Hadrons and direct photon in pp and pA collisions at LHC and saturation effects

TL;DR

This work analyzes hadron and direct-photon production in $pp$ and $pA$ collisions at RHIC and LHC within a unified color-dipole framework, testing a set of saturation models against DIS data. It demonstrates that saturation effects are most pronounced at very forward rapidities, predicting large photon-to-pion ratios $R_{ m gamma/ pi}$ (up to $\sim$10–20 at $\eta\approx7$–$8$) and distinctive forward-rapidity enhancements in the spectra. Nuclear effects are studied via $R_{pA}$, showing general suppression at midrapidity due to gluon shadowing, with the Cronin enhancement potentially surviving for direct photons in models with higher $Q_s$, while CGC-based predictions tend toward suppression. The paper compares GBW, CGC/AAMS-BK, b-CGC, KLR-AdS/CFT, and Semi-Sat dipole models, highlighting sizable model-to-model differences that future LHC data can discriminate, and emphasizes forward photons as clean probes of small-$x$ dynamics.

Abstract

We investigate hadrons and direct photon production in pp and pA collisions at the energies of RHIC and LHC within the color-dipole approach employing various saturation models. We show that greatest sensitivity to saturation effects is reached at very forward rapidities for pp collisions at LHC (\sqrt{s}=14 TeV). The ratio of direct-photon to pion production can be about 20-10 (at η=7-8). Therefore, direct photon production at forward rapidities should provide a rather clean probe. We calculate the rapidity dependence of the invariant cross-section and find some peculiar enhancement at forward rapidities which is more pronounced for direct photon production. We show that this peak is further enhanced by saturation effects. We provide predictions for the nuclear modification factor R_{pA} for pions and direct photon production in pA collisions at LHC energy at midrapidity. We show within various saturation models that the pion Cronin enhancement at RHIC is replaced by a moderate suppression at LHC energy at midrapidity due to gluon shadowing effects. Cronin enhancement of direct photons can survive at LHC energy within models with a larger saturation scale.

Figures (17)

• Figure 1: Gluon radiation (top panel) and direct photon production (bottom panel) for a projectile quark interacting with the target.
• Figure 2: Top panel: Saturation scale defined via Eq. (\ref{['defs']}) as a function of $1/x$ for various color-dipole models. Lower panel: The total dipole-proton cross section $\sigma_{q\bar{q}}(r,x)$ at fixed $x=10^{-5}$ in the various color-dipole models introduced in Sec. VI.
• Figure 3: The hadrons spectra at the RHIC energy at midrapidity. The dashed lines are calculated with the GBW model. The solid lines are the pQCD calculation results taken from Ref. me-croin. Note that the color dipole approach is valid at very small $x_2$ corresponding to $p_T<2$ GeV at RHIC energy and midrapidity(shown by a line). The experimental data are from rhic2006star-data.
• Figure 4: Pions and protons spectra in $pp$ collisions at the LHC energies $\sqrt{s}=5.5$ and $14$ TeV at midrapidity. Theory curves are calculated with the GBW model. Note that our model for particle productions is not reliable at midrapidity, for explanation see the end of Sec. III.
• Figure 5: Pion spectra obtained from various dipole models at forward rapidities and LHC energies for $pp$ collisions.