Impact of new results from the ultraperipheral collision on modeling the proton and neutron emission in photon-induced nuclear processes

TL;DR

This work confronts the proton and neutron yields in ultraperipheral $ ext{Pb}^{208}+ ext{Pb}^{208}$ collisions with a hybrid framework that combines EPA photon flux, a pre-equilibrium phase (via HIPSE/TCM and GiBUU) for $E_ ext{γ}>200$ MeV, and statistical de-excitation (GEMINI++ or GEM2) to predict final remnants and multiplicities. By linking pre-equilibrium dynamics to the de-excitation stage, the authors quantify the contributions to single-proton emission and the neutron-energy tails in a cohesive, energy-dependent picture, and they assess the capabilities and limitations of several model combinations against ALICE data. A central finding is that reproducing the experimentally observed large $ ext{1p}$ cross section (~40 b) requires substantial pre-equilibrium input from photon interactions with individual nucleons across a wide photon-energy spectrum, something not achieved by GiBUU+GEMINI++ or GEM2 alone. The results imply that a comprehensive account of pre-equilibrium processes and high-energy, partonic mechanisms is essential for accurate UPC predictions, and they motivate targeted experiments (e.g., at JLab or EIC) to further constrain the underlying photoproduction dynamics.

Abstract

The ultrarelativistic collisions of heavy ions provide rich spectrum of possibilities to discuss the response of the nucleus to photons. Newly published neutron and proton multiplicities measured in the ALICE experiment in ultraperipheral collisions allow investigating the influence of the electromagnetic fields on colliding nuclei for the $^{208}$Pb+$^{208}$Pb at $\sqrt{s_{NN}}$=5.02~TeV. The theoretical predictions are done within hybrid model including equivalent photon approximations (EPA), GiBUU modeling of pre-equilibrium processes and generation of the exited nuclear remnants, which decay is modeled by statistical approach: GEM2 or GEMINI++. The cross-sections of the mass-charge distributions of nuclear remnants as well as the neutron, proton and other charged particle multiplicities are estimated. We concentrate on production of protons and isotopes coming from the electromagnetic dissociation. Special attention is devoted to emission of a single proton. The cross section for $1p$ emission is very close to maximal available one based on reactions of photon with individual nucleons. Our pre-equilibrium processes explain simultaneously the tail of neutron energy distributions in the nuclear rest frame observed in $γ+ A$ collisions.

Figures (13)

• Figure 1: Photoabsorption cross section for $\gamma$+Pb reaction in energy range 10 MeV- 100 GeV (full line) compared with the total cross section obtained with the GiBUU model (dots).
• Figure 2: Distribution of the primary mass and its excitation energy predicted in the naively adopted HIPSE ($E_{\gamma}$=200 MeV - top), and GiBUU ($E_{\gamma}$=200 MeV - middle and $E_{\gamma}$=2000 MeV - bottom.)
• Figure 3: Average excitation energy as a function of energy of the photon in $\gamma+^{208}$Pb collisions in different models.
• Figure 4: Cross section for production of primary fragments: GiBUU for collisions of 200 MeV and 2000 MeV photons in $\gamma+^{208}$Pb reaction.
• Figure 5: Cross section for production of final nuclei: GiBUU + GEMINI++ for 200 MeV and 2000 MeV photons in $\gamma+^{208}$Pb reactions.