### Speakers

### Description

We will report on our results for light-by-light scattering in

ultraperipheral Pb-Pbcollisions at the LHC.

We calculate cross section for the elementary $\gamma \gamma \to \gamma \gamma$

subprocess taking into account the following contributions:

(a) box mechanisms with leptons and quarks in the loops,

(b) VDM-Regge mechanism (fluctuation of both photons to vector mesons

and their interaction) and

(c) two-gluon exchange.

Our nuclear calculations are based on equivalent photon approximation in the impact parameter space.

We use realistic charge form factor of nuclei which is a Fourier transform

of the charge distribution in nuclei.

Our estimate has shown that ultraperipheral Pb-Pb

collisions can be measured at the LHC. This opened a possibility

to study the $\gamma\gamma \to \gamma\gamma$ scattering at the LHC.

Our rather optimistic predictions became a motivation for experimental groups

to perform corresponding experimental studies.

Very recently, the ATLAS Collaboration observed 13 events for light-by-light scattering in ultraperipheral Pb-Pb collisions.

They obtained the cross section of 70$\pm$20(stat.)$\pm$17(syst.) nb.

Simultaneously, our Standard Model predictions gave 49$\pm$10 nb.

Our theoretical calculations were a source of ATLAS Monte Carlo simulation.

The ATLAS measurement is a first experimental observation of the

$\gamma \gamma \to \gamma \gamma$ mechanism.

In the talk, we shall present many differential distributions.

We shall discuss a possibility of a separation and identification of different components

(three subprocesses mentioned above). We shall present a similar

analysis for the pp$\to$pp$\gamma\gamma$ reaction.

This talk will be based mainly on our analyses which were presented in Ref. [1] and [2].

[1] M. K{\l}usek-Gawenda, P. Lebiedowicz and A. Szczurek,

Phys. Rev. C93 (2016) 044907,

[2] M. K{\l}usek-Gawenda, W. Sch\"afer and A. Szczurek,

Phys. Lett. B761 (2016) 399.

[3] The ATLAS Collaboration,

ATLAS-CONF-2016-111 (2016).