Investigation of the Spectator Effect on Light Nuclei Production in Nucleus-Nucleus Collisions at High Baryon Density Region

TL;DR

This study investigates how spectator nucleons affect light-nuclei production in Au+Au collisions at $\sqrt{s_{\rm NN}} = 3$ GeV, a high-baryon-density region relevant to the QCD phase diagram. Using the AMPT-HC hadron-cascade model with an after-burner coalescence mechanism, it demonstrates a substantial low-$p_{\rm T}$ enhancement from spectators, especially in peripheral collisions and forward/backward rapidities. Conventional extrapolations of limited $p_{\rm T}$ acceptance (e.g., Blast-Wave fits) underestimate total light-nuclei yields because they miss the spectator contribution; when the same acceptance is applied, model results agree with STAR data. The findings highlight the critical role of spectator dynamics for precise yield extraction of light nuclei in high-density nuclear matter and imply that future analyses must explicitly account for spectator effects.

Abstract

The light nuclei yields and yield ratios, as sensitive probes of the QCD phase transition, have been precisely measured at various collision energies.However, due to limited detector acceptance, the full $p_{\rm T}$ integral yield often requires extrapolation of the measured transverse momentum spectra to the unmeasured low-$p_{\rm T}$ region via functional fitting.Simulations with the AMPT-HC transport model and an after-burner coalescence approach indicate a significant low-$p_{\rm T}$ enhancement in peripheral collisions or forward rapidities, primarily originating from spectator nucleons.Consequently, conventional extrapolations tend to underestimate the true light nuclei yields.

Figures (5)

• Figure 1: Transverse momentum spectra of different components (top panels) and participant component fraction (bottom panels) of $p, d$ and $t$ in Au+Au collisions at $\sqrt{s_{\rm{NN}}}$ = 3 GeV. The different color bands are the AMPT-HC model simulation results, black bands are total component, red bands are participant component, blue bands are spectator component, green bands are mixing component. The solid marker points are STAR published data STAR:2023uxk. (a) centrality 0-10% and mid-rapidity $-0.1 < y < 0$, (b) centrality 0-10% and backward rapidity $-1.0 < y < -0.9$, (c) centrality 40-80% and backward rapidity $-1.0 < y < -0.9$.
• Figure 2: Blast-Wave function fitting at limited acceptance region in peripheral Au+Au collisions at centrality 40-80% at $\sqrt{s_{\rm{NN}}}$ = 3 GeV. Black lines are the AMPT-HC model simulation results, red dash lines are Blast-Wave function fitting with $p_{\rm T} > 0.5$ GeV for proton, $p_{\rm T} > 1.0$ GeV for deuteron and $p_{\rm T} > 1.2$ GeV for triton.
• Figure 3: $d{\rm N}/dy$ of $p, d$ and $t$ in peripheral Au+Au collisions at centrality 40-80% at $\sqrt{s_{\rm{NN}}}$ = 3 GeV. The black lines are integral of total $p_{\rm T}$ spectra in full $p_{\rm T}$ range and the red lines are the sum of integral of Blast-Wave fitting function in low $p_{\rm T}$ range and integral of total $p_{\rm T}$ spectra in high $p_{\rm T}$ range from the AMPT-HC model. The black solid markers are STAR published data STAR:2023uxk.
• Figure 4: Total (4$\pi$) and mid-rapidity ($-0.5 < y < 0$) yield as a function of the number of participants nucleons $<\rm N_{part.}>$ for each nucleon of $p, d$ and $t$ in Au+Au collision at $\sqrt{s_{\rm{NN}}}$ = 3 GeV. The black and red color represent total and mid-rapidity yield respectively. The solid and dash lines represent without or with Blast-Wave function extrapolation of $p_{\rm T}$ spectra. The star points are STAR published data STAR:2023uxk.
• Figure 5: Different components of total yield as a function of the number of participants nucleons $<\rm N_{part.}>$ for each nucleon of $p, d$ and $t$ in Au+Au collision at $\sqrt{s_{\rm{NN}}}$ = 3 GeV. The black lines are total component, the red lines are participant component, the blue lines are spectator component, the green lines are mixing component.