Ultrarelativistic Heavy Ions of large charge Z are accompanied by a large flux of Weizsacker–Williams photons. This opens up the opportunity to study a variety of single photon induced nuclear processes, as well as photon-photon processes.
We would like to present a formalism which allows to calculate differential distributions of dileptons produced via photon-photon fusion in semi-central (impact parameter $<$ 2 $\times$ nuclear radius) nucleus-nucleus collisions for a given centrality. In this approach the differential cross section is calculated using the complete polarization density matrix of photons resulting from the Wigner distribution formalism. We will present several differential distributions such as invariant mass of dileptons, dilepton transverse momentum and acoplanarity for different regions of centrality. The results of the calculations will be compared to experimental data of the STAR, ALICE and ATLAS collaborations. Very good agreement with the data is achieved without free parameters in all cases. Additional final state rescattering of leptons in the quark-gluon plasma is not needed. Our new approach based on Wigner distributions of photons gives a much better agreement with experimental data than the previous approaches used in the literature.
M. Kłusek-Gawenda, W. Schaefer and A. Szczurek,
``Centrality dependence of dilepton production via $\gamma \gamma$ processes from Wigner distributions of photons in nuclei,''
Phys. Lett. B814, 136114 (2021), [arXiv:2012.11973 [hep-ph]].
|Collaboration / Activity||Theory|