Kinetic Freeze-Out Conditions and Net Baryon Density in Au+Au Collisions at $\sqrt{s_{NN}} = 7.7$--$39$ GeV within a Collective Flow Fireball Model
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| Formato: | Preprint |
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2026
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| author | Alam, Sk Noor Roy, Victor |
| author_facet | Alam, Sk Noor Roy, Victor |
| contents | We investigate the effects of transverse and longitudinal collective flow on kinetic freeze-out conditions and net baryon density in 0--5\% central Au+Au collisions at $\sqrt{s_{NN}} = 7.7$--$39$ GeV within the RHIC Beam Energy Scan program. Using a covariant statistical fireball model, we fit the transverse momentum spectra of protons and positive pions from STAR data to simultaneously extract the kinetic freeze-out temperature $T$, baryon chemical potential $μ_B$, and transverse flow velocity $v_T$, for three fixed longitudinal velocities $v_z = 0.0$, $0.2$, and $0.4$. Longitudinal flow induces a systematic upward shift in $T$, spanning $143$--$171$ MeV, $150$--$180$ MeV, and $175$--$209$ MeV for $v_z = 0$, $0.2$, and $0.4$, respectively, arising from a kinematic degeneracy between $v_z$ and $T$ in the Lorentz-invariant distribution function rather than from any hardening of the $p_T$ spectra. While temperatures extracted at $v_z = 0$ and $0.2$ are broadly consistent with the QCD crossover temperature $T_c \approx 155$--$160$ MeV expected from lattice QCD, the values obtained at $v_z = 0.4$ significantly exceed $T_c$, suggesting that the system would no longer be described by a hadron resonance gas at these conditions and indicating that large longitudinal velocities may be physically disfavored at these collision energies. The baryon chemical potential decreases monotonically from $μ_B \sim 420$ MeV at $7.7$ GeV to $\sim 200$ MeV at $39$ GeV, independently of $v_z$. Reconstructing the freeze-out trajectory in the $(ρ_B, \varepsilon)$ plane, we identify a maximum in net baryon density at $\sqrt{s_{NN}} \lesssim 11.5$ GeV, with dynamical flow enhancing the inferred baryon compression by up to $\sim$20\% relative to the static limit. These results provide refined freeze-out benchmarks for future hydrodynamic modeling and for experiments at FAIR and low-energy RHIC runs. |
| format | Preprint |
| id |
arxiv_https___arxiv_org_abs_2603_07160 |
| institution | arXiv |
| publishDate | 2026 |
| record_format | arxiv |
| spellingShingle | Kinetic Freeze-Out Conditions and Net Baryon Density in Au+Au Collisions at $\sqrt{s_{NN}} = 7.7$--$39$ GeV within a Collective Flow Fireball Model Alam, Sk Noor Roy, Victor Nuclear Theory We investigate the effects of transverse and longitudinal collective flow on kinetic freeze-out conditions and net baryon density in 0--5\% central Au+Au collisions at $\sqrt{s_{NN}} = 7.7$--$39$ GeV within the RHIC Beam Energy Scan program. Using a covariant statistical fireball model, we fit the transverse momentum spectra of protons and positive pions from STAR data to simultaneously extract the kinetic freeze-out temperature $T$, baryon chemical potential $μ_B$, and transverse flow velocity $v_T$, for three fixed longitudinal velocities $v_z = 0.0$, $0.2$, and $0.4$. Longitudinal flow induces a systematic upward shift in $T$, spanning $143$--$171$ MeV, $150$--$180$ MeV, and $175$--$209$ MeV for $v_z = 0$, $0.2$, and $0.4$, respectively, arising from a kinematic degeneracy between $v_z$ and $T$ in the Lorentz-invariant distribution function rather than from any hardening of the $p_T$ spectra. While temperatures extracted at $v_z = 0$ and $0.2$ are broadly consistent with the QCD crossover temperature $T_c \approx 155$--$160$ MeV expected from lattice QCD, the values obtained at $v_z = 0.4$ significantly exceed $T_c$, suggesting that the system would no longer be described by a hadron resonance gas at these conditions and indicating that large longitudinal velocities may be physically disfavored at these collision energies. The baryon chemical potential decreases monotonically from $μ_B \sim 420$ MeV at $7.7$ GeV to $\sim 200$ MeV at $39$ GeV, independently of $v_z$. Reconstructing the freeze-out trajectory in the $(ρ_B, \varepsilon)$ plane, we identify a maximum in net baryon density at $\sqrt{s_{NN}} \lesssim 11.5$ GeV, with dynamical flow enhancing the inferred baryon compression by up to $\sim$20\% relative to the static limit. These results provide refined freeze-out benchmarks for future hydrodynamic modeling and for experiments at FAIR and low-energy RHIC runs. |
| title | Kinetic Freeze-Out Conditions and Net Baryon Density in Au+Au Collisions at $\sqrt{s_{NN}} = 7.7$--$39$ GeV within a Collective Flow Fireball Model |
| topic | Nuclear Theory |
| url | https://arxiv.org/abs/2603.07160 |