Diphoton Signatures from Heavy Axion Decays at the CERN Large Hadron Collider

Recently, the LHC collaborations, ATLAS and CMS, have announced an excess in the diphoton channel with local significance of about 3σ around an invariant mass distribution of ∼750 GeV, after analyzing new data collected at centre-of-mass energies of √s = 13 TeV. In this Letter, we present a possible interpretation of such a signature, within the framework of a minimal UV-complete model with a massive singlet pseudo-scalar state a that couples to a new TeV-scale coloured vector-like fermion F, which has a large exotic hypercharge quantum number. The pseudo-scalar state a might be a heavy pseudo-Goldstone boson, such as a heavy axion, which decays into two photons and whose mass lies around the excess region. The mass of the CP-odd state a and its coupling to F may be due to non-perturbative topological effects, which break the original Goldstone shift symmetry dynamically. The possible role of the heavy axion a in the radiative generation of the seesaw Majorana scale and in the solution to the so-called strong CP problem are briefly discussed.