Nuclear $\gamma$-radiation as a signature of ultra-peripheral ion collisions at the LHC

Abstract.We study the peripheral ion collisions at LHC energies where a nucleus is excited to a discrete state and then emits $\gamma$-rays. Large nuclear Lorenz factors allow the observation of high-energy photons up to a few tens GeV and in the angular region of a few hundred micro-radians from the beam direction. These photons can be used to tagg events with particle production in the central rapidity region in ultra-peripheral collisions. To detect these photons it is necessary to have an electromagnetic detector in front of the zero-degree calorimeter in LHC experiments.