Azimuthal asymmetries in lepton and heavy-quark pair production in UPCs

Abstract

Azimuthal modulations in lepton and heavy-quark pair production in ultraperipheral collisions (UPCs) of highly charged ions are investigated. The modulations in the azimuthal angles of the sum and difference of the transverse momenta of the pair of particles in the final state, as well as of the transverse impact parameter, arise from the collisions of unpolarized and polarized photons. A full description of the cross section in terms of Generalized Transverse Momentum Dependent parton distributions (GTMDs) for photons is given including a careful consideration of the Fourier transform to impact parameter space. In particular, this leads to a feed-in mechanism among harmonics of different orders, which in principle generates harmonics of all (even) orders. Wherever comparable, our analytical results for the azimuthal modulations agree with those presented in other papers on this topic. Compared to these other works, we separate effects that arise from the anisotropies of the GTMDs from those that do not and retain terms proportional to the mass of the produced particles, as they are relevant for muon, charm and bottom quark production. We show that the normalized differential cross section changes considerably with the produced particle mass, which should be discernible in UPCs at RHIC and LHC. For the numerical results we adopt several models for the photon GTMD correlator, and find that all of them are in fairly good agreement with each other and with UPC data from STAR. We also present results for various azimuthal modulations for RHIC kinematics, where we compare $e^{+}{e}^{-}$ production with the production of heavier particles, and for LHC kinematics, focusing on ${\mu }^{+}{\mu }^{-}$ production. These results exhibit interesting mass-dependent features in the asymmetries that may help study the anisotropies arising from the underlying photon GTMD description.

22 pages, 9 figures

References in corpus (12)

• Generalized parton correlation functions for a spin-1/2 hadron
• Quark Wigner Distributions and Orbital Angular Momentum
• Diffractive Dijet Production and Wigner Distributions from the Color Glass Condensate
• Probing the linear polarization of photons in ultraperipheral heavy ion collisions
• Lepton Pair Production Through Two Photon Process in Heavy Ion Collisions
• Impact parameter dependence of the azimuthal asymmetry in lepton pair production in heavy ion collisions
• Initial state angular asymmetries in high energy p+A collisions: spontaneous breaking of rotational symmetry by a color electric field and C-odd fluctuations
• Momentum Anisotropy of Leptons from Two Photon Processes in Heavy Ion Collisions
• Estimation of Impact Parameter and Transverse Spherocity in heavy-ion collisions at the LHC energies using Machine Learning
• Azimuthal asymmetries of muon pair production in ultraperipheral heavy ion collisions
• Electromagnetic interaction of low transverse momentum electrons and positrons with heavy nuclei in ultra-peripheral ultra-relativistic heavy-ion collisions
• Validity of equivalent photon spectra and the photoproduction processes in p-p collisions