Gluon induced contributions to Z gamma production at hadron colliders

TL;DR

The paper evaluates whether NNLO gluon-induced contributions can significantly affect Zγ production at hadron colliders. It computes two classes of gluon-initiated processes: tree-level gg→q q̄ Zγ and loop gg→Zγ, including leptonic decays and anomalous ZZγ and Zγγ couplings, with a careful treatment of singularities and photon isolation. Using modern PDFs and MSbar factorization, it finds that these gluon-induced contributions are modest (loop < 5%), even after accounting for gluon density enhancements, and the bremsstrahlung piece is negligible. Consequently, NLO predictions for Zγ production remain reliable for phenomenology, and gluon-induced NNLO effects do not provide useful constraints on anomalous couplings.

Abstract

We study the contribution of gluon induced partonic subprocesses to Z gamma pair production at hadron colliders. These processes contribute only at next-to-next-to-leading order but are potentially enhanced by two factors of the gluon parton densities. However, we find that their contribution is modest and that next-to-leading order calculations give reliable predictions.

Figures (1)

• Figure 1: The contribution to the $p_T^\gamma$ distribution in $Z\gamma$ production at the LHC separated into the various partonic initial states: $q\bar{q}$ (solid line), $g q$ (dashed) and $gg$. The gluon induced contribution is separated into the loop and bremsstrahlung part.