Abstract
Light-by-light (LbyL) scattering, ! , is a quantum-mechanical process, forbidden by the classical theory of electrodynamics, but possible in Quantum Electrodynamics via a loop diagram. Despite the small cross section, it is theoretically possible to observe this process in ultra-peripheral high-energy heavy-ion collisions. Based on 0.48 nb1 of 2015 Pb+Pb data, a first direct evidence of LbyL scattering was established by the ATLAS Collaboration in 2017. In total, 13 events were found in the signal region with a background expectation of 2:60:7 events. The excess corresponds to 4.4 significance over the background-only hypothesis. In November 2018, the new dataset of Pb+Pb collisions was collected by the ATLAS experiment with an integrated online luminosity of 1.7 nb1. This recent dataset has been employed to perform a preliminary study using the control sample from ! e+e process.
Highlights
Within the Color Glass Condensate (CGC) framework, the hybrid formalism [1] is used to study single inclusive particle production at next-toleading order [2,3,4,5,6,7,8,9,10] and heavy-quark production [11] at forward rapidity
The dense target is treated in the CGC, i.e. it is defined as a distribution of strong color fields, which during the scattering transfer transverse momentum to the propagating partonic configuration
The usual approach is to calculate the cross section in the CGC, after which one takes the small dipole or correlation limit which corresponds to a leading-power expansion in the ratio p2t /Q2 of the hard scales with Λ2QCD p2t Q2
Summary
Within the Color Glass Condensate (CGC) framework, the hybrid formalism [1] is used to study single inclusive particle production at next-toleading order [2,3,4,5,6,7,8,9,10] and heavy-quark production [11] at forward rapidity. In this set-up, the wave function of the projectile proton is treated in the spirit. Non-perturbative models such as the McLerran–Venugopalan (MV) [23] or Golec-Biernat–Wüsthoff (GBW) model [24] can be used to write down analytical expressions
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