Search for the Standard Model Higgs boson produced in association with a vector boson and decaying to a b-quark pair with the ATLAS detector

TL;DR

This ATLAS Letter addresses the challenge of detecting the SM Higgs boson in the H→bb decay by exploiting Higgs production in association with a vector boson (VH) across three leptonic channels. The analysis uses data from 7 TeV collisions corresponding to about 4.6–4.7 fb^-1, and performs a binned fit to the bb̄ invariant mass in boosted kinematic regions to suppress backgrounds, with extensive data-driven background constraints. No significant excess is observed, and the paper sets 95% CL upper limits on the VH(H→bb) production rate relative to the SM prediction, ranging from 2.5 to 5.5 times the SM cross section over 110–130 GeV. The results provide important early constraints on Higgs production in the bb̄ final state and inform subsequent searches at higher energies and luminosities.

Abstract

This Letter presents the results of a direct search with the ATLAS detector at the LHC for a Standard Model Higgs boson of mass 110 < m(H) < 130 GeV produced in association with a W or Z boson and decaying to bb. Three decay channels are considered: ZH->llbb, WH->lvbb, and ZH->vvbb, where l corresponds to an electron or a muon. No evidence for Higgs boson production is observed in a dataset of 7 TeV pp collisions corresponding to 4.7/fb of integrated luminosity collected by ATLAS in 2011. Exclusion limits on Higgs boson production, at the 95% confidence level, of 2.5 to 5.5 times the Standard Model cross section are obtained in the mass range 110 - 130 GeV. The expected exclusion limits range between 2.5 and 4.9 for the same mass interval.

Figures (5)

• Figure 1: (a) The dilepton invariant mass distribution in the $ZH \to \ell^+\ell^- b\bar{b}$ channel, (b) the missing transverse energy without the $m_\mathrm{T}$ requirement in the $WH \to \ell\nu b\bar{b}$ channel, (c) the azimuthal angle separation between $E_{\mathrm{T}}^{\mathrm{miss}}$ and $p_{\mathrm{T}}^{\mathrm{miss}}$ and (d) the minimum azimuthal separation between $E_{\mathrm{T}}^{\mathrm{miss}}$ and any jet in the $ZH \to \nu\bar{\nu} b\bar{b}$ channel. All distributions are shown for events containing two $b$-tagged jets. The various Monte Carlo background distributions are normalized to data sidebands and control distributions and the multi-jet background is entirely estimated from data as described in the text. The vertical dashed lines correspond to the values of the cuts applied in each analysis, and the horizontal arrows indicate the events selected by each cut.
• Figure 2: The invariant mass $m_{b\bar{b}}$ for $ZH \to \ell^+\ell^- b\bar{b}$ shown for the different $p_{\mathrm{T}}^{Z}$ bins: (a) $0 < p_{\mathrm{T}}^{Z} < 50\mathrm{\ Ge V} \textrm{Ge V}$, (b) $50 \leq p_{\mathrm{T}}^{Z} < 100\mathrm{\ Ge V} \textrm{Ge V}$, (c) $100 \leq p_{\mathrm{T}}^{Z} < 200\mathrm{\ Ge V} \textrm{Ge V}$, (d) $p_{\mathrm{T}}^{Z} \geq 200\mathrm{\ Ge V} \textrm{Ge V}$ and (e) for the combination of all $p_{\mathrm{T}}^{Z}$ bins. The signal distributions are shown for $m_H = 120\mathrm{\ Ge V} \textrm{Ge V}$ and are enhanced by a factor of five for visibility. The shaded area indicates the total uncertainty on the background prediction. For better visibility, the signal histogram is stacked onto the total background, unlike the various background components which are simply overlaid in the figure.
• Figure 3: The invariant mass $m_{b\bar{b}}$ for $WH \to \ell\nu b\bar{b}$ shown for the different $p_{\mathrm{T}}^{W}$ bins: (a) $0 < p_{\mathrm{T}}^{W} < 50\mathrm{\ Ge V} \textrm{Ge V}$, (b) $50 \leq p_{\mathrm{T}}^{W} < 100\mathrm{\ Ge V} \textrm{Ge V}$, (c) $100 \leq p_{\mathrm{T}}^{W} < 200\mathrm{\ Ge V} \textrm{Ge V}$, (d) $p_{\mathrm{T}}^{W} \geq 200\mathrm{\ Ge V} \textrm{Ge V}$ and (e) for the combination of all $p_{\mathrm{T}}^{W}$ bins. The signal distributions are shown for $m_H = 120\mathrm{\ Ge V} \textrm{Ge V}$ and are enhanced by a factor of five for visibility. The shaded area indicates the total uncertainty on the background prediction. For better visibility, the signal histogram is stacked onto the total background, unlike the various background components which are simply overlaid in the figure.
• Figure 4: The invariant mass $m_{b\bar{b}}$ for $ZH \to \nu\bar{\nu} b\bar{b}$ shown for the different $p_{\mathrm{T}}^{Z}$ bins: (a) $120 < p_T^Z < 160\mathrm{\ Ge V} \textrm{Ge V}$, (b) $160 \leq p_T^Z < 200\mathrm{\ Ge V} \textrm{Ge V}$, (c) $p_T^Z \geq 200\mathrm{\ Ge V} \textrm{Ge V}$ and (d) for the combination of all $p_T^Z$ bins. The signal distributions are shown for $m_H = 120\mathrm{\ Ge V} \textrm{Ge V}$ and are enhanced by a factor of five for visibility. The shaded area indicates the total uncertainty on the background prediction. For better visibility, the signal histogram is stacked onto the total background, unlike the various background components which are simply overlaid in the figure.
• Figure 5: Expected (dashed) and observed (solid line) exclusion limits for (a) the $ZH \to \ell^+\ell^- b\bar{b}$ , (b) $WH \to \ell\nu b\bar{b}$ and (c) $ZH \to \nu\bar{\nu} b\bar{b}$ channels expressed as the ratio to the SM Higgs boson cross section, using the profile-likelihood method with $CL_s$. The dark (green) and light (yellow) areas represent the 1$\sigma$ and 2$\sigma$ ranges of the expectation in the absence of a signal. (d) shows the 95% confidence level exclusion limits obtained from the combination of the three channels.