  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 \varphi
(associated with low mass scale string theory) that has a coupling with gauge
kinetic terms. We reevaluate the production of \varphi by photon fusion to
accommodate recent developments on additional contributions to relativistic
light-light scattering. We also study the production of \varphi 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 \alt M_s/TeV \alt 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 \sim 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 \varphi, but
without altering the range of possible string mass scales.

High Energy Physics - Phenomenology;High Energy Physics - Theory