Constraints on the off-shell Higgs boson signal strength in the high-mass $ZZ$ and $WW$ final states with the ATLAS detector

TL;DR

The paper exploits the high-mass, off-shell region of Higgs decays to ZZ and WW to constrain the off-shell signal strength μ_off-shell and, under model assumptions, the Higgs total width Γ_H. It combines three channels (ZZ→4ℓ, ZZ→2ℓ2ν, WW→eνμν) with state-of-the-art simulations of signal, interference, and backgrounds, accounting for large theoretical uncertainties in gg-initiated processes and unknown gg→VV K-factors. A matrix-element discriminant in ZZ→4ℓ, plus transverse-mass-based methods in ZZ→2ℓ2ν and WW→eνμν, feed into profile likelihood fits and CLs limits, which are then combined with on-shell measurements to constrain Γ_H/Γ_H^SM and R_gg. The results yield upper limits on off-shell Higgs yields and, assuming SM-like energy dependence of couplings, translate into a Higgs width of order tens of MeV, consistent with SM expectations and providing a complementary indirect width constraint. Overall, the work demonstrates the power of off-shell Higgs analyses, including interference effects, to probe Higgs properties beyond on-shell measurements.

Abstract

Measurements of the $ZZ$ and $WW$ final states in the mass range above the $2m_Z$ and $2m_W$ thresholds provide a unique opportunity to measure the off-shell coupling strength of the Higgs boson. This paper presents constraints on the off-shell Higgs boson event yields normalised to the Standard Model prediction (signal strength) in the $ZZ \to 4\ell$, $ZZ\to2\ell2ν$ and $WW\to eνμν$ final states. The result is based on $pp$ collision data collected by the ATLAS experiment at the LHC, corresponding to an integrated luminosity of $20.3$ fb$^\text{-1}$ at a collision energy of $\sqrt{s}$=8 TeV. Using the $CL_s$ method, the observed 95% confidence level (CL) upper limit on the off-shell signal strength is in the range 5.1-8.6, with an expected range of 6.7-11.0. In each case the range is determined by varying the unknown $gg\to ZZ$ and $gg\to WW$ background K-factor from higher-order QCD corrections between half and twice the value of the known signal K-factor. Assuming the relevant Higgs boson couplings are independent of the energy scale of the Higgs production, a combination with the on-shell measurements yields an observed (expected) 95\% CL upper limit on $Γ_H/Γ_H^{\mathrm{SM}}$ in the range 4.5- -7.5 (6.5- -11.2) using the same variations of the background K-factor. Assuming that the unknown $gg\rightarrow VV$ background K-factor is equal to the signal K-factor,this translates into an observed (expected) 95% CL upper limit on the Higgs boson total width of 22.7 (33.0) MeV.

Figures (12)

• Figure 1: The leading-order Feynman diagrams for \ref{['fig:feynman_diagram_ggHVV']} the $gg \to H^* \to VV$ signal, \ref{['fig:feynman_diagram_ggVV']} the continuum $gg \to VV$ background and \ref{['fig:feynman_diagram_qqVV']} the $q \bar{q} \to VV$ background.
• Figure 2: \ref{['fig:m4l_ggzz4l_lhelevel:m4l']} Differential cross-sections as a function of the four-lepton invariant mass $m_{4\ell}$ in the range of 100 $<~m_{4\ell} <$ 1000 for the $gg\to (H^*\to) ZZ\to 2e2\mu$ channel at the parton level, for the $gg\to H^*\to ZZ$ signal (solid line), $gg\to ZZ$ continuum background (dots), $gg\to (H^*\to) ZZ$ with SM Higgs boson coupling (long-dashed line, including signal plus background plus interference) and $gg\to (H^* \to) ZZ$ with $\mu_\text{off-shell}=10$ (dashed line). \ref{['fig:m4l_ggzz4l_lhelevel:interference']} Differential cross-section as a function of $m_{4\ell}$ in the range of 130 $< m_{4\ell} <$ 1000 for the SM $gg\to H^*\to ZZ\to 2e2\mu$ signal (solid line) and its interference with the $gg\to ZZ\to 2e2\mu$ continuum background (dashed line).
• Figure 3: Observed distributions for \ref{['fig:m4l_data_and_exp:m4l']} the four-lepton invariant mass $m_{4\ell}$ in the range of 220 $< m_{4\ell} <$ 1000 and \ref{['fig:m4l_data_and_exp:ME']} the ME-based discriminant combining all lepton final states for the ME-based analysis signal region, compared to the expected contributions from the SM including the Higgs boson (stack). The dashed line corresponds to the total expected event yield, including all backgrounds and the Higgs boson with $\mu_\text{off-shell}=10$. A relative $gg\to ZZ$ background K-factor of $\text{R}^B_{H^*}$=1.0 is assumed. The $Z$+jets and top-quark backgrounds are barely visible in the plot since they are very small (<1% of the total background).
• Figure 4: Observed distribution of the $ZZ$ transverse mass $m_{\mathrm{T}}^{ZZ}$ in the range 380 $< m_{\mathrm{T}}^{ZZ} <$ 1000 combining the $2e2\nu$ and $2\mu2\nu$ channels, compared to the expected contributions from the SM including the Higgs boson (stack). The first bin only contains events in the range 380 $< m_{\mathrm{T}}^{ZZ} <$ 400 . The hatched area shows the combined statistical and systematic uncertainties. The dashed line corresponds to the total expected event yield, including all backgrounds and the Higgs boson with $\mu_\text{off-shell}=10$. A relative $gg\to ZZ$ background K-factor of $\text{R}^B_{H^*}$=1 is assumed.
• Figure 5: Observed distributions of R$_8$, constructed from the dilepton invariant mass and transverse mass, Eq. \ref{['eq:r8']}, in the $WW\rightarrow e\nu\mu\nu$ channel for \ref{['fig:hww_topcr']} the top control region, \ref{['fig:hww_wwcr']}$WW$ control region (the CRs start at 160 ), and \ref{['fig:hww_r8']} the signal region for R$_8$ above 450 , compared to the expected contributions from the SM including the Higgs boson (stack). The dashed line corresponds to the total expected event yield, including all backgrounds and the Higgs boson with $\mu_\text{off-shell}=10$. The last bin in \ref{['fig:hww_topcr']} and \ref{['fig:hww_r8']} includes the overflow. A relative $gg\rightarrow WW$ background K-factor of R$^B_{H^*}=1$ is assumed. The top-quark and $WW$ backgrounds are normalised to data as described in Sect. \ref{['sec:hww_event_selection']}. The stacking order follows the legend in each plot.