Resonance-Continuum Interference in the Diphoton Higgs Signal at the LHC

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, $gg\to H\to \gamma \gamma$, with the continuum QCD background, $gg\to \gamma \gamma$, induced by quark loops. Helicity selection rules suppress the effect, which is dominantly due to the imaginary part of the two-loop $gg\to \gamma \gamma$ 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.

Figure 1

Sample Feynman diagrams contributing to the interference of $gg\to H\to \gamma \gamma$ 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 the center-of-mass scattering angle $\theta =45°$. 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 $m_H=140\mathrm{G}\mathrm{e}\mathrm{V}$. The vertical dotted line indicates that an event with $\theta <34.9°$ will not pass the standard ATLAS and CMS photon $p_{\mathrm{T}}$ cuts.