Measuring the diphoton coupling of a 750 GeV resonance

TL;DR

The paper proposes a model‑independent method to measure the photon–photon coupling of a hypothetical 750 GeV resonance by tagging elastic γγ production pp → ppγγ with forward protons. Using AFP/CT‑PPS detectors and central photon measurements, the approach isolates photon fusion events and determines f_γ directly, independent of whether gluon or quark fusion dominates other production modes. The authors compute the elastic γγ cross section, assess backgrounds, and derive projected f_γ sensitivities for 300 and 3000 fb⁻¹, showing that this method probes the strong‑photon‑coupling region complementary to dijet constraints. They also discuss prospects for elastic production in other diboson channels (ZZ, Zγ, WW) and the role of timing detectors in enabling these analyses, highlighting the method’s potential to disentangle production mechanisms at the LHC.

Abstract

A slight excess has been observed in the first data of photon-photon events at the 13 TeV LHC, that might be interpreted has a hint of physics beyond the Standard Model. We show that a completely model-independent measurement of the photon-photon coupling of a putative 750 GeV resonance will be possible using the forward proton detectors scheduled at ATLAS and CMS.

Figures (5)

• Figure 1: Schematic representation of the resonant inelastic process $pp\to\gamma\gamma X$ with gluon and quark fusion (above) and photon fusion (below).
• Figure 2: Schematic representation of the resonant elastic process $pp\to \gamma\gamma pp$. The elastic gluon fusion process requires an additional exchange of a virtual gluon.
• Figure 3: Bounds and sensitivities in the $f_\gamma-f_g$ plane, in case of production via photon and gluon fusion. Purple: $68\%$ C.L. and $95\%$ C.L. credible regions corresponding to the observed diphoton event rate. Green lines: Limit of the region above which $\Gamma_{EW}+\Gamma_{gg} \leq\Gamma^{\rm tot}$. Dotted (dashed) lines correspond to $\Gamma_{EW}/\Gamma_{\gamma\gamma}=1.64$ (53.9) respectively. Blue: Excluded region from Run 1 dijet searches ATLAS_dijet_8TeVCMS_dijet_8TeV. Red: Sensitivity region from the potential measurement of $pp\rightarrow \gamma\gamma pp$ using forward proton detectors, for $300$ fb$^{-1}$ of integrated luminosity, see Eq. (\ref{['eq:sens']}).
• Figure 4: Inferred value of $f_\gamma$ (68% C.L.) as a function of the observed number of events for 300 fb$^{-1}$ (purple) and 3000 fb$^{-1}$ (orange) of data. We have assumed $\Gamma^{\rm tot}=45$ GeV.
• Figure 5: Partial decay width for $\phi \to ZZ, Z\gamma, W^+W^-$ normalised to the $\phi \to \gamma\gamma$ width. The black line corresponds to $\Gamma_{\rm EW}/\Gamma_{\gamma\gamma}$. The grey region is excluded at $95\%$ C.L. by the $Z\gamma$ search from Run 1 Aad:2014fha.