String consistency, heavy exotics, and the 750 GeV diphoton excess at the LHC

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 vectorlike 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 375 GeV and vector-like quarks with masses up to 3 TeV. However, a large width (Γ/M ∼ 0.06), as suggested by the ATLAS data, cannot be easily accommodated in this framework. Renormalization group equations with GUTscale 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.

Precision tools and models to narrow in on the 750 GeV diphoton resonance

“…In supersymmetry, the scalar partner of the goldstino could provide an explanation to the diphoton signal [97,147,167,335]. Other ideas, slightly more exotic, include: a model with a space-time varying electromagnetic coupling constant [135], Gluinonia [337], Squarkonium/Diquarkonium [299], flavons [244], axions in various incarnations [8,24,63,246,274,336], a natural Coleman-Weinberg theory [22,307], radiative neutrino mass models [264,325,327], and string-inspired models [19,132,188,240,254].…”

750 GeV diphotons and supersymmetric grand unification

“…In figure 6 we depict the maximal diphoton cross section as a function of the lightest charged vector particle mass (either a lepton or a slepton) and of the lightest colored vector particle mass (right panel). 11 In the left panel, we required (s)quark masses of at least 700 GeV, in the right panel slepton masses of at least 380 GeV. In addition, the fermion mass ratios of table 3 were imposed.…”

“…It remains to be shown that the part of parameter space, in which a sufficient diphoton signal arises, is accessible within grand unified theories. Therefore, we have chosen to implement the model with a singlet and one 10 + 10 into 12 More restrictive numbers which circulate in the literature have been obtained under the assumption of a larger diphoton cross section. Table 5.…”

“…Vector-like 'exotics' are very common in string model constructions such as D-brane theories [8] or heterotic string compactifications [9]. The possibility that these exotics mediate the diphoton signal has been considered in [10][11][12][13][14][15][16][17]. However, in UV derived settings, there exist strong constraints on the possible charges and couplings of the new states.…”

Diphoton excess at 750 GeV: gluon–gluon fusion or quark–antiquark annihilation?

“…Though further data is required to establish the existence of a new resonance or other beyond the SM (BSM) mechanism responsible for the diphoton excess, significant theoretical efforts have been made to explain the possible diphoton excess in various BSM scenarios [4-9,11-13,15-a e-mail: jgao@anl.gov b e-mail: zhanghao@physics.ucsb.edu c e-mail: zhuhx@mit.edu 19,[26][27][28][29][30][32][33][34]38,[41][42][43][44][45][48][49][50][51][52][53][54][55][56][57]59,63,[65][66][67][68][69]71,72,[75][76][77][78][79][80][83][84][85][86][87]89,90,[93]…”