Interference Effects in the Decays of Spin-Zero Resonances into $\gamma \gamma$ and $t\bar{t}$

TL;DR

This paper investigates interference effects in the production and decay of hypothetical scalar or pseudoscalar resonances at the LHC, focusing on how these effects influence observed signals in gamma gamma and top-antitop channels, with implications for new physics searches.

Contribution

It provides a detailed analysis of how interference impacts the line-shape and apparent mass of resonances, including the effects of additional vector-like quarks, and constrains models based on current experimental data.

Findings

Interference can cause a ~20% enhancement in the signal rate for large resonance widths.

Interference effects can produce dips or peaks in the top-antitop mass distribution depending on model details.

Current LHC data constrains the presence of large interference effects in resonance production.

Abstract

We consider interference effects in the production via gluon fusion in LHC collisions at 13 TeV and decays into $\gamma \gamma$ and $t {\bar t}$ final states of one or two putative new resonant states $\Phi$, assumed here to be scalar and/or pseudo scalar particles. Although our approach is general, we use for our numerical analysis the example of the putative $750$ GeV state for which a slight excess was observed in the initial LHC $13$ TeV data. We revisit previous calculations of the interferences between the heavy-fermion loop-induced $gg \to \Phi \to \gamma \gamma$ signal and the continuum $gg\to \gamma\gamma$ QCD background, which can alter the production rate as well as modify the line-shape and apparent mass. We find a modest enhancement by $\sim 20$% under favorable circumstances, for a large $\Phi$ width. The effect of interference on the apparent scalar-pseudoscalar mass difference in a two-Higgs-doublet model is found to be also modest. An exploratory study indicates that similar effects are to be expected in the $gg \to \Phi \to Z \gamma$ channel. In this and other models with a large $\Phi$ total width, the dominant $\Phi$ decays are expected to be into $t \bar t$ final states. We therefore also study the effects of interference of the $gg\to \Phi \to t\bar t$ signal with the $gg\to t \bar t$ continuum QCD background and show that in the presence of standard fermions only in the $gg\to \Phi$ loops, it is destructive causing a dip in the $t \bar t$ mass distribution. Including additional vector-like quarks leads to a different picture as peaks followed by dips can then occur. We use the absence of such effects in ATLAS and CMS data to constrain models of the production and decays of the $\Phi$ state(s).