Prompt photon production and photon-hadron correlations at RHIC and the LHC from the Color Glass Condensate

TL;DR

This paper applies the Color Glass Condensate framework with running-coupling BK evolution to forward-rapidity proton–nucleus collisions, focusing on inclusive prompt photon production and semi-inclusive photon–hadron correlations. By separating direct and fragmentation photon contributions, it shows direct photons are more sensitive to gluon saturation through the dipole amplitude N_F, and it predicts a strong suppression of away-side photon–hadron correlations at high energies and forward rapidities. The study provides predictions for the nuclear modification factor $R_{pA}^{\gamma}$ and for photon–hadron azimuthal correlations at RHIC and the LHC, highlighting distinctive saturation signatures that differ from collinear factorization approaches. Overall, it argues that photon observables offer cleaner, more robust tests of CGC dynamics and small-x saturation than di-hadron measurements, motivating targeted experimental tests.

Abstract

We investigate inclusive prompt photon and semi-inclusive prompt photon-hadron production in high energy proton-nucleus collisions using the Color Glass Condensate (CGC) formalism which incorporates non-linear dynamics of gluon saturation at small x via Balitsky-Kovchegov equation with running coupling. For inclusive prompt photon production, we rewrite the cross-section in terms of direct and fragmentation contributions and show that the direct photon (and isolated prompt photon) production is more sensitive to gluon saturation effects. We then analyze azimuthal correlations in photon-hadron production in high energy proton-nucleus collisions and obtain a strong suppression of the away-side peak in photon-hadron correlations at forward rapidities, similar to the observed mono-jet production in deuteron-gold collisions at forward rapidity at RHIC. We make predictions for the nuclear modification factor R_{p(d)A} and photon-hadron azimuthal correlations in proton(deuteron)-nucleus collisions at RHIC and the LHC at various rapidities.

Figures (9)

• Figure 1: Nuclear modification factor for direct photon production in minimum-bias pA (dashed line) and dA (solid line) collisions at RHIC ($\sqrt{S}=0.2$ TeV) at $\eta = 2, 3$. The curves are obtained from Eq. (\ref{['pho4']}) using the solution to rcBK equation with the initial saturation scale $Q_{0p}^2=0.168~\text{GeV}^2$ for proton and $Q_{0A}^2=3Q_{0p}^2$ for a nucleus (gold).
• Figure 2: Nuclear modification factor for direct, fragmentation and inclusive prompt photon production in minimum-bias p(d)A collisions at RHIC $\sqrt{S}=0.2$ TeV (right) and the LHC $\sqrt{S}=4.4$ TeV (left) energy at various rapidities. The curves are the results obtained from Eq. (\ref{['pho4']}) and the solution to rcBK equation with the initial saturation scale $Q_{0p}^2=0.168~\text{GeV}^2$ for a proton and $Q_{0A}^2=3Q_{0p}^2$ for a nucleus (gold), corresponding to set II in Eq. (\ref{['set']}).
• Figure 3: Nuclear modification factor for direct photon production in p(d)A collisions at various rapidities at RHIC $\sqrt{S}=0.2$ TeV (right) and the LHC $\sqrt{S}=4.4$ TeV energy (left). The curves are the results obtained from Eq. (\ref{['pho4']}) and the solution to rcBK equation using different initial saturation scales for a proton $Q_{0p}$ and a nucleus $Q_{0A}$. The band shows our theoretical uncertainties arising from allowing a variation of the initial saturation scale of the nucleus in a range consistent with previous studies of DIS structure functions as well as particle production in minimum-bias pp, pA and AA collisions in the CGC formalism, see the text for the details.
• Figure 4: Nuclear modification factor for direct photon (right) and inclusive prompt photon (left) production in pA collisions at various rapidities a the LHC $\sqrt{S}=8.8$ TeV energy. The band (CGC-rcBK-av) similar to Fig. \ref{['fig-r2']} corresponds to the results obtained from Eq. (\ref{['pho4']}) and the solutions to the rcBK evolution equation using different initial saturation scales for a proton $Q_{0p}$ and a nucleus $Q_{0A}$, see the text for the details.
• Figure 5: Right: Nuclear modification factor for direct photon production at $\eta=3$ in minimum-bias dA $\sqrt{S}=0.2$ TeV (RHIC) and pA $\sqrt{S}=4.4, 8.8$ TeV (LHC) collisions. The curves are the results obtained from Eq. (\ref{['pho4']}) and the solution to rcBK equation with the initial saturation scale $Q_{0p}^2=0.168~\text{GeV}^2$ for a proton and $Q_{0A}^2=3Q_{0p}^2$ for a nucleus. Left: Comparison of the inclusive prompt photon nuclear modification factor predictions from the CGC (in this paper) and the standard collinear factorization approach pqcd-models. The band CGC-rcBK-av is the same as in Fig. \ref{['fig-r22']}.