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Probing fully coherent radiation and parton densities using (virtual) photons at the LHC

François Arleo, Djessy Bourgeais, Maxime Guilbaud, Greg Jackson, Víctor Valencia Torres

TL;DR

This work analyzes fully coherent radiation effects on prompt photon production in proton–nucleus collisions at the LHC, incorporating medium-induced FCEL and FCEG within LO direct and fragmentation photon processes and linking their impact to the nuclear modification factor $R_{pA}(y,p_\top)$. The study shows that FCEL suppresses prompt photons at backward rapidity while FCEG can slightly enhance yields at forward rapidity, with fragmentation photons contributing but not dramatically altering the results. In parallel, Drell–Yan production, being colorless at LO, remains largely insensitive to FCEL and serves as a clean probe of nuclear PDFs; pseudo-data and Bayesian reweighting using the nNNPDF3.0 set demonstrate that DY measurements at LHCb Run 3 can substantially tighten small-$x$ constraints on both quark and gluon densities. Overall, the work highlights the complementary roles of prompt photons and DY in constraining nPDFs and clarifies the distinct signatures of FCEL/FCEG in high-energy pA collisions.

Abstract

Prompt photon production in pA collisions has long been suggested as a sensitive probe of the nuclear parton distribution functions (nPDFs). In this study, we present recent results on another cold nuclear matter effect, namely fully coherent radiation induced by parton multiple scattering, which may influence the nuclear dependence of prompt photon production. Medium-induced radiation effects, implemented in leading-order direct and fragmentation photon processes, are computed for pPb collisions at the LHC. At backward rapidity, photons are sensitive to fully coherent energy loss (FCEL), while at forward rapidity, fully coherent energy gain (FCEG) plays a crucial role due to the dominance of the $qg \to qγ$ scattering channel. In contrast, for virtual photon production, the impact of fully coherent radiation is marginal, making Drell-Yan (DY) one of the best ways to probe nuclear PDFs. The power of the DY process is demonstrated by reweighting nPDF sets at next-to-leading order using realistic pseudo-data for LHC Run 3.

Probing fully coherent radiation and parton densities using (virtual) photons at the LHC

TL;DR

This work analyzes fully coherent radiation effects on prompt photon production in proton–nucleus collisions at the LHC, incorporating medium-induced FCEL and FCEG within LO direct and fragmentation photon processes and linking their impact to the nuclear modification factor . The study shows that FCEL suppresses prompt photons at backward rapidity while FCEG can slightly enhance yields at forward rapidity, with fragmentation photons contributing but not dramatically altering the results. In parallel, Drell–Yan production, being colorless at LO, remains largely insensitive to FCEL and serves as a clean probe of nuclear PDFs; pseudo-data and Bayesian reweighting using the nNNPDF3.0 set demonstrate that DY measurements at LHCb Run 3 can substantially tighten small- constraints on both quark and gluon densities. Overall, the work highlights the complementary roles of prompt photons and DY in constraining nPDFs and clarifies the distinct signatures of FCEL/FCEG in high-energy pA collisions.

Abstract

Prompt photon production in pA collisions has long been suggested as a sensitive probe of the nuclear parton distribution functions (nPDFs). In this study, we present recent results on another cold nuclear matter effect, namely fully coherent radiation induced by parton multiple scattering, which may influence the nuclear dependence of prompt photon production. Medium-induced radiation effects, implemented in leading-order direct and fragmentation photon processes, are computed for pPb collisions at the LHC. At backward rapidity, photons are sensitive to fully coherent energy loss (FCEL), while at forward rapidity, fully coherent energy gain (FCEG) plays a crucial role due to the dominance of the scattering channel. In contrast, for virtual photon production, the impact of fully coherent radiation is marginal, making Drell-Yan (DY) one of the best ways to probe nuclear PDFs. The power of the DY process is demonstrated by reweighting nPDF sets at next-to-leading order using realistic pseudo-data for LHC Run 3.

Paper Structure

This paper contains 4 sections, 14 equations, 5 figures, 1 table.

Table of Contents

  1. Introduction
  2. Fully coherent radiation effects on prompt photons
  3. nPDF constraints from virtual photons
  4. Conclusion

Figures (5)

  • Figure 1: Prompt photon differential cross section as a function of $y$ at fixed $p_{_\perp}=5$ GeV, and for each individual partonic sub-processes.
  • Figure 2: $R_{\textnormal{pPb}}\xspace(y, p_{_\perp}=5\,\textnormal{GeV})$ for the partonic sub-processes $\ell \in \{1,2,3,4\}$ and when all processes are added (dashed line).
  • Figure 3: $R_{\textnormal{pPb}}\xspace$ of direct photon as a function of $y$ at $p_{_\perp}=5$ GeV (left) and as a function of $p_{_\perp}$ at $y=4$ (right). The uncertainties reflect the variation of the transport coefficient and that of the factorization scale.
  • Figure 4: $R_{\textnormal{pPb}}\xspace$ of direct (green band), fragmentation (yellow band) and total prompt photons (blue band) as a function of $y$ at $p_{_\perp}=5$ GeV (left) and as a function of $p_{_\perp}$ at $y=4$ (right).
  • Figure 5: Left: relative error in the predicted $R^\textnormal{DY}_{\textnormal{pA}}$ as a function of rapidity, before and after reweighting. Right: the same, but for the gluon nPDF $f^\textnormal{A}_g(x,Q=9~{\rm GeV})$ as a function of $x\,$. Note that in both panels, only the uncertainty in the nPDFs is shown and calculated according to Eq. (\ref{['old']}) and Eq. (\ref{['new']}).