750 GeV composite axion as the LHC diphoton resonance

Abstract

We propose that the 750 GeV resonance, presumably observed in the early LHC Run 2 data, could be a heavy composite axion that results from condensation of a hypothetical quark in a high-colour representation of conventional QCD. The model, motivated by a recently proposed solution to the strong CP problem, is very economical and is essentially defined by the properties of the additional quark -its colour charge, hypercharge and mass. The axion mass and its coupling to two photons (via axial anomaly) can be computed in terms of these parameters. The axion is predominantly produced via photon fusion (γγ → A) which is followed by Z vector boson fusion and associated production at the LHC. We find that the total diphoton cross section of the axion can be fitted with the observed excess. Combining the requirement on the cross-section, such that it reproduces the diphoton excess events, with the bounds on the total width (Γ tot 45 GeV), we obtain the effective coupling in the range 1.6 × 10 −4 GeV −1 C A 6.5 × 10 −5 GeV −1 . Within this window of allowed couplings the model favours a narrow width resonance and y 2 Q ∼ O(10). In addition, we observe that the associated production qq → Aγ → γγγ can potentially produce a sizeable number of three photon events at future LHC. However, the rare decay Z → A * γ → γγγ is found to be too small to be probed at the LHC and e + e − colliders.

The production of a diphoton resonance via photon-photon fusion

“…One interesting, and in some sense natural possibility, given that this possible excess is only seen so far in the γγ decay channel (and not, for example, in the dijet mass spectrum), is that the resonance may couple dominantly to photons, with the coupling to gluons and other coloured particles being either suppressed or absent entirely. This has been discussed in [3,4] (see also [5][6][7][8][9][10][11][12][13][14][15]), where the γγ induced process in both the usual inclusive and the exclusive modes has been considered. In the latter case the incoming photons are emitted coherently from the colliding protons, which then remain intact after the collision; this will naturally occur in a non-negligible fraction of events.…”

Precision tools and models to narrow in on the 750 GeV diphoton resonance

“…As more data becomes available and the evidence for new physics becomes more substantial, one might want to UV complete these mod-els, at which point the tools we are advertising become relevant and necessary. Other authors [42,52,60,73,114,122,131,233,235,273,293,297,308,315,320,323,346] considered strongly coupled models, in which the resonance is a composite state. This possibility would be favoured by a large width of the resonance, as first indications seem to suggest.…”

One-loop chiral perturbation theory with two fermion representations

“…Any additional singlet NGB which appears in a theory with multiple fermion representations is a particularly interesting object. It can play the role of a composite axion [40][41][42][43][44], offering a potential solution to the strong CP problem. In various extensions of the Standard Model, the singlet may provide a candidate to explain the 750 GeV diphoton excess observed by ATLAS and CMS [45,46]; within composite Higgs models it appears quite naturally as a relatively isolated light state with anomaly-induced couplings to pairs of Standard-Model vector bosons [29,47].…”