Baryon and electric charge stoppings in nuclear collisions and the role of strangeness

TL;DR

The paper investigates how baryon stopping and net electric charge stopping differ in high-energy nuclear collisions by analyzing the observable $B/Q×Z/A$ and its rapidity dependence. It derives parton-level expressions for B and Q and shows that full phase-space conservation yields $B/Q×Z/A=1$, while a nonzero strangeness asymmetry between strange and anti-strange quarks can drive deviations. Using AMPT-SM simulations for Zr+Zr, Ru+Ru, and Cd+Cd (isobar) collisions at $200A$ GeV, it shows that mid-rapidity $B/Q×Z/A$ is below unity due to strangeness asymmetry; symmetrizing the strange and anti-strange distributions can push the ratio toward or above unity. It also analyzes $B/ΔQ×ΔZ/A$, finding it more sensitive to net-light-quark stoppings and generally below unity, while STAR data indicate larger values, revealing a puzzle. The results emphasize the role of strangeness in baryon transport and point to the need for more differential, species-resolved measurements to constrain baryon-number transport mechanisms.

Abstract

It has been challenging to quantitatively understand the stopping of incoming nucleons in nuclear collisions, and recently it has been proposed that comparing the baryon stopping with electric charge stopping can help address the question. Here we focus on the $B/Q\times Z/A$ ratio, which can strongly depend on rapidity although its value is one for the full phase space. We find that this ratio is very sensitive to the difference between strange and anti-strange rapidity distributions (the $s-\bar s$ asymmetry), and slightly more anti-strange quarks at mid-rapidity would lead to a ratio well below one. This is the case for Zr+Zr and Ru+Ru isobar collisions at $200A$ GeV from a multi-phase transport (AMPT) model. Without the $s-\bar s$ asymmetry, the AMPT model would give a mid-rapidity $B/Q\times Z/A$ ratio at or above one. In addition, the AMPT model gives $B/ΔQ\times ΔZ/A<1$ at mid-rapidity for isobar collisions at all centralities, which strongly contradicts the recent data from the STAR Collaboration. We further find that the $B/ΔQ\times ΔZ/A$ ratio is very sensitive to the net-light quark ($u,d$) stoppings, but it is less sensitive to the $s-\bar s$ asymmetry than the $B/Q\times Z/A$ ratio by a factor of 3.

Figures (8)

• Figure 1: The rapidity distributions of (a) the net-baryon number and (b) the net-electric charge in minimum bias isobar collisions at $\sqrt{s_{\rm NN}}=200A$ GeV from the AMPT-SM model from initial partons (dotted), initial hadrons (dashed) and final state hadrons (solid).
• Figure 2: The $B/Q \times Z/A$ ratio from the AMPT-SM model as a function of rapidity at different stages of the evolution of minimum bias Zr+Zr collisions; the ratio from the default AMPT model (curve with circles) is also shown.
• Figure 3: Rapidity distributions of initial quarks from the AMPT-SM model for minimum bias (a) Zr+Zr and (b) Cd+Cd isobar collisions at $200A$ GeV.
• Figure 4: The $B/Q\times Z/A$ ratio as a function of rapidity for minimum bias collisions of three isobar systems at $200A$ GeV from the AMPT-SM model, including results for initial partons and final partons with and without the inclusion of strange quarks as well as final state hadrons.
• Figure 5: Same as Fig. \ref{['fig4']} but from a modified AMPT-SM model where the distributions of initial strange and anti-strange quarks are symmetrized to be the same.