Gluon versus photon production of a 750 GeV diphoton resonance

The production mechanism of a 750 GeV diphoton resonance, either via gluon or photon fusion, can be probed by studying kinematic observables in the diphoton events. We perform a detector study of the two production modes of a hypothetical scalar or tensor diphoton resonance in order to characterize the features of the two scenarios. The nature of the resonance production can be determined from the jet multiplicity, the jet and diphoton rapidities, the rate of central pseudorapidity gaps, or the possible detection of forward protons from elastic photoproduction for events in the signal region. Kinematic distributions for both signals and expected irreducible diphoton background events are provided for comparison along with a study of observables useful for distinguishing the two scenarios at an integrated luminosity of $20{\mathrm{fb}}^{-1}$. We find that decay photons from a 750 GeV scalar resonance have a preference for acceptance in the central detector barrel, while background events are more likely to give accepted photons in the detector end caps. This disfavors the interpretation of the large number of excess events found by the Run-2 CMS diphoton search with one photon detected in the end cap as a wide spin-0 resonance signal. However, one expects more end cap photons in the case of spin-2 resonance.

Figure 1

Elastic-elastic, elastic-inelastic and inelastic-inelastic contributions to the photoproduction of the resonance $R$.

Figure 2

The required relationship between the branching ratio of $R\to \gamma \gamma$ and total width $\mathrm{\Gamma }$ to match the observed event rate, varied between 3–6 fb, assuming photon fusion dominates. The central value (red) corresponds to a 4.5 fb production cross section.

Figure 3

The required relationship between the branching ratio of $R\to \gamma \gamma$ and gluon coupling $c_{gg}$ to match the observed event rate, varied between 3–6 fb, assuming gluon fusion dominates. The central value (red) corresponds to a 4.5 fb production cross section.

Figure 4

Left: Jet multiplicity per accepted diphoton event for $gg\mathrm{F}$ (black), $\gamma \gamma \mathrm{F}$ (red), and irreducible $\gamma \gamma$ background (blue). Right: Sample of accepted $gg\mathrm{F}$ signal (black) and $\gamma \gamma \mathrm{F}$ signal (red) both combined with 50% $\gamma \gamma$ background contamination with statistics corresponding to $20{\mathrm{fb}}^{-1}$. Uncertainties are statistical only.

Figure 5

Left: $H_T$ distribution per accepted diphoton event for $gg\mathrm{F}$ (black), $\gamma \gamma \mathrm{F}$ (red), and irreducible $\gamma \gamma$ background (blue). Right: Sample of accepted $gg\mathrm{F}$ signal (black) and $\gamma \gamma \mathrm{F}$ signal (red) both combined with 50% $\gamma \gamma$ background contamination with statistics corresponding to $20{\mathrm{fb}}^{-1}$. Uncertainties are statistical only.

Figure 6

Left: Jet pseudorapidity distribution per accepted diphoton event for $gg\mathrm{F}$ (black), $\gamma \gamma \mathrm{F}$ (red), and irreducible $\gamma \gamma$ background (blue). Right: Sample of accepted $gg\mathrm{F}$ signal (black) and $\gamma \gamma \mathrm{F}$ signal (red) both combined with 50% $\gamma \gamma$ background contamination with statistics corresponding to $20{\mathrm{fb}}^{-1}$. Uncertainties are statistical only.

Figure 7

Left: Fraction of accepted diphoton events containing a central pseudorapidity gap (see text for definition) of size $\mathrm{\Delta }\eta$ for $gg\mathrm{F}$ (black), $\gamma \gamma \mathrm{F}$ (red), and irreducible $\gamma \gamma$ background (blue). Right: Sample of accepted $gg\mathrm{F}$ signal (black) and $\gamma \gamma \mathrm{F}$ signal (red) both combined with 50% $\gamma \gamma$ background contamination with statistics corresponding to $20{\mathrm{fb}}^{-1}$. Uncertainties are statistical only.

Figure 8

Left: Pseudorapidity of accepted diphotons for $gg\mathrm{F}$ (black), $\gamma \gamma \mathrm{F}$ (red), and irreducible $\gamma \gamma$ background (blue). Right: Pseudorapidity of photons in accepted diphoton events, normalized by the number of accepted diphoton events, $N_{acc}$, for $gg\mathrm{F}$ (black), $\gamma \gamma \mathrm{F}$ (red), and irreducible $\gamma \gamma$ background (blue). Uncertainties are statistical only.

Figure 9

Left: Pseudorapidity of photons in accepted diphoton events, normalized by the number of accepted diphoton events, $N_{acc}$, for $\gamma \gamma \mathrm{F}$ production of a spin-0 (red) or spin-2 (purple) resonance and irreducible $\gamma \gamma$ background (blue). Right: Pseudorapidity of photons in accepted diphoton events for $gg\mathrm{F}$ production of a spin-0 (black) or spin-2 (green) resonance and irreducible $\gamma \gamma$ background (blue). Uncertainties are statistical only.