Rapidity dependence of mean transverse momentum fluctuation and decorrelation in baryon-dense medium

Abstract

I study the event-by-event fluctuation and rapidity decorrelation of the mean transverse momentum $\spt$, which has recently been proposed as a sensitive probe of the equation of state at finite baryon density. The investigation reveals that, in a baryon-rich medium, the event-by-event fluctuation of the mean transverse momentum is driven by the combined effects of energy-density and net-baryon-density fluctuations. Consequently, the rapidity dependence of this observable provides a promising handle to probe the three-dimensional structure of both energy and baryon density profiles. Previous studies have shown that $\spt$ decorrelation along rapidity is largely insensitive to shear and bulk viscosity; however, its dependence on baryon diffusion, another key transport coefficient in baryonic matter, has not been explored. I find that baryon diffusion has a negligible impact, establishing this observable as a robust probe of the equation of state. Furthermore, I present predictions for identified hadrons and observe a pronounced splitting in the rapidity decorrelation of mean transverse momentum between protons and antiprotons, indicating different transverse flow dynamics for baryons and antibaryons.

Figures (5)

• Figure 1: Space–time rapidity ($\eta_s$) dependence of the transverse-plane–integrated energy density and net-baryon density for 0–10% central Au+Au collisions at $\sqrt{s_{NN}} = 19.6$ GeV.
• Figure 2: Pseudo-rapidity dependence of the mean-$p_T$ fluctuation $v_0(\eta)/v_0(\eta=0)$, scaled by its respective midrapidity value, for smooth and fluctuating initial conditions (IC). The comparison illustrates the competing roles of energy-density and net-baryon-density fluctuations in driving rapidity dependence.
• Figure 3: Pseudorapidity dependence of (a) $\langle p_T\rangle$, (b) $v_0 \equiv \sigma_{p_T}/\langle p_T\rangle$, (c) $R_{p_T}$, and (d) $r_{p_T}$ for charged hadrons in 0–10% Au+Au collisions at $\sqrt{s_{NN}} = 19.6$ GeV. Model results with vanishing ($C_B = 0.0$) and finite ($C_B = 0.5$) baryon diffusion are shown for comparison. STAR midrapidity measurements are plotted where available STAR:2017salSTAR:2019dow.
• Figure 4: (Color Online) Rapidity dependence of observables related to mean transverse momentum correlations and fluctuations for identified hadrons in 0–10% central Au+Au collisions at $\sqrt{s_{NN}} = 19.6$ GeV. Panels (a)–(d) show $\langle p_T\rangle$, $v_0=\sigma_{p_T}/\langle p_T\rangle$, $R_{p_T}$, and $r_{p_T}$ for $\pi^\pm$; panels (e)–(h) for $K^\pm$; and panels (i)–(l) for protons and antiprotons. Positively charged particles are shown with black lines and negatively charged particles with colored lines. Results with vanishing baryon diffusion ($C_B=0.0$, solid lines) are compared to those with finite baryon diffusion ($C_B=0.5$, dashed lines). STAR data of $\langle p_T \rangle$ at 5-10% centrality are plotted for comparison.
• Figure 5: (Color Online) Rapidity dependence of $R_{p_T}$ for different identified-particle combinations in 0–10% Au+Au collisions at $\sqrt{s_{NN}} = 19.6$ GeV. Results are shown for pion, kaon, and proton. Solid lines represents combinations with zero conserved quantum numbers, including $(\pi^+ + \pi^-)$, $(K^+ + K^-)$, and $(p + \bar{p})$, where dashed lines represent combinations of nonzero conserved quantum numbers $(\pi^+ - \pi^+)$, $(K^+ - K^+)$, and $(p - \bar{p})$. The comparison highlights mass-dependent decorrelation as well as the splitting induced by finite conserved charges, particularly the baryon-density–driven separation between protons and antiprotons.