HEP Colloquia 2024

by Luca Maxia (University of Groningen)

Wednesday 27 Nov 2024, 14:30 → 16:00 Europe/Rome

Sala Consiliare (Department of Physics)

Dilepton production in UPCs: Probing the photon one function that rules them all

 

In recent years, the description of lepton-pair production in ultra-peripheral collisions (UPCs) of heavy nuclei has attracted much attention, particularly with the release of new RHIC data. The description of this process requires the introduction of the so-called Wigner distribution of the photons. They are related via Fourier transform to the generalised transverse momentum dependent distributions (GTMDs), which depend on two internal transverse momenta: $k_{ T}$ and $\Delta_{T}$. The former is the intrinsic transverse momentum also found in the transverse momentum dependent distributions, whereas the latter corresponds to the (transverse) momentum transfer. 

Note that if one is interested in exploring the gluonic content of nuclear matter (e.g. by considering heavy quarkonium production), the QED channel corresponds to the background of the process. Therefore, understanding the photon GTMDs is crucial to single out the QCD channel and probe the gluon GTMDs. However, recent theoretical works on this subject failed to address the sometimes counter-intuitive features that the presence of the two aforementioned transverse momenta generates.

In this talk I will present the theoretical framework that describes UPCs of heavy ions in terms of photon GTMDs, and I will try to clarify its counter-intuitive features. I will also discuss how the mass of the produced system modifies the cross section and some of the various asymmetries that can be measured in this process.