Diphoton Resonance from a Warped Extra Dimension

Abstract: We argue that extensions of the Standard Model (SM) with a warped extra dimension, which successfully address the hierarchy and flavor problems of elementary particle physics, can provide an elegant explanation of the 750 GeV diphoton excess recently reported by ATLAS and CMS. A gauge-singlet bulk scalar with ${\cal O}(1)$ couplings to fermions is identified as the new resonance $S$, and the vector-like Kaluza-Klein excitations of the SM quarks and leptons mediate its loop-induced couplings to photons and gluons. The electroweak gauge symmetry almost unambiguously dictates the bulk matter content and hence the hierarchies of the $S\to \gamma\gamma$, $WW$, $ZZ$, $Z\gamma$, $t\bar t$ and dijet decay rates. We find that the $S\to Z\gamma$ decay mode is strongly suppressed, such that $\mbox{Br}(S\to Z\gamma)/\mbox{Br}(S\to\gamma\gamma)<0.1$. The hierarchy problem for the new scalar boson is solved in analogy with the Higgs boson by localizing it near the infrared brane. The infinite sums over the Kaluza-Klein towers of fermion states are finite and can be calculated in closed form with a remarkably simple result. Reproducing the observed $pp\to S\to\gamma\gamma$ signal requires Kaluza-Klein masses in the multi-TeV range, consistent with bounds from flavor physics and electroweak precision observables. Useful side products of our analysis, which can be adapted to almost any model for the diphoton resonance, are the calculation of the gluon-fusion production cross section $\sigma(pp\to S)$ at NNLO in QCD, an exact expression for the inclusive $S\to gg$ decay rate at N$^3$LO, a study of the $S\to t\bar t h$ three-body decay and a phenomenological analysis of portal couplings connecting $S$ with the Higgs field.