Improved constraints on the mixing and mass of $Z'$ bosons from resonant diboson searches at the LHC at $\sqrt{s}=13$ TeV and predictions for Run II

TL;DR

This paper uses LHC data to set new stringent limits on the mass and mixing angle of hypothetical $Z'$ bosons decaying into $W^+W^-$ pairs, improving previous constraints and providing predictions for future searches.

Contribution

It provides updated constraints on $Z'$ boson parameters using diboson resonance searches at 13 TeV LHC data, combining these with Drell-Yan results for extended exclusion regions.

Findings

Constraints on $Z$-$Z'$ mixing angle are of order a few $ imes 10^{-4}$.

Limits on $Z'$ mass and mixing are significantly improved over previous electroweak data analyses.

Predictions for future Run II data analysis are provided.

Abstract

New neutral vector bosons $Z'$ decaying to charged gauge boson pairs $W^+W^-$ are predicted in many scenarios of new physics, including models with an extended gauge sector such as $E_6$, left-right symmetric $Z^\prime_{\rm LRS}$ and the sequential standard model $Z'_{\rm SSM}$. For these benchmark models we calculate and present theoretical expectations for different values of the $Z^\prime$ mass $M_2$ and mixing parameter $\xi$. Our results are based on the narrow width approximation which allows to make a convenient comparison of experiment to theoretical benchmark models. The diboson production allows to place stringent constraints on the $Z$-$Z'$ mixing angle and the $Z'$ mass, which we determine by using data from $pp$ collisions at $\sqrt{s}=13$ TeV recorded by the ATLAS detector at the CERN LHC, with integrated luminosity of $\sim$ 36 fb$^{-1}$. By comparing the experimental limits to the theoretical predictions for the total cross section of $Z'$ resonant production and its subsequent decay into $W^+W^-$ pairs, we show that the derived constraints on the mixing angle for the benchmark models are of the order of a few $\times 10^{-4}$, i.e., greatly improved with respect to those derived from the global analysis of electroweak data. We combine the limits derived from diboson production data with those obtained from the Drell--Yan process in order to significantly extend the exclusion region in the $M_{2}$-$\xi$ parameter plane. Also, we demonstrate that further improvement on the constraining of this mixing can be achieved through analysis of the full set of Run~II data.