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Non-monotonic energy dependence of net-proton number fluctuations

Updated on Mon, 2020-12-28 21:17 by yuzhang1. Originally created by yangzz on 2019-07-20 18:44.

Title: Non-monotonic energy dependence of net-proton number fluctuations

PAs(alphabetically): ShinIchi Esumi, Shu He, Xiaofeng Luo, Bedanga Mohanty, Toshihiro Nonaka, Ashish Pandav,  Nu Xu, Zhenzhen Yang, Yu Zhang 
Target Journal: Phys. Rev. Lett.
Paper draft : https://www.star.bnl.gov/protected/bulkcorr/luoxf/shortpaper/paper_draft/
Supplemental:   https://www.star.bnl.gov/protected/bulkcorr/luoxf/shortpaper/paper_draft/PRL-NP-Supplemental-revised-2.pdf
Analysis note : https://drupal.star.bnl.gov/STAR/blog/luoxf/analysis-note-net-proton-papers

 Long paper webpage: https://drupal.star.bnl.gov/STAR/blog/yangzz/measurements-cumulants-net-proton-multiplicity-distributions-star
Webpage for 2014 net-proton PRL paper : https://www.star.bnl.gov/protected/bulkcorr/luoxf/PaperProposal/PaperProposal.htm
Paper proposal presentation in PWG: https://www.star.bnl.gov/protected/bulkcorr/luoxf/PWG_discussion/PaperProposal.pdf

ABSTRACT

    Observations from collisions of heavy-ion at relativistic energies have established the formation of a new phase of matter, Quark Gluon Plasma (QGP), a deconfined state of quarks and gluons in a specific region of the temperature versus baryonic chemical potential phase diagram of strong interactions. A program to study the features of the phase diagram, such as a possible critical point, by varying the collision energy (√sNN), is performed at the Relativistic Heavy-Ion Collider (RHIC) facility. Non-monotonic variation with √sNN of moments of the net-baryon number distribution, related to the correlation length and the susceptibilities of the system, is suggested as a signature for a critical point. We report the first evidence of a non-monotonic variation in kurtosis × variance of the net-proton number (proxy for net-baryon number) distribution as a function of √sNN with 3.1σ significance, for head-on (central) gold-on-gold (Au+Au) collisions measured using the STAR detector at RHIC. Non-central Au+Au collisions and models of heavy-ion collisions without a critical point show a monotonic variation as a function of √sNN.

FIGURES


Figure 1: Event-by-event net-proton number distributions for head-on (0-5% central) Au+Au colli- sions for nine √sNN values measured by the STAR detector at RHIC. The distributions are normal- ized to the total number of events at each √sNN. The statistical uncertainties are smaller than the symbol sizes and the lines are to guide the eye. The distributions in this figure are not corrected for proton and anti-proton detection efficiency.The deviation of the distribution for √sNN = 54.4 GeV from the general energy dependence trend is understood to be due to the reconstruction efficiency of protons and anti-protons being different compared to other energies.


Figure 2: Cumulants (Cn) of the net-proton distributions for central (0-5%) and peripheral (70-80%) Au+Au collisions as a function of collision energy. The transverse momentum (pT) range for the measurements is from 0.4 to 2 GeV/c and the rapidity (y) range is ± 0.5. The vertical narrow and wide bars represent the statistical uncertainties and systematic uncertainties, respectively. The energy range for the STAR fixed-target (FXT) program is shown as arrows in panel (3).



Figure3: Upper panels: Sσ (1) and κσ2 (2) of net-proton distributions  for 0-5% central Au+Au collisions from √sNN = 7.7 - 62.4 GeV. The bar on the data points are statistical and systematic uncertain- ties added in quadrature. The black solid lines are polynomial fit functions which best describes the data. The black dashed lines are the Poisson baselines. Lower panels: Derivative of the fitted polynomial as a function of . The bar and the shaded band on the derivatives represent the statistical and systematic uncertainties, respectively.


Figure 4: Sσ (1) and κσ2 (2) as a function of collision energy for net-proton distributions measured in Au+Au collisions. The results are shown for central (0-5%, filled circles ) and peripheral (70- 80%, open squares) collisions within 0.4 < pT (GeV/c) < 2.0 and |y| < 0.5. The vertical narrow and wide bars represent the statistical and systematic uncertainties, respectively. Shaded green band is the estimated statistical uncertainty for BES-II and the energy range for STAR fixed-target (FXT) program is shown as arrows in panel (2). The peripheral data points have been shifted for clarity of presentation. Results from a hadron resonance gas (HRG) model and a transport model calculation (UrQMD) for central collisions (0-5%) are shown as black and gold bands, respectively. These model calculations utilize the experimental acceptance, and incorporate conservation laws for strong interactions, but do not include a phase transition or a critical point.


METHODS

 

Figure 5: Left panel: Square of the mass of the charged particles, requiring timing information from the TOF, as a function of the product of the momentum (p) and the ratio of the particle’s charge to the elementary charge e (q), both measured using the TPC in Au+Au collisions at √sNN = 39 GeV. The white dashed lines correspond to the expected square of the mass of each particle species. Right panel: The transverse momentum (pT ) versus the rapidity (y) for protons measured in the STAR detector for Au+Au collisions.




 

SUMMARY

In conclusion, we have presented measurements of net-proton cumulant ratios with the STAR detector at RHIC over a wide range in μB (20 to 420 MeV) which are relevant to a QCD critical point search in the QCD phase diagram. We have observed a non-monotonic behaviour, as a function of √sNN, in net-proton κσ2 in central Au-on-Au collisions with a significance of 3.1σ. In contrast, monotonic behaviour with √sNN is observed for the statistical hadron gas model, and for a nuclear transport model without a critical point, as observed in experimentally in peripheral collisions. The deviation of the measured κσ2 from several baseline calculations with no critical point, and its non-monotonic dependence on √sNN is qualitatively consistent with expectations  from a QCD-based model which includes a critical point. Our measurements can also be  compared to the baryon number susceptibilities computed from QCD to understand the various other features of the QCD phase structure as well as to obtain the freeze-out conditions in heavy-ion collisions. Higher event statistics, which will allow for a more differential measurement of these experimental observables in y-pT along with comparison to theoretical QCD calculations which includes the dynamics associated with heavy-ion collisions, will help in establishing the critical point.

    

REFERENCE

[1]A.Bzdak, V.Koch, V.Skokov, Eur.Phys.J., C77, 288(2017)
[2]X.Luo, Phys. Rev. C 91, 034907 (2015)
[3]T. Nonaka et al., PRC95, 064912 (2017)
[4]M. Kitazawa and X. Luo, PRC96, 024910 (2017)
[5]X. Luo and N. Xu, Nucl. Sci. Tech. 28, 112 (2017)
[6] B. Ling and M. A. Stephanov, Phys. Rev. C 93, no. 3, 034915 (2016) doi:10.1103/PhysRevC.93.034915 [arXiv:1512.09125 [nucl-th]].
[7] J. Brewer, S. Mukherjee, K. Rajagopal and Y. Yin, Phys. Rev. C 98, no. 6, 061901 (2018) doi:10.1103/PhysRevC.98.061901 [arXiv:1804.10215 [hep-ph]].
More references can be found at : https://www.star.bnl.gov/protected/bulkcorr/luoxf/PaperProposal2018/Papers_talks.htm







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