Impact of inelastic corrections on $\gamma \gamma \to \gamma \gamma$ scattering from UPC at the LHC

Abstract

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 CTEQ18QED 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.
}