Is a Large Intrinsic k_T Needed to Describe Photon + Jet Photoproduction at HERA?

TL;DR

This work delivers a full NLO QCD calculation for isolated photon+jet photoproduction at HERA, incorporating direct, resolved, fragmentation, and the $\gamma g\to\gamma g$ box, with the Weizsäcker-Williams photon flux and a jet definition via the $k_T$-algorithm. By examining $k_T$-sensitive observables and applying ZEUS-like cuts, the study finds no need for an additional intrinsic $\langle k_T\rangle$ to describe the data; scale dependence is mild and most distributions agree with measurements, though some discrepancies persist in specific bins likely due to detector or hadronization effects. The results reinforce the sufficiency of fixed-order NLO QCD for this process and inform modeling of initial-state $k_T$ in photon-induced events. The analysis also highlights the sensitivity of certain observables to cut choices and the importance of asymmetric cuts to avoid infrared complications. Overall, the findings challenge the necessity of large intrinsic $k_T$ in photon+jet photoproduction at HERA and are consistent with similar conclusions from inclusive prompt photon studies.

Abstract

We study the photoproduction of an isolated photon and a jet based on a code of partonic event generator type which includes the full set of next-to-leading order corrections. We compare our results to a recent ZEUS analysis in which an effective k_T of the incoming partons has been determined. We find that no additional intrinsic k_T is needed to describe the data.

Figures (12)

• Figure 1: Examples of contributing subprocesses at leading order
• Figure 2: Nonisolated total $\gamma+$ jet cross section for $E_T^{\gamma}> 5$ GeV as a function of $E_{T,\rm{min}}^{jet}$. The photon energy range is $0.2<y<0.9$, the rapidities are integrated over $-1.5<\eta^{jet}<1.8$ and $-0.7<\eta^{\gamma}<0.9$.
• Figure 3: Behaviour of $d\sigma/dx_{\gamma}^{LL}$, $d\sigma/dx_{\gamma}^{meas}$ and $d\sigma/dx_{\gamma}^{obs}$ for the isolated $\gamma$ + jet cross section.
• Figure 4: Magnitude of leading order and NLO results for the isolated cross section $d\sigma^{\gamma+jet}/d\eta^{\gamma}$. Isolation with $\epsilon_c=0.1, R=1$, jet rapidities integrated over $-1.5<\eta^{jet}<1.8$ .
• Figure 5: Magnitude of LO and NLO contributions for resolved and direct parts separately, with isolation ($\epsilon_c=0.1, R=1)$.