Photons as Probes of Gluon Saturation in Dilute + Dense Collisions

Sanjin Benić,Oscar Garcia-Montero,Raju Venugopalan,Kenji Fukushima

doi:10.3390/proceedings2019010033

Sanjin Benić, Oscar Garcia-Montero + Show 2 more

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https://doi.org/10.3390/proceedings2019010033

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Abstract

We use the Color Glass Condensate (CGC) effective field theory (EFT) to calculate inclusive photon production to leading order q g → q γ , (LO), and next-to leading order g g → q q ¯ γ (NLO) at LHC energies. These processes dominate the photon production at small-x , where x ≲ 0 . 01 in the target and projectile protons. We show that the NLO contribution dominates at values of x typical at the LHC, since its cross-section is sensitive to the gluon distributions in both protons. We perform a comparison of our results to the available inclusive photon data, from ATLAS and CMS at center-of-mass energies of 2 . 76 and 7 TeV . This data lies in the range k ⊥ > 20 GeV . We show that for this range, the k ⊥ -factorized cross-section converges to the full CGC EFT result, and can be used for the comparison. We find that it gives good agreement with experimental results. Our results are to be considered as a first step towards constraining unintegrated gluon distributions, which will be continued for larger systems, where coherent scatterings are enhanced.

Highlights

Photons produced in pp collisions are excellent probes of the internal structures of the hadronic waveforms
The next-to-leading order (NLO) channel gg → qqγ is dominant for high energies, in mid-rapidities, as the gluon distributions are enhanced at small-x
Dipole evolution was given by the running coupling Balitsky-Kovchegov equation, which is a good approximation to the general expression given by the Balitsky-JIMWLK hierarchy

Summary

Introduction

Photons produced in pp collisions are excellent probes of the internal structures of the hadronic waveforms. In this limit the parametrical leading order (LO) is given by the qg → qγ process [2,3],which is dominant when a hard photon is emitted from a large-x valence quark before or after this scatters coherently off the target. The next-to-leading order (NLO) channel gg → qqγ is dominant for high energies, in mid-rapidities, as the gluon distributions are enhanced at small-x.

Results

Conclusion