Speaker
Description
The current theoretical estimations lead to cross-sections for $AA \to \gamma \gamma AA$ which are somewhat smaller than the measured ones by the ATLAS and CMS Collaborations, In our recent paper, we estimated the contribution of inelastic channels to the Light-by-Light (LbL) scattering in ultraperipheral collisions (UPC) of heavy ions, in which one or both of the incident nuclei dissociate ($A A \to \gamma \gamma X Y$ where $X, Y = A, A'$) due to the photon emission. These new mechanisms are related to extra emissions that are difficult to identify and may be misinterpreted as enhanced $\gamma \gamma \to \gamma \gamma$ scattering.
We include processes of coupling of photons to individual nucleons in addition to coherent coupling to the whole nuclei. Both elastic (nucleon in the ground state) and inelastic (nucleon in an excited state) are taken into account. The inelastic nucleon fluxes are calculated using CT18qed photon distribution in nucleon. The inelastic photon fluxes are shown and compared to standard photon fluxes in the nucleus.
We present the ratio of the inelastic corrections to the standard contribution to the nuclear cross section. We find that for the ATLAS kinematics the inelastic corrections grow with $M_{\gamma \gamma}$ and rapidity difference. Our results indicate that the inelastic contributions can be of the order of 20-40 \% of the traditional (no nuclear excitation) predictions. We discuss also uncertainties due to the choice of factorization scale.
We calculate cross sections for the production of different neutron classes 0n0n, 0nXn+Xn0n and XnXn, possible to measure in Zero Degree Calorimeters (ZDCs), associated with photon-photon scattering in UPC of lead on lead. The calculations are performed for the ATLAS kinematics. Our calculations for neutron classes are tested against existing data for $\rho^0$ production in UPC. We get a good agreement for absolute cross section as well as very good results for fractional cross section which gives confidence to the analogous calculation for diphoton production, corresponding to $\gamma \gamma \to \gamma \gamma$ scattering. We present both absolute as well as fractional cross sections for the different neutron classes. We also show different distributions in impact parameter, photon rapidity, transverse momentum, diphoton invariant mass and photon rapidity difference. The shapes of the photon distributions depend on the neutron classes which we quantify. It would be valuable to test our predictions using the ATLAS main detector and the ATLAS ZDCs.
