Boosting the $H\to$ invisibles searches with $Z$ boson polarization

TL;DR

This paper demonstrates that analyzing Z boson polarization in associated Higgs production can significantly improve the sensitivity of H to invisible decays searches at the LHC, leading to about 20% better bounds.

Contribution

It introduces a detailed polarization analysis of the Z boson to enhance the discrimination between signal and background in H to invisible searches.

Findings

20% improvement in the upper bound for Higgs invisible branching ratio

Complete set of angular coefficients $A_i$ in the Collins-Soper frame obtained

Realistic Monte Carlo study confirms polarization analysis effectiveness

Abstract

It is argued that, in $H \to $ invisibles searches with $Z(\ell\ell)H$ associated production at the LHC, the signal efficiency can be sensibly improved via a detailed study of the $Z$ boson polarization, discriminating between the signal and the dominant-irreducible $Z(\ell\ell)Z(\nu\nu)$ background. We first present a comprehensive polarization study, obtaining the complete set of angular coefficients $A_i$ in the Collins-Soper reference frame and identifying the dominant phenomenological effects. Then, we show the results for a realistic Monte Carlo study to $H\to$ invisibles, taking the polarization analysis into account. We obtain about $20\%$ improvement in the upper bound for the branching ratio of the Higgs boson to invisible particles, assuming $300\ \mathrm{fb}^{-1}$ of data at the 13 TeV LHC.