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

It is argued that, in $H\to \text{invisibles}$ 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 $H\to \text{invisibles}$, 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.

Figure 1

Angular coefficients, $A_4$ for $ZH$ (red solid), $A_2$ for $ZZ$ (blue dashed) and $A_4$ for $ZZ$ (blue dotted), calculated in binned $p_{\mathrm{T}\ell \ell }$ regions at the LO accuracy.

Figure 2

Ratio of the normalized $(\cos \theta ,\varphi )$ distribution for the $ZH$ process to that for the $ZZ$ background process at the LO.

Figure 3

Normalized distributions of the softer lepton $p_{\mathrm{T}\ell }$ (left), $\mathrm{\Delta }{y}_{\ell \ell }$ (middle) and $\cos \mathrm{\Delta }{\varphi }_{\ell \ell }$ (right), for $ZH$ (red solid curves) and $ZZ$ (blue $\times$) at the LO, imposing the selection in Eq. (2).

Figure 4

Signal (red) and background (blue/green) ${\cancel{E}}_T$ (left) and $\mathrm{\Delta }{\varphi }_{\ell \ell }$ (right) distributions. The background (signal) histograms are (non)stacked. We apply all the selections described in the text but ${\cancel{E}}_T>200\mathrm{GeV}$ and $\mathrm{\Delta }{\varphi }_{\ell \ell }<\pi /2$ (left panel) and $\mathrm{\Delta }{\varphi }_{\ell \ell }<\pi /2$ (right panel). We consider the $\sqrt{s}=13\mathrm{TeV}$ LHC.

Figure 5

Expected 95% CL upper bound on $\sigma \times \mathcal{B}\mathcal{R}(H\to \mathrm{inv})/{\sigma }_{\mathrm{SM}}$, as a function of the 13 TeV LHC luminosity, based on the rate analysis (blue) and on our polarization analysis (red). While for the polarization analysis we perform a binned log-likelihood study exploring the $(\theta ,\varphi )$ distribution, the rate analysis only accounts for the rate information. The cut-flow and the samples used in these two analysis hypotheses are the same.