arXiv subjects

Mason Alexander Ross

Publications and source records attributed to Mason Alexander Ross.

4 recordsLinked to original sources

arXiv2026

The role of strangeness in baryon and electric charge stoppings

Recently it has been proposed that comparing the net-baryon ($B$) stopping with net-electric charge ($Q$) stopping can help studies of the baryon stopping mechanism in nuclear collisions. Here we find the $B/Q\times Z/A$ ratio to be very sensitive to the difference between $s$ and $\bar s$ quark rapidity distributions. For mid-rapidity of isobar collisions at 200A GeV, a multi-phase transport (AMPT) model gives slightly more $\bar s$ than $s$, which leads to $B/Q\times Z/A<1$, while the model without the $s-\bar s$ asymmetry gives $B/Q\times Z/A \geq 1$. Comparing Ru+Ru and Zr+Zr isobar collisions, the AMPT (and UrQMD) model gives $B/ΔQ\times ΔZ/A<1$ at mid-rapidity at all centralities, which strongly contradicts the recent STAR data. We also 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. These results are expected to help us resolve the stopping puzzle and better understand the baryon stopping mechanism in the future.

nucl-th↗

arXiv2026

The shear viscosity of quark-gluon matter calculated with parton transport and comparisons with the Chapman-Enskog results

We numerically calculate the shear viscosity of quark-gluon matter via the Green-Kubo relation with an improved ZPC model. We include all $2\leftrightarrow 2$ parton cross sections at finite temperature, which are based on perturbative QCD and screened with thermal masses, and consider massless quark-gluon systems with Boltzmann statistics in chemical equilibrium. We then compare the Green-Kubo results with the analytical results from the leading-order Chapman-Enskog method for the same parton cross sections over the temperature range $150-600$ MeV. We also examine the simpler case of isotropic and constant parton cross sections. Overall, we find that the two methods agree rather well. Specifically, the Green-Kubo results are greater than the Chapman-Enskog results by an average of $\sim 9\%$ for isotropic and constant cross sections and by an average of $\sim 3\%$ for finite-temperature pQCD cross sections, where the difference between the two methods is presumably due to higher-order corrections to the leading-order Chapman-Enskog results.

hep-ph↗

arXiv2025

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

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.

nucl-th↗

arXiv2024

Search for baryon junctions in photonuclear processes and isobar collisions at RHIC

During the early development of Quantum Chromodynamics, it was proposed that baryon number could be carried by a non-perturbative Y-shaped topology of gluon fields, called the gluon junction, rather than by the valence quarks as in the QCD standard model. A puzzling feature of ultra-relativistic nucleus-nucleus collisions is the apparent substantial baryon excess in the midrapidity region that could not be adequately accounted for in most conventional models of quark and diquark transport. The transport of baryonic gluon junctions is predicted to lead to a characteristic exponential distribution of net-baryon density with rapidity and could resolve the puzzle. In this context we point out that the rapidity density of net-baryons near midrapidity indeed follows an exponential distribution with a slope of $-0.61\pm0.03$ as a function of beam rapidity in the existing global data from A+A collisions at AGS, SPS and RHIC energies. To further test if quarks or gluon junctions carry the baryon quantum number, we propose to study the absolute magnitude of the baryon vs. charge stopping in isobar collisions at RHIC. We also argue that semi-inclusive photon-induced processes ($γ+p$/A) at RHIC kinematics provide an opportunity to search for the signatures of the baryon junction and to shed light onto the mechanisms of observed baryon excess in the mid-rapidity region in ultra-relativistic nucleus-nucleus collisions. Such measurements can be further validated in A+A collisions at the LHC and $e+p$/A collisions at the EIC.

hep-ph↗