Hard photon production and matrix-element parton-shower merging

TL;DR

Addresses the challenge of prompt-photon production by unifying the direct and fragmentation components within a single Monte Carlo framework. The authors develop an interleaved QCD+QED dipole-like parton shower and extend matrix-element–parton-shower merging to include photons, enabling higher-order real-emission corrections for hard photons. The approach describes the photon fragmentation function measured at LEP and improves hadron-collider predictions for isolated photons and diphotons at Tevatron, validating the democratic treatment of photons and partons. The work lays groundwork for improved LHC predictions with a unified treatment of strong and electroweak radiation, without introducing new free parameters.

Abstract

We present a Monte-Carlo approach to prompt-photon production, where photons and QCD partons are treated democratically. The photon fragmentation function is modelled by an interleaved QCD+QED parton shower. This known technique is improved by including higher-order real-emission matrix elements. To this end, we extend a recently proposed algorithm for merging matrix elements and truncated parton showers. We exemplify the quality of the Monte-Carlo predictions by comparing them to measurements of the photon fragmentation function at LEP and to measurements of prompt photon and diphoton production from the Tevatron experiments.

Figures (6)

• Figure 1: The $z_\gamma$ distribution measured in hadronic $Z^0$ decays by ALEPH Buskulic:1995au for $2$-jet, $3$-jet and $\geq 4$-jet events at different Durham resolution $y_{\rm cut}$. The theory result corresponds to QCD+QED shower evolution of the leading-order $q\bar{q}$ process, taking into account hadronisation corrections.
• Figure 2: Inclusive photon transverse energy distribution compared to data from CDF Aaltonen:2009ty (left) and DØ Abazov:2005wc (right). The contributions of different classes of leading-order core processes are also displayed. For the notation used cf. the main text.
• Figure 3: The inclusive photon transverse energy obtained with a QCD+QED shower simulation supplemented by real-emission matrix elements with up to two additional QCD partons or photons (denoted $2\to {2,3,4}$) is compared to data from CDF Aaltonen:2009ty (left) and DØ Abazov:2005wc (right).
• Figure 4: Properties of diphoton events measured by CDF Acosta:2004sn. Displayed are the sub-contributions from different leading-order matrix elements and their sum.
• Figure 5: Properties of diphoton events measured by the CDF collaboration Acosta:2004sn. Figure \ref{['fig:tev_diphoton_ps']} compares the influence of different parton-shower kinematics when using leading-order matrix elements. Figure \ref{['fig:tev_diphoton_me']} shows the same comparison for merged event samples with up to two additional particles in the matrix element-final state. Scheme 1 refers to the algorithm outlined in Appendix \ref{['sec:kinematics']}, scheme 2 stands for the original implementation Schumann:2007mg.