How resonance-continuum interference changes 750 GeV diphoton excess: signal enhancement and peak shift

A bstract A hypothetical new scalar resonance, a candidate explanation for the recently observed 750 GeV diphoton excess at the LHC 13 TeV, necessarily interferes with the continuum background gg → γγ . The interference has two considerable effects: (1) enhancing or suppressing diphoton signal rate due to the imaginary-part interference and (2) distorting resonance shape due to the real-part interference. We study them based on the best-fit analysis of two benchmark models: two Higgs doublets with ∼50 GeV width (exhibiting the imaginary-part interference effect) and a singlet scalar with 5 GeV width (exhibiting the real-part one), both extended with vector-like fermions. We find that the resonance contribution can be enhanced by a factor of 2 (1.6) for 3 (6) fb signal rate, or the 68% CL allowed mass region is shifted by O (1) GeV. If the best-fit excess rate decreases in the future data, the interference effects will become more significant.