Logo
ScienceStack
Table of Contents
Fetching ...
Sign In

Precision Measurements of Kinematic Scan for Fluctuations of (Net-)proton Multiplicity Distributions in Au+Au Collisions from RHIC-STAR

Yige Huang

TL;DR

This work addresses locating the QCD critical end point by exploiting rapidity scans of event-by-event net-proton fluctuations in STAR BES-II Au+Au collisions across $\sqrt{s_{NN}}=7.7$--$27$ GeV. It combines precise proton identification in extended rapidity $|y|<0.6$ with factorial cumulant analysis, finite-size scaling of susceptibility, and Binder-cumulant extrapolations to map critical behavior in a finite, expanding system. The results show a power-law-like dependence of proton factorial cumulants on the rapidity window with exponents below CEP expectations, and yield a consistent picture of a finite-size critical region around $\mu_B \approx 550$--$650$ MeV, supported by a $\mu_{B,c}\approx 648$ MeV from FSS and a Binder-crossing near $\mu_B \approx 549$ MeV. These findings constrain the CEP location and motivate further high-$\mu_B$ measurements.

Abstract

This work presents measurements of the rapidity-window dependence of event-by-event net-proton cumulants and proton factorial cumulants in Au+Au collisions at $\sqrt{s_\mathrm{NN}}=$7.7 -- 27 GeV, using high-statistics data from RHIC BES-II. Protons and antiprotons are identified with improved detector performance within $0.4<p_\mathrm{T}<2.0$ GeV/$c$ and $|y|<0.6$, enabling a wide coverage in momentum space to probe long-range correlations near the QCD critical point. In the most central collisions, the proton number $κ_2/κ_1$ and $κ_3/κ_1$ exhibit power-law scaling with the rapidity window, but with exponents below the theoretical expectation, suggesting that the critical point, if it exists, may lie at higher baryon densities. A finite-size scaling analysis of the susceptibility and Binder cumulant study points out a critical baryon chemical potential region in 550 -- 650 MeV.

Precision Measurements of Kinematic Scan for Fluctuations of (Net-)proton Multiplicity Distributions in Au+Au Collisions from RHIC-STAR

TL;DR

This work addresses locating the QCD critical end point by exploiting rapidity scans of event-by-event net-proton fluctuations in STAR BES-II Au+Au collisions across -- GeV. It combines precise proton identification in extended rapidity with factorial cumulant analysis, finite-size scaling of susceptibility, and Binder-cumulant extrapolations to map critical behavior in a finite, expanding system. The results show a power-law-like dependence of proton factorial cumulants on the rapidity window with exponents below CEP expectations, and yield a consistent picture of a finite-size critical region around -- MeV, supported by a MeV from FSS and a Binder-crossing near MeV. These findings constrain the CEP location and motivate further high- measurements.

Abstract

This work presents measurements of the rapidity-window dependence of event-by-event net-proton cumulants and proton factorial cumulants in Au+Au collisions at 7.7 -- 27 GeV, using high-statistics data from RHIC BES-II. Protons and antiprotons are identified with improved detector performance within GeV/ and , enabling a wide coverage in momentum space to probe long-range correlations near the QCD critical point. In the most central collisions, the proton number and exhibit power-law scaling with the rapidity window, but with exponents below the theoretical expectation, suggesting that the critical point, if it exists, may lie at higher baryon densities. A finite-size scaling analysis of the susceptibility and Binder cumulant study points out a critical baryon chemical potential region in 550 -- 650 MeV.

Paper Structure

This paper contains 7 sections, 4 figures.

Table of Contents

  1. Introduction
  2. Proton Identification
  3. Results
  4. Proton Factorial Cumulants
  5. Finite-Size Scaling with Susceptibility
  6. Extrapolation with Binder Cumulant
  7. Summary

Figures (4)

  • Figure 1: The TPC (outer band) and TOF (inner band) acceptance of proton candidates in $y$ -- $p_\mathrm{T}$ plane. Detector upgrades in BES-II extend the coverage in rapidity up to $|y| < 0.6$ and in transverse momentum of $0.4 < p_\mathrm{T}<2.0$ GeV/$c$. The protons are identified by energy loss from TPC and mass information from TOF (in high momentum region). Tracks are taken from Au+Au collisions at $\sqrt{s_\mathrm{NN}}=$ 19.6 GeV, with 39 < $V_z$ < 40 cm for positive rapidity part, and -40 < $V_z$ < -39 cm for negative rapidity part.
  • Figure 2: Rapidity window size dependence of proton factorial cumulants normalized by $\kappa_1$. Circles denote STAR data: solid for central and open for peripheral collisions. UrQMD results are shown as bands. Red curves indicate power-law fits to solid circles, with third-order exponents labeled in dark green. Gray dashed curves present power function fits with fixed exponent to the expectation Ling:2015yau.
  • Figure 3: The finite-size scaling analysis of $\chi_2$ presents good scaling for data at $\sqrt{s_\mathrm{NN}}$ =7.7 -- 27 GeV. STAR data within various rapidity windows $W$ collapse onto the red dashed power-law curve well with critical baryon chemical potential $\mu_\mathrm{Bc}=648$ MeV.
  • Figure 4: Binder cumulant as a function of $\mu_\mathrm{B}$. Markers represent different rapidity windows: $W=0.4$ (red stars), $W=0.6$ (blue squares), and $W=0.8$ (black dots). Linear fits yield 68% confidence bands, with their overlap beginning around 549 MeV, consistent with the finite-size scaling estimate shown by the orange band.