Abstract
We compute the differential cross-section for inclusive prompt photon+quark production in deeply inelastic scattering of electrons off nuclei at small x (e + A DIS) in the framework of the Color Glass Condensate effective field theory. The result is expressed as a convolution of the leading order (in the strong coupling αs) impact factor for the process and universal dipole matrix elements, in the limit of hard photon transverse momentum relative to the nuclear saturation scale Qs,A(x). We perform a numerical study of this process for the kinematics of the Electron-Ion Collider (EIC), exploring in particular the azimuthal angle correlations between the final state photon and quark. We observe a systematic suppression and broadening pattern of the back-to-back peak in the relative azimuthal angle distribution, as the saturation scale is increased by replacing proton targets with gold nuclei. Our results suggest that photon+jet final states in inclusive e + A DIS at high energies are in general a promising channel for exploring gluon saturation that is complementary to inclusive and diffractive dijet production. They also provide a sensitive empirical test of the universality of dipole matrix elements when compared to identical measurements in proton-nucleus collisions. However because photon+jet correlations at small x in EIC kinematics require jet reconstruction at small k⊥, it will be important to study their feasibility relative to photon-hadron correlations.
Highlights
It is conjectured [1, 2] that the proliferation of soft gluons in hadron wavefunctions at high energies via bremsstrahlung is saturated by many-body screening and recombination effects when gluon occupancies are parametrically O(1/αs), where αs is the QCD coupling
The discovery and characterization of this gluon saturation phenomenon is a major goal of deeply inelastic scattering (DIS) experiments at the Electron-Ion Collider (EIC) and several signatures of gluon saturation have been discussed in this context [3, 4]
We derived in this paper analytic expressions for inclusive prompt photon+quark production in e + A DIS at small x and at leading order (LO) in the Color Glass Condensate (CGC) effective field theory (EFT) power counting
Summary
It is conjectured [1, 2] that the proliferation of soft (small x) gluons in hadron wavefunctions at high energies via bremsstrahlung is saturated by many-body screening and recombination effects when gluon occupancies are parametrically O(1/αs), where αs is the QCD coupling. As we noted, with the onset of gluon saturation, the momentum transfer from the hadron is given by k⊥ ∼ Qs(x) This contributes to a systematic suppression and broadening of the back-to-back or “away side” peak in the ∆φ relative azimuthal distribution of the hard photon-jet cross-section or of dijets. Appendix G contains expressions for inclusive quark production in DIS at small xBj for different polarizations of the virtual photon.
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