Diphoton excess in the 2HDM: Hastening towards instability and the nonperturbative regime

In December 2015, the ATLAS and CMS collaborations presented a possible signal in the diphoton channel. Although more data have shown that the signal was likely to be only a statistical fluctuation, it is interesting to ask what can be learned if any diphoton excess would show up in future data. Here we challenge the interpretation of any possible diphoton excess in a two-Higgs-doublet framework and find results that are valid whenever a large number of colored and charged vectorlike fermions are needed to boost the production cross section. In a broad region of parameter space, the one-loop effects of these fermions abruptly drive the scalar potential to instability even before the hypercharge gauge coupling becomes nonperturbative.