Kinetic Freeze-Out Properties from Transverse Momentum Spectra of kaon, Pion, and (anti-)proton production in U+U collisions at $\sqrt{s_{NN}}$ = 193 GeV

TL;DR

By fitting mid-rapidity $p_T$ spectra of π, K, and p in U+U collisions at $\sqrt{s_{NN}}=193$ GeV with a Tsallis-based multi-source thermal model, the paper extracts the kinetic freeze-out properties and their centrality dependence. It finds that the kinetic freeze-out temperature $T_{kin}$ is largely centrality-independent, while the non-equilibrium parameter $q$ grows with centrality and particle mass, indicating stronger stopping and non-equilibrium in central events. The study also shows that the mean transverse momentum $\langle p_T\rangle$ increases with the number of participants and heavier hadrons have harder spectra, with good agreement to STAR data. These results illuminate the interplay of thermal excitation, collective dynamics, and nuclear deformation in the late-stage evolution of QCD matter and provide a baseline for comparing deformed and spherical collision systems.

Abstract

In the framework of the multi-source thermal model employing the Tsallis distribution, the transverse momentum distributions of kaon, pion, and (anti -)proton production in U+U collisions at $\sqrt{s_{NN}}$ = 193 GeV with varying centrality are investigated. The transverse momentum spectra are appropriately characterized. The dependencies of parameters (average transverse momenta, effective temperature, and entropy index) on event centrality are determined. It is observed that the q parameters increase as the average number of particles participating in the collisions rises, which implies that the nuclear stopping degree elevates with the increase of collision centrality. The T value remains fundamentally consistent for the same particle under different collision parameters, suggesting that the kinetic freezing temperature of particle ejection in this collision system is independent of the collision parameters.

Figures (6)

• Figure 1: Transverse momentum spectra of $\pi^{+}$ and $\pi^{-}$ are calculated at mid-rapidity ($\left | y \right | < 0.1$) in U+U collisions at $\sqrt{s_{NN}}$=193 GeV for $0-5\%$, $5-10\%$, $10-20\%$, $20-30\%$, $30-40\%$, $40-50\%$, $50-60\%$, $60-70\%$ and $70-80\%$ centrality. The theoretical calculation results based on the Tsallis distribution are represented by lines. Experimental data taken from the STAR Collaboration Abdallah21 are represented by the symbols.
• Figure 2: Transverse momentum spectra of $K^{+}$ and $K^{-}$ are calculated at mid-rapidity ($\left | y \right | < 0.1$) in U+U collisions at $\sqrt{s_{NN}}$=193 GeV for $0-5\%$, $5-10\%$, $10-20\%$, $20-30\%$, $30-40\%$, $40-50\%$, $50-60\%$, $60-70\%$ and $70-80\%$ centrality. The theoretical calculation results based on the Tsallis distribution are represented by lines. Experimental data taken from the STAR Collaboration Abdallah21 are represented by the symbols.
• Figure 3: The transverse momentum spectra of $p$ and $\bar{p}$ are computed at mid-rapidity ($\left | y \right | < 0.1$) in $U+U$ collisions at $\sqrt{s_{NN}}$=193 GeV for centrality intervals of $0-5\%$, $5-10\%$, $10-20\%$, $20-30\%$, $30-40\%$, $40-50\%$, $50-60\%$, $60-70\%$ and $70-80\%$. The theoretical calculation results based on the Tsallis distribution are represented by lines. The experimental data sourced from the STAR Collaboration Abdallah21 are denoted by the symbols.
• Figure 4: The $\left \langle p_{T} \right \rangle$ as a function of $\left \langle N_{part} \right \rangle$ at mid-rapidity ($\left | y \right | < 0.1$) of $\pi^{+}$, $K^{+}$ and $p$ for $U+U$ collisions at $\sqrt{s_{NN}}$=193 GeV. The red solid circles represent data collected by the STAR Collaboration Abdallah21. The black diamonds represent the calculations based on the Tsallis distribution.
• Figure 5: Dependence of $T$ on the average number of participants for $\pi^{+}/\pi^{-}$, $K^{+}/K^{-}$, $p$ and $\bar{p}$ in events with different centrality intervals. The symbols represent the parameter values listed in Table I, II and III.