  The CMS and ATLAS reports on a possible excess of diphoton events at $750$
GeV are the cause of great excitement and hope. We show that a pseudoscalar
axion coupled to the topological density of hypercharge, suggested in the past
by Brustein and Oaknin as a candidate for inducing baryogenesis, can explain
the signal in the diphoton channel. The hypercharge axion (HCA) can also decay
to $Z\gamma$ and $ZZ$. The expected number of events in these channels is too
small to have been detected, but such decays should be observed in the future.
The HCA can be produced via vector boson fusion (VBF) or via associated
production (AP). The latter should be manifested by a characteristic decay to 3
photons which should be observed soon. We adapt the previous analysis of
possible detection of the HCA at the LHC by Brustein and Oaknin and by Elfgren
to the case that the mass of the HCA is 750 GeV and find the expected cross
section and decay width in terms of the HCA coupling. We find that the expected
cross-section fits well the observed number of excess events and that there is
tension between the measured cross-section and the reported partial decay
width. The tension is caused because for both VBF and AP the production
cross-section and the decay width depend linearly on each other. We therefore
expect that if the new resonance is indeed the HCA, either its width should be
significantly smaller than the reported width or the production cross-section
should be significantly higher than the reported one.
