String Consistency, Heavy Exotics, and the $750$ GeV Diphoton Excess at the LHC

TL;DR

This paper explores how string consistency conditions necessitate exotic particles that could explain the 750 GeV diphoton excess observed at the LHC, analyzing their properties, couplings, and implications within string-inspired models.

Contribution

It demonstrates that string consistency conditions naturally lead to exotics capable of explaining the diphoton excess, and analyzes their couplings, masses, and phenomenological viability.

Findings

Exotics are prevalent in string-derived quivers satisfying consistency conditions.

Certain exotics can produce a narrow diphoton resonance consistent with LHC data.

Large width scenarios are difficult to realize within this framework.

Abstract

String consistency conditions are stronger than anomaly cancellation and can require the addition of exotics in the visible sector. We study such exotics and demonstrate that they may account for the modest excess at $750$ GeV in recent diphoton resonance searches performed by the ATLAS and CMS collaborations. In a previous analysis of type II MSSM D-brane quivers we systematically added up to five exotics for the sake of satisfying string consistency conditions. Using this dataset, we demonstrate that 89780 of the 89964 quivers have exotics, 78155 of which include singlets that may couple to MSSM or exotic multiplets with coupling structures governed by $U(1)$ symmetries that are often anomalous. We demonstrate that certain sets of exotics are far preferred over others and study the structure of singlet couplings to heavy exotics carrying standard model charges. Typical possibilities include singlets that may decay to vector-like quarks and / or vector-like leptons and subsequently to two photons. We show that a narrow width diphoton excess can be accounted for while evading existing bounds if multiple exotics are added, with vector-like leptons of mass $M_L\lesssim 375$ GeV and vector-like quarks with masses up to $\simeq 3$ TeV. However, a large width $(\Gamma/M \sim 0.06)$, as suggested by the ATLAS data, cannot be easily accommodated in this framework. Renormalization group equations with GUT-scale boundary conditions show that these supersymmetric models are perturbative and stable. Type IIA compactifications on toroidal orbifolds allow for $O(10)$ Yukawa couplings in the ultraviolet. We also discuss the possibility of accounting for the diphoton excess in a low string scale scenario via the decay of string axions.