Gluons to Diphotons via New Particles with Half the Signal’s Invariant Mass

Abstract

Any new particle charged under SU (3)C and carrying electric charge will leave an imprint in the di-photon invariant mass spectrum as it can mediate gg → γγ process through loops. The combination of properties of loop functions, threshold resummation and gluon pdfs can result in a peak-like feature in the di-photon invariant mass around twice the mass of a given particle even if the particle is short-lived and thus it doesn't form a narrow bound state. Using recent ATLAS analysis, we set upper limits on the combined SU (3)C and electric charge of new particles and indicate future prospects. We also discuss the possibility that the excess of events in the di-photon invariant mass spectrum around 750 GeV originates from loops of a particle with mass around 375 GeV.Introduction. As demonstrated by discoveries of the Z boson and the Higgs boson, a resonance is the cleanest signal of a new particle as long as its branching ratios to visible modes are nonzero. However, many popular models including minimal supersymmetric standard model or models with various top-partners predict particles that can be produced in pairs. For the pair production, the searches highly depend on decay modes of a given particle and there are known scenarios in well motivated models which are difficult to see directly even if production cross sections are sizable. In principle, a model can always be constructed in which a new particle cascade decays to complex final states consisting of soft particles and possibly missing energy, or the particle has a large number of possible final states with small branching ratios to individual ones. Signatures that are less model dependent or do not depend on decay modes at all are therefore an integral part of searches for new physics.

Top-quark mass from the diphoton mass spectrum

“…The result with the oneloop amplitude is also plotted for comparison. In the plots, we observe that the distributions show a characteristic structure near m γ γ 2m t = 346 GeV; it shows a dip and then a small bump below the threshold [26]. We find that, if we employ the LO Green function, the shape of the distribution changes by the scale choice.…”

“…Our framework follows the preceding studies on h → γ γ [28,29], and some of our numerical results overlap with that in Ref. [26].…”

“…Our framework follows the preceding studies on h → γ γ [28,29], and some of our numerical results overlap with that in Ref. [26].We discuss further to utilize the predicted mass spectrum for a precise determination of the top-quark mass, which is one of the fundamental parameters in the SM. Although the top-quark mass has been measured with an error of sub-GeV level [30], its interpretation in terms of well-defined mass parameters is not settled yet in perturbative QCD.…”

“…The analytic expression of the one-loop amplitude has been known for a long time for both the massless-and the massive-quark loops [16][17][18][19][20][21][22]. The two-loop amplitude has been calculated only for the massless-quark loops [23][24][25].The threshold structure of the massive-quark-loop amplitude deserves particular interests [21,26,27] where the massive quark is regarded as the top quark or a hypothetical particle beyond the SM. Beyond the one-loop level, the amplitude receives large QCD corrections due to the Coulomb-gluon exchanges between the nearly on-shell and low-velocity heavy quarks in s-channel.…”

“…The threshold structure of the massive-quark-loop amplitude deserves particular interests [21,26,27] where the massive quark is regarded as the top quark or a hypothetical particle beyond the SM. Beyond the one-loop level, the amplitude receives large QCD corrections due to the Coulomb-gluon exchanges between the nearly on-shell and low-velocity heavy quarks in s-channel.…”