Unified Resummation of Soft Gluon Radiation in Heavy Meson Pair Photoproduction

TL;DR

This paper introduces a unified resummation framework for heavy-meson pair photoproduction that accurately models soft-gluon radiation effects across different mass regimes and enhances the understanding of gluon saturation phenomena in high-energy collisions.

Contribution

The paper develops a comprehensive resummation approach for soft-gluon radiation in heavy-meson pair production, bridging massive and massless quark regimes, and applies it within the Color Glass Condensate framework to phenomenologically analyze saturation effects.

Findings

Resummation accurately describes H1 azimuthal correlation data.

Angular asymmetries are sensitive to gluon saturation effects.

Heavy-quark masses suppress soft radiation, highlighting saturation signals.

Abstract

We develop a unified resummation framework for heavy-meson pair photoproduction that treats soft-gluon radiation in a massive scheme for $|\boldsymbol q| \lesssim m_Q$ and is consistent with the massless limit $m_Q \ll |\boldsymbol q|$, where $\boldsymbol q$ denotes the transverse momentum of the pair and $m_Q$ the quark mass. This framework describes the full correlation regime $|\boldsymbol q| \ll |\boldsymbol P|$, with $\boldsymbol P$ the pair's relative transverse momentum. Next, within the Color Glass Condensate framework, we quantify the impact of soft gluon radiation on two important phenomenological observables, namely, the azimuthal $\Delta\phi$ correlation and the harmonic asymmetries $\langle \cos(2n\phi_{qP})\rangle$. Our results show that the resummation provides an excellent description of the H1 $\Delta\phi$ data. We then find that the angular-asymmetry harmonics are sizable and their ratios in $\gamma A$ to $\gamma p$ collisions are highly sensitive to gluon saturation. We further find a clear mass hierarchy in both observables, indicating that heavy-quark masses suppress soft radiation and thereby enhance genuine small-$x$ saturation signals. Our work demonstrates that heavy-quark pair photoproduction provides a quantitative probe of gluon saturation, directly measurable in forthcoming EIC and UPC measurements. Finally, the unified resummation is readily extendable to other processes involving massive particles and jets in both $eA$ and $pA$ collisions.