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).
