Abstract
This paper investigates the electromagnetic production of lepton pairs with low transverse momentum in relativistic heavy ion collisions. We estimate the initial photons' transverse momentum contributions by employing models where the average transverse momentum squared of the incoming photon can be calculated in the equivalent photon approximation. We further derive an all order QED resummation for the soft photon radiation, which gives an excellent description of the ATLAS data in ultra-peripheral collisions at the LHC. For peripheral and central collisions, additional $p_T$-broadening effects from multiple interaction with the medium and the magnetic field contributions from the quark-gluon plasma are also discussed.
Highlights
There have been strong interests in the electromagnetic process of dilepton production in two-photon scattering in heavy-ion collisions in the last few years [1,2,3,4,5]
This paper investigates the electromagnetic production of lepton pairs with low total transverse momentum in relativistic heavy-ion collisions
We introduce the so-called generalized transverse momentum dependent (GTMD) photon distributions in the momentum space, Z Γijðx; kT ; Δ⊥Þ 1⁄4
Summary
There have been strong interests in the electromagnetic process of dilepton production in two-photon scattering in heavy-ion collisions in the last few years [1,2,3,4,5]. The STAR Collaboration studied pair production at low pT, pT < 200 MeV/c in gold-gold and uranium-uranium collisions [2], found evidence for momentum broadening, with pT spectra different from calculations without inmedium effects [32,33,34,35] This broadening could be attributed to in-medium scattering of the produced leptons, but first, it is necessary to account for possible changes to the pT spectrum due to changes in the impact-parameter distribution, as induced by either additional photon exchange or hadronic interactions. To better understand these developments from STAR and ATLAS, we study the acoplanarity of lepton pairs in heavyion collisions.
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