Resonance-continuum interference in the di-photon Higgs signal at the LHC

TL;DR

A low mass standard model Higgs boson should be visible at the Large Hadron Collider through its production via gluon-gluon fusion and its decay to two photons through the interference of this resonant process with the continuum QCD background, gg-->gammagamma, induced by quark loops.

Abstract

A low mass Standard Model Higgs boson should be visible at the Large Hadron Collider through its production via gluon-gluon fusion and its decay to two photons. We compute the interference of this resonant process, gg -> H -> gamma gamma, with the continuum QCD background, gg -> gamma gamma induced by quark loops. Helicity selection rules suppress the effect, which is dominantly due to the imaginary part of the two-loop gg -> gamma gamma scattering amplitude. The interference is destructive, but only of order 5% in the Standard Model, which is still below the 10-20% present accuracy of the total cross section prediction. We comment on the potential size of such effects in other Higgs models.

Figures (2)

• Figure 1: Sample Feynman diagrams contributing to the interference of $gg \rightarrow H \rightarrow \gamma\gamma$ with the continuum background. Only one diagram is shown at each loop order, for each amplitude. The blob contains $W$ and $t$ loops, and small contributions from lighter charged fermions.
• Figure 2: Top panel: the percentage reduction of the SM Higgs $\gamma\gamma$ signal as a function of the Higgs mass, for CM scattering angle $\theta = 45^\circ$. The solid curve gives the result with all phases turned on; the other curves turn on one of the component phases at a time. Bottom panel: the same quantities, plotted as a function of the scattering angle, for $m_H = 140$ GeV. The vertical dotted line indicates that an event with $\theta < 34.9^\circ$ will not pass the standard ATLAS and CMS photon $p_{\rm T}$ cuts.