Update on 750 GeV diphotons from closed string states

TL;DR

This paper explores the possibility that a 750 GeV diphoton resonance observed at the LHC could originate from a closed string excitation, analyzing production mechanisms and consistency with experimental bounds.

Contribution

It reevaluates the production of a string-inspired resonance via photon and gluon fusion, demonstrating compatibility with LHC data and constraining string mass scales and couplings.

Findings

Both photon and gluon fusion can produce the resonance within the allowed string mass range.

The resonance can explain the LHC diphoton excess without conflicting with LHC8 null results.

String mass scales between 7 and 30 TeV are consistent with experimental bounds.

Abstract

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.