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

TL;DR

This paper proposes that a heavy axion-like pseudo-scalar particle could explain the 750 GeV diphoton excess observed at the LHC, by decaying into two photons within a minimal UV-complete model involving new vector-like fermions.

Contribution

It introduces a minimal UV-complete model with a heavy axion-like particle coupling to new vector-like fermions to interpret the diphoton excess at 750 GeV.

Findings

The heavy axion can decay into two photons matching the excess.

The model links the axion to non-perturbative effects and Goldstone symmetry breaking.

Potential implications for the strong CP problem and Majorana scale generation.

Abstract

Recently, the LHC collaborations, ATLAS and CMS, have announced an excess in the diphoton channel with local significance of about $3\,\sigma$ around an invariant mass distribution of $\sim 750$ GeV, after analyzing new data collected at centre-of-mass energies of $\sqrt{s} = 13~{\rm TeV}$. We present a possible physical 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 $F$, whose hypercharge quantum number is a non-zero integer. 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 effects, which can break the original Goldstone shift symmetry dynamically. The possible role that the heavy axion $a$ can play in the radiative generation of a seesaw Majorana scale and in the solution to the so-called strong CP problem is briefly discussed.