Neutron emission from the photon-induced reactions in ultraperipheral ultrarelativistic heavy-ion collisions

Ultraperipheral collisions are a source of various interesting phenomena based on photon-induced reactions. We calculate cross sections for single and any number of $n$, $p$, $\ensuremath{\alpha}$, and $\ensuremath{\gamma}$ rays in ultraperipheral heavy-ion collision for Large Hadron Collider energies. We analyze the production of a given number of neutrons relevant for a recent ALICE experiment, for $\sqrt{{s}_{NN}}=5.02\phantom{\rule{0.16em}{0ex}}\mathrm{TeV}$. In our approach, we include both single and multiple photon exchanges as well as the fact that not all photon energies are used in the process of equilibration of the residual nucleus. We propose a simple two-component model in which only part of photon energy ${E}_{\ensuremath{\gamma}}$ is changed into the excitation energy of the nucleus $({E}_{\mathrm{exc}}\ensuremath{\ne}{E}_{\ensuremath{\gamma}})$ and compare its results with outcomes of hipse and empire codes. The role of high photon energies for small neutron multiplicities is discussed. Emission of a small number of neutrons at high photon energies seems to be crucial to understand the new ALICE data. All effects work in the desired direction, but the description of the cross section of four- and five-neutron emission cross sections from first principles is rather demanding. The estimated emission of charged particles such as protons, deuterons, and $\ensuremath{\alpha}$ is briefly discussed and confronted with very recent ALICE data, obtained with a proton zero degree calorimeter.

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