Mass-degenerate Higgs bosons at 125 GeV in the two-Higgs-doublet model

The analysis of the Higgs boson data by the ATLAS and CMS Collaborations appears to exhibit an excess of h→γγ events above the Standard Model (SM) expectations, whereas no significant excess is observed in h→ZZ*→four lepton events, albeit with large statistical uncertainty due to the small data sample. These results (assuming they persist with further data) could be explained by a pair of nearly mass-degenerate scalars, one of which is an SM-like Higgs boson and the other is a scalar with suppressed couplings to W⁺W^- and $ZZ$. In the two-Higgs-doublet model, the observed γγ and ZZ*→four lepton data can be reproduced by an approximately degenerate $CP$-even (h) and $CP$-odd (A) Higgs boson for values of sin(β-α) near unity and 0.70tanβ1. An enhanced γγ signal can also arise in cases where mₕmH, mHmA, or mₕmHmA. Since the ZZ*→4 leptons signal derives primarily from an SM-like Higgs boson whereas the γγ signal receives contributions from two (or more) nearly mass-degenerate states, one would expect a slightly different invariant mass peak in the ZZ*→four lepton and γγ channels. The phenomenological consequences of such models can be tested with additional Higgs data that will be collected at the LHC in the near future.