ATLAS Diboson Excess from Stueckelberg Mechanism

TL;DR

This paper proposes a model where a leptophobic Z' boson, arising from a Stueckelberg mechanism, explains the ATLAS diboson excess at 2 TeV, predicting specific decay modes and helicity structures for future tests.

Contribution

It introduces a novel Stueckelberg extension model for the Z' boson that accounts for the diboson excess and predicts distinctive decay patterns and helicity amplitudes.

Findings

Model explains the ATLAS diboson excess at 2 TeV.

Predicts Z' decays into WW, ZZ, and Zγ with specific ratios.

Forecasts observable signals at LHC run II with 13 TeV energy.

Abstract

We discuss the diboson excess seen by the ATLAS detector around 2 TeV in the LHC run I at $\sqrt{s}=8$ TeV. We explore the possibility that such an excess can arise from a $Z'$ boson which acquires mass through a $U(1)_X$ Stueckelberg extension. The corresponding $Z'$ gauge boson is leptophobic with a mass of around 2 TeV and has interactions with $SU(2)_L$ Yang-Mills fields and gauge fields of the hypercharge. The analysis predicts $Z'$ decays into $WW$ and $ZZ$ as well as into $Z\gamma$. Further three-body as well as four-body decays of the $Z'$ such as $WWZ, WW\gamma, WWZZ$ etc are predicted. In the analysis we use the helicity formalism which allows us to exhibit the helicity structure of the $Z'$ decay processes in an transparent manner. In particular, we are able to show the set of vanishing helicity amplitudes in the decay of the massive $Z'$ into two vector bosons due to angular momentum conservation with a special choice of the reference momenta. The residual set of non-vanishing helicity amplitudes are identified. The parameter space of the model compatible with the diboson excess seen by the ATLAS experiment at $\sqrt s=8$ TeV is exhibited. Estimate of the diboson excess expected at $\sqrt s= 13$ TeV with 20 fb$^{-1}$ of integrated luminosity at LHC run II is also given. It is shown that the $WW$, $ZZ$ and $Z\gamma$ modes are predicted to be in the approximate ratio $1:\cos^2\theta_W (1+ \alpha \tan^2\theta_W)^2/2: (1-\alpha)^2\sin^2\theta_W/2$ where $\alpha$ is the relative strength of the couplings of hypercharge gauge fields to the couplings of the Yang-Mills gauge fields. Thus observation of the $Z\gamma$ mode as well as three-body and four-body decay modes of the $Z'$ will provide a definite test of the model and of a possible new source of interaction beyond the standard model.