arXiv subjects

Alex Akridge

Publications and source records attributed to Alex Akridge.

3 recordsLinked to original sources

arXiv2025

Probing the Neutron Skin with Extreme Collision Geometries in Heavy-Ion Collisions

Understanding how protons and neutrons are located differently in an atomic nucleus can provide fundamental information on nuclear structure and have far-reaching implications for astrophysics. A precise determination of this important difference, often quantified by the so-called neutron skin thickness, is challenging both theoretically and experimentally. Here we show how one can use a new category of observables in heavy ion collisions to probe the neutron skin thickness of nuclei like $^{208}$Pb and $^{48}$Ca, by utilizing the asymmetry between neutrons and protons of spectator nucleons in super-central collisions as well as that of participant nucleons in peripheral collisions. Using quantitative simulations, we demonstrate their sensitivity and great potential in constraining neutron skin thickness for both $^{208}$Pb and $^{48}$Ca nuclei in these extreme event geometries. Furthermore, we propose the asymmetric collisions between $^{48}$Ca and $^{40}$Ca nuclei as a unique and powerful way to nail down the neutron skin thickness.

nucl-th↗

arXiv2025

Initial Baryon Stopping and Angular Momentum in Heavy-Ion Collisions

Noncentral heavy-ion collisions create fireballs with large initial orbital angular momentum that is expected to induce strong vorticity in the hot bulk fluid and generate global spin polarization of the produced particles. As the collision beam energy $\sqrt{s_{\rm NN}}$ decreases to approach the two-nucleon-mass threshold, this initial angular momentum approaches zero. One may thus expect that the observed global spin polarization should reach a maximum and then drop to zero as increased stopping competes with decreased initial momentum. Recent experimental measurements, however, appear to show a continual rise of hyperon polarization even down to $\sqrt{s_{\rm NN}} =$ 2.42 GeV, suggesting a peak very near threshold which is difficult to interpret and calls for a better understanding of angular momentum initial conditions, especially at low energy. Here, we develop a new Glauber-based initial state model ("Glauber+") to investigate the initial distribution of angular momentum with respect to rapidity as well as the dependence of this distribution on initial baryon stopping across a wide range of collisional beam energy. We estimate that the angular momentum per produced final charged particle at mid-rapidity peaks around 5 GeV, which may present a potential challenge to an interpretation of the spin polarization measurements near threshold as being a consequence of the initial angular momentum of the colliding system.

nucl-th↗

arXiv2024

Studying $\pi^+\pi^-$ photoproduction beyond Pomeron exchange

Forward photoproduction of $\pi^+\pi^-$ pairs with invariant mass of the order of $m_\rho\sim 770$ MeV is traditionally understood to be produced via Pomeron exchange. Based on a detailed analysis of the CLAS photoproduction data, it is shown that the dynamics of two-pion photoproduction for $|t|\gtrsim 0.5$ GeV$^2$ cannot be explained by Pomeron exchange alone. This motivates the development of a new theoretical model of two-pion photoproduction which incorporates both two-pion and pion-nucleon resonant contributions. After fitting free parameters, the model provides an excellent description of the low moments of the angular distribution measured at CLAS, and enables an assessment of the relative contributions of particular production mechanisms and an interpretation of the various features of the data in terms of these mechanisms.

hep-ph↗