**Can raw net-proton cumulants be used as a probe of QCD phase boundary?**
Phase transitions in quantum chromodynamics, particularly the critical point, could be probed by higher net-proton cumulants measured in relativistic heavy ion collisions (RHIC). However, due to the finite number of produced protons, raw net-proton cumulants are dominated by Poisson-like fluctuations. Subtracting this statistical part, we demonstrate that the dynamical part is more relevant to the critical related fluctuations.
A QCD phase diagram is usually drawn in the plane of temperature and baryon chemical potential. At high enough temperature and/or density, the nuclear matter may undergo a phase transition to quark gluon plasma (QGP). QGP is believed to have existed in the very early universe, following the Big Bang, and now it is re-discovered in small bangs - relativistic heavy ion collisions.
It has been suggested that the scale of QCD phase diagrams can be set by the comparison of the measured net-baryon cumulants with lattice QCD calculations. The related experimental observables, higher net-proton cumulants, have been measured at RHIC and have been extensively studied by various models. However, a transport model (AMPT), a statistical model (Therminator) and a hadron resonance gas model (HRG) can all well describe the data from RHIC, as shown in figure 1.
It was soon realized that the Poisson-like statistical fluctuations are not negligible at RHIC, where the mean net-proton is usually less than 10 at RHIC Beam Energy Scan (BES). The centrality dependency of net-proton kurtosis and the energy dependency of net-proton ratios are actually dominated by the statistical fluctuations (SK) as shown in figure 1, and so are the RHIC data. The dynamical kurtosis, related to the critical phenomenon, is obtained after subtracting the statistical part. It is zero in the pure statistical model, Therminator, and non-zero in transport models as shown in figure 2. Recently, QCD based models have shown that higher dynamical net-proton cumulants should change sign near the critical temperature of QCD phase transition. Their incident energy dependence at RHIC/BES and the future FAIR will serve as a good locator of the critical point of QCD phase transition. The experimental data analysis at RHIC and parallel model investigation are ongoing.
__Yuanfang Wu, Zhiming Li, Xue Pan and Fengbo Xiong__
**About the author**
Lizhu Chen, Xue Pan, Fengbo Xiong, and Lin Li are all Ph D students of Professor Yuanfang Wu. Lizhu Chen has analyzed finite-size behavior of QCD related observables at RHIC and calculated critical behavior of higher cumulants of order parameters using the 3D-Ising model. She is currently working on the data analysis of higher cumulants at RHIC/BES using the STAR detector at BNL. Xue Pan has calculated the critical behavior of higher cumulants of order parameters using 3D-O(2) and O(4) models. Fengbo Xiong and Lin Li are studying higher net-proton cumulants using various models in relativistic heavy ion collisions. Dr Zhiming Li and Dr Na Li have found longitudinal boost-invariance of charge balance function in nuclear and hadron-hadron collisions. Professor Yuanfang Wu has studied correlation and fluctuation in high energy physics and nuclear physics for years. Now she focuses on the location of QCD phase boundary and properties of elliptic flow in relativistic heavy ion collisions. Dr Gang Wang has been working on experimental analyses of collective motion and heavy quark physics in STAR at RHIC. |