Next-to-Leading-Order Monte Carlo Simulation of Diphoton Production in Hadronic Collisions

TL;DR

Addresses the need for precise NLO predictions in diphoton production at hadron colliders by adapting the POWHEG positive-weight matching to include both QCD and QED emissions and photon fragmentation. The method constructs barB(ΦB), handles real and virtual corrections with Catani-Seymour subtraction, and generates the hardest emission via QCD/QED Sudakovs with veto/rejection. Results show improved radiation modeling and finite low-pT behavior for the γγ system, with good agreement to fixed-order calculations where applicable and to collider data, validating the approach. This work delivers the first NLO photon-involved process simulation and provides a practical tool implemented in Herwig++ for prompt photon production and Higgs-background studies.

Abstract

We present a method, based on the positive weight next-to-leading-order matching formalism (POWHEG), to simulate photon production processes at next-to-leading-order (NLO). This technique is applied to the simulation of diphoton production in hadron-hadron collisions. The algorithm consistently combines the parton shower and NLO calculation, producing only positive weight events. The simulation includes both the photon fragmentation contribution and a full implementation of the truncated shower required to correctly describe soft emissions in an angular-ordered parton shower.

Figures (8)

• Figure 1: Diphoton production at leading-order.
• Figure 2: Diphoton production at next-to-leading-order. In (a) the real and virtual Feynman diagrams contributing to the $q\bar{q}\to\gamma\gamma$ subprocess are shown while in (b) the real diagrams for $gq$ initiated process are given.
• Figure 3: Bremsstrahlung contribution for diphoton production.
• Figure 4: Rapidity of the $\gamma\gamma$-pair at NLO. The distribution from the Herwig++ parton shower with POWHEG correction (solid black line) is compared with NLO cross section from DIPHOX (dashed red line). At LO the Herwig++ distribution is given by the dotted black line while the cross section from DIPHOX by the dash-dotted red line.
• Figure 5: The (a) invariant mass and (b) transverse momentum of the $\gamma\gamma$-pair. The solid blue line shows the POWHEG approach, while the dashed red curve shows the result of the Herwig++ shower at LO. We show the NLO cross section provided by DIPHOX (magenta dotted line) and RESBOS (green dashed-dotted line). The data are from Ref. Acosta:2004sn and the curves are plotted with RivetBuckley:2010ar. In the lower panel, the yellow band describes the one sigma variation of data.