Update on 750 GeV diphotons from closed string states

Motivated by the recent update on LHC searches for narrow and broad resonances decaying into diphotons we reconsider the possibility that the observed peak in the invariant mass spectrum at M-gamma gamma = 750 GeV originates from a closed string (possibly axionic) excitation phi (associated with low mass scale string theory) that has a coupling with gauge kinetic terms. We reevaluate the production of phi [ by photon fusion to accommodate recent developments on additional contributions to relativistic light light scattering. We also study the production of phi via gluon fusion. We show that for both a narrow and a broad resonance these two initial topologies can accommodate the excess of events, spanning a wide range of string mass scales 7 less than or similar to M-s/TeV less than or similar to 30 that are consistent with the experimental lower bound: M-s > 7 TeV, at 95% CL. We demonstrate that for the two production processes the LHC13 data is compatible with the lack of a diphoton excess in LHC8 data within similar to 1 sigma. We also show that if the resonance production is dominated by gluon fusion the null results on dijet searches at LHC8 further constrain the coupling strengths of phi, but without altering the range of possible string mass scales. (C) 2016 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license