Bounding the Higgs width at the LHC: complementary results from $H \to WW$

TL;DR

Direct measurement of the Higgs width $Γ_H$ at the LHC is challenging due to the narrow SM width and detector resolution. The authors extend the off-shell width constraint approach, previously proposed for $H \to ZZ$, to the $H \to WW$ channel by analyzing the high-$M_T$ tail and including Higgs–continuum interference, providing width-dependent cross sections at $8$ and $13$ TeV. Using ATLAS 8 TeV data in the $N_{jet}=0$ category with various cuts, they derive width bounds ranging from $Γ_H < 365^{+118}_{-79} Γ_H^{SM}$ (full cuts) to $Γ_H < 45^{+9}_{-7} Γ_H^{SM}$ (with $M_T>300$ GeV) under plausible systematics; the bounds are weaker than the $H \to ZZ$ channel but can become competitive with improved control of systematics. The study demonstrates the potential for WW-based width constraints and motivates a dedicated high-$M_T$ analysis to maximize sensitivity and complement existing ZZ results.

Abstract

We investigate the potential of the process $gg \to H \to WW$ to provide bounds on the Higgs width. Recent studies using off-shell $H\rightarrow ZZ$ events have shown that Run 1 LHC data can constrain the Higgs width, $Γ_H < (25-45) Γ_{H}^{\rm SM}$. Using 20 fb-1 of 8 TeV ATLAS data, we estimate a bound on the Higgs boson width from the WW channel between $Γ_H < (100-500) Γ_H^{SM}$. The large spread in limits is due to the range of cuts applied in the existing experimental analysis. The stricter cuts designed to search for the on-shell Higgs boson limit the potential number of off-shell events, weakening the constraints. As some of the cuts are lifted the bounds improve. We show that there is potential in the high transverse mass region to produce upper bounds of the order of $(25-50) Γ_H^{SM}$, depending strongly on the level of systematic uncertainty that can be obtained. Thus, if these systematics can be controlled, a constraint on the Higgs boson width from the $H \to WW$ decay mode can complement a corresponding limit from $H \to ZZ$.

Figures (5)

• Figure 1: Overall picture at 8 TeV (colour online), with the full ATLAS cuts described in the text imposed.
• Figure 2: Overall picture at 8 TeV (colour online), under the basic ATLAS cuts, i.e. when neither the $m_{\ell\ell}$ nor the $\Delta\phi_{\ell\ell}$ cut in Eq. (\ref{['eq:mphicuts']}) has been applied.
• Figure 3: Overall picture at 8 TeV (colour online). In this figure the ATLAS cuts described in the text have been imposed, including the $\Delta\phi_{\ell\ell}$ cut in Eq. (\ref{['eq:mphicuts']}) but removing the constraint on the maximum invariant mass of the dilepton pair.
• Figure 4: Limits on the Higgs width obtained using the results reported by ATLAS. The solid line represents the limit obtained using an estimate of the systematic uncertainty obtained from the results presented in ref. ATLAS:2013wla ($\delta_B=7.4$%). The dashed and dotted lines represent limits obtained using the more conservative choices $\delta_B=10$% (dashed) and $\delta_B=12$% (dotted).
• Figure 5: Estimated limits on the Higgs width obtained using a cut on the transverse mass, $M_T > 300$ GeV. The limits are computed using an estimate of the expected background cross section, as described in the text, and for different projected experimental uncertainties. The number of signal events is also estimated from the ATLAS 20 fb$^{-1}$ expectation, with the corresponding theoretical uncertainty represented by the dashed curves.