Abstract
We show that contrary to naive expectations the recently observed diphoton excess can be explained by a vector resonance, which decays to a photon and a light scalar $s$, followed by a decay of the scalar into two photons: $Z' \rightarrow \gamma s \rightarrow 3 \gamma$. As the two photons from the scalar decay are highly boosted, the experimental signature is an apparent diphoton final state. In fact all the necessary ingredients are naturally present in $Z'$ models: Additional fermions with electroweak quantum numbers are required in order to render the theory anomaly free and naturally induce the required effective couplings, while the hidden Higgs which gives mass to the $Z'$ can be very light. In particular no new coloured states are required in this framework. We also show that in such a setup the width of the resonance can be rather large, while all couplings remain perturbative.
Highlights
We have shown that extending the Standard Model (SM) with a new U (1) gauge group can provide an explanation for the recently observed diphoton excess around ∼750 GeV by both ATLAS and CMS
The central idea is that a very light scalar decaying into two photons can mimic a single photon if it is produced with sufficiently large boost factor
It turns out that loops of new extra fermions give rise to trilinear vertices allowing the Z to decay into sγ and the light scalar s to decay into γ γ respectively
Summary
The recently observed excess in the diphoton channel reported by ATLAS [1] and (with slightly less significance) CMS [2], has triggered a large amount of interest in the particle physics community [3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48]. In this letter we show that the observed excess can be due to a new vector resonance, which decays into three photons (see [51]). As an example we will consider the case of a new U (1) under which only the third generation quarks are charged This is motivated by the observation that no resonance has been observed at the LHC Run 1, implying that the increase of production cross section needs to be sizeable. In the case of b quarks in the initial state, the ratio of production cross sections at 8 and 13 TeV is sufficiently large (5.4) due to the proton PDFs. As couplings of O(1) are needed in order to have a large enough production cross section, the width due to decays into bband ttis sizeable and can be as large as 45 GeV as indicated by the ATLAS result, while being consistent with searches for pp → Z → bb and pp → Z → tt. ✩ This article is registered under preprint number: DESY 15-256, arXiv:1512
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