Z. M. Li

Boost invariance and multiplicity dependence of the charge balance function in π+p and K+p collisions at s=22 GeV

Abstract Boost invariance and multiplicity dependence of the charge balance function are studied in π + p and K + p collisions at 250 GeV/ c incident beam momentum with full acceptance coverage. Charge balance, as well as charge fluctuations, are found to be boost invariant over the whole rapidity region, but both depend on the size of the rapidity window. It is also found that the balance function becomes narrower with increasing multiplicity, which is consistent with the narrowing of the balance function with increasing centrality and/or system size, as observed in current relativistic heavy ion experiments.

Influences of statistics and initial size fluctuation on high-order cumulants of conserved quantities in relativistic heavy ion collisions

Through the generation of the UrQMD model, event statistics for the products of kurtosis (κ) and variance (σ2) of net-proton and net-charge multiplicity distributions are carefully studied. It is shown that the statistics at RHIC/BES at GeV are not sufficient to use the method of centrality bin width correction (CBWC). Corresponding results are systematically underestimated. A way to improve the CBWC method is proposed. It can remove the statistical dependence of the data and also reduce the initial size fluctuation.

Influence of statistics on the measured moments of conserved quantities in relativistic heavy ion collisions

We study statistics dependence of the probability distributions and the means of measured moments of conserved quantities, respectively. The required statistics of all interested moments and their products are estimated based on a simple simulation. We also explain why the measured moments are underestimated when the statistics are insufficient.With the statistics at RHIC/BES, the second and third order moments can be reliably obtained based on the method of Centrality bin width correction (CBWC), which can not be applied for the fourth order moments at low energy. With planning statistics at RHIC/BES II, and improved CBWC method,

Statistical and dynamical fluctuations in the ratios of higher net-proton cumulants in relativistic heavy-ion collisions

kappasigma^2

Longitudinal boost invariance of the charge balance function in hadron-hadron and nucleus-nucleus collisions

in a finer centrality bin scale should be measurable. This will help us to understand the current observation of energy and centrality dependence of high-order moments.

Statistical and dynamical parts of the cumulants of conserved charges in relativistic heavy ion collisions

With the help of transport and statistical models, we find that the ratios of higher net-proton cumulants measured at RHIC are dominated by Poisson-like statistical fluctuations. A way to eliminate this statistical fluctuation is suggested. The obtained dynamical ratios of higher net-proton cumulants are demonstrated to be more relevant to the underlying physics, i.e. the correlations between proton and antiproton, or the critical fluctuations.

Measurement of the ratio of the sixth order to the second order cumulant of net-proton multiplicity distributions in relativistic heavy-ion collisions

Using Monte Carlo generators of the PYTHIA model for hadron-hadron collisions and a multiphase transport (AMPT) model for nucleus-nucleus collisions, the longitudinal boost-invariance of charge balance function and its transverse momentum dependence are carefully studied. It shows that the charge balance function is boost-invariant in both p+p and Au+Au collisions in these two models, consistent with experimental data. The balance function properly scaled by the width of the pseudorapidity window is independent of the position or the size of the window and is corresponding to the balance function of the whole pseudorapidity range. This longitudinal property of balance function also holds for particles in small transverse momentum ranges in the PYTHIA and the AMPT default models, but is violated in the AMPT with string melting. The physical origin of the results are discussed.

High cumulants of conserved charges and their statistical uncertainties

The Poisson-liked statistical fluctuations, which are caused by the finite number of produced particles, are firstly estimated for the cumulants of conserved charges, i.e., the cumulants of net-baryon, net-electric charge, and net-strangeness. They turn out to be the same as those baselines derived from Hadron Resonance Gas (HRG) model. The energy and centrality dependence of net-proton cumulants at the Relativistic Heavy-Ion Collider (RHIC) are demonstrated to be mainly caused by statistical fluctuations. Subtracting the statistical fluctuations, the dynamical kurtosis of net- and total-proton from two versions of the AMPT model and the UrQMD model at current RHIC beam energies are presented. It is found that the observed sign change in the kurtosis of net-proton can not be reproduced by these three transport models. There is no significant difference between net- and total-proton kurtosis in model calculations, in contrary to the data at RHIC.

Statistical and dynamical fluctuations of Binder ratios in heavy ion collisions

Abstract We investigate the measurement of the sixth order cumulant and its ratio to the second order cumulant ( C 6 / C 2 ) in relativistic heavy-ion collisions. The influence of statistics and different methods of centrality bin width correction on C 6 / C 2 of net-proton multiplicity distributions is demonstrated. There is no satisfactory method to extract C 6 / C 2 with the current statistics recorded at lower energies by STAR at RHIC. With statistics comparable to the expected statistics at the planned future RHIC Beam Energy Scan II (BES II), no energy dependence of C 6 / C 2 is observed in central collisions using the UrQMD model. We find that if the transition signal is as strong as predicted by the PQM model, then it is hopefully observed at the upcoming RHIC BES II.

The study of longitudinal properties of the charge balance function

We study the influence of measured high cumulants of conserved charges on their associated statistical uncertainties in relativistic heavy-ion collisions. With a given number of events, the measured cumulants randomly fluctuate with an approximately normal distribution, while the estimated statistical uncertainties are found to be correlated with corresponding values of the obtained cumulants. Generally, with a given number of events, the larger the cumulants we measure, the larger the statistical uncertainties that are estimated. The error-weighted averaged cumulants are dependent on statistics. Despite this effect, however, it is found that the three sigma rule of thumb is still applicable when the statistics are above one million.