Inadequacy of zero-width approximation for a light Higgs boson signal

TL;DR

The paper investigates the validity of the zero-width approximation for a light Standard Model Higgs and demonstrates that off-shell contributions and Higgs–continuum interference can significantly affect Higgs signal normalisation, even when Gamma_H/M_H is tiny. It quantifies these effects in inclusive production as well as with realistic experimental cuts, showing that the high-mass tail above 2M_V can contribute roughly 10% corrections in gg->H->VV and induce sizable interference. By introducing ratios and analyzing multiple channels (including 4-lepton ZZ decays and WW channels with transverse-mass observables), the authors show that experimental cuts can suppress the problematic region but that off-shell effects must be accounted for in precision Higgs coupling measurements. The results underscore the need to move beyond the ZWA in future LHC analyses, particularly for accurate normalization and coupling determinations, and indicate that weak-boson-fusion H->VV channels are similarly impacted.

Abstract

In the Higgs search at the LHC, a light Higgs boson (115 GeV <~ M_H <~ 130 GeV) is not excluded by experimental data. In this mass range, the width of the Standard Model Higgs boson is more than four orders of magnitude smaller than its mass. The zero-width approximation is hence expected to be an excellent approximation. We show that this is not always the case. The inclusion of off-shell contributions is essential to obtain an accurate Higgs signal normalisation at the 1% precision level. For gg (-> H) -> VV, V= W,Z, O(10%) corrections occur due to an enhanced Higgs signal in the region M_VV > 2 M_V, where also sizable Higgs-continuum interference occurs. We discuss how experimental selection cuts can be used to exclude this region in search channels where the Higgs invariant mass cannot be reconstructed. We note that the H -> VV decay modes in weak boson fusion are similarly affected.

Figures (18)

• Figure 1: Representative Feynman graphs for the Higgs signal process (left) and the $q\bar{q}$- (center) and $gg$-initiated (right) continuum background processes at LO.
• Figure 2: The NNLO $ZZ$ (black) and $WW$ (red) invariant mass distributions in $gg \to VV$ for ${\mu_{H}} = 125{\,\text{GeV}}$.
• Figure 3: The LO $ZZ$ invariant mass distribution $gg \to ZZ$ for ${\mu_{H}} = 125{\,\text{GeV}}$. The black line is the total, the red line gives the signal while the cyan line gives signal plus background; the blue line includes the $q\bar{q} \to ZZ$ contribution.
• Figure 4: $M_{WW}$ distributions for $gg\ (\to H)\to W^-W^+\to \ell\bar{\nu}_\ell \bar{\ell}\nu_\ell$ in $pp$ collisions at $\sqrt{s} = 8$ TeV for $M_H=125$ GeV and $\Gamma_H = 0.004434$ GeV calculated at LO with gg2VV. The ZWA distribution (black, dashed) as defined in Eq. (\ref{['eq:ZWA_MVV']}) in the main text, the off-shell Higgs distribution (black, solid), the $d\sigma(|{\cal M}_H + {\cal M}_\text{cont}|^2)/dM_{WW}$ distribution (blue) and the $d\sigma(|{\cal M}_H|^2 + |{\cal M}_\text{cont}|^2)/dM_{WW}$ distribution (red) are shown. Standard cuts are applied: $p_{T\ell} > 20$ GeV, $|\eta_\ell| < 2.5$, $=\hbox{$p$} p \hbox{/} _T > 30$ GeV, $M_{\ell\ell} >$ 12 GeV. Differential cross sections for a single lepton flavour combination are displayed. No flavour summation is carried out for charged leptons or neutrinos.
• Figure 5: $M_{WW}$ distributions for $gg\ (\to H)\to W^-W^+\to \ell\bar{\nu}_\ell \bar{\ell}\nu_\ell$ for $M_H=125$ GeV. Interference effects in the region of the Higgs resonance and the $W$-pair threshold are shown. Details as in Fig. \ref{['fig:lvlv_MWW_l']}.