the C / C of net-protons from 54.4 and 200 GeV Au + Au collisionsat RHIC. The results from peripheral collisions at both energies are consistent with each other. The C / C Nuclear Physics A 00 (2020) 1–4 C / C of net-proton multiplicity distributions in Au + Au collisions at √ s NN = . T =
160 MeV.Fig. 2. Rapidity ( y ) and transverse momentum ( p T ) acceptance dependence of C / C of net-proton multiplicity distributions Au + Aucollisions √ s NN =
200 GeV in 0-40% centrality. / Nuclear Physics A 00 (2020) 1–4 measured in 0-40% Au + Au collisions at √ s NN =
200 GeV is found to be negative and consistent with thelattice QCD prediction. This consistency implies that the nuclear matter created at 200 GeV collisions mightundergo a smooth crossover transition from the QGP to hadronic matter. On the other hand, the result from54.4 GeV shows a positive value. The negative and positive signs at 200 and 54.4 GeV in 0-40% Au + Aucollisions are at 2 sigma e ff ect. High statistics data are needed in order to understand the phase structure atvanishing baryonic density.
6. Acknowledgement
This work was supported by the National Key Research and Development Program of China (2018YFE0205201),the National Natural Science Foundation of China (No.11828501, 11575069, 11890711, 11861131009 and11950410505), and China Postdoctoral Science Foundation funded project 2018M642878.
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