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 $\sim 750\mathrm{GeV}$, after analyzing new data collected at center-of-mass energies of $\sqrt{s}=13\mathrm{TeV}$. We present a possible physical interpretation of such a signature, within the framework of a minimal UV-complete model with a massive singlet pseudoscalar state $a$ that couples to a new TeV-scale colored vectorlike 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 nonperturbative 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.

• Received 16 December 2015

DOI:https://doi.org/10.1103/PhysRevD.93.015017