Combined search for the Standard Model Higgs boson in pp collisions at sqrt(s) = 7 TeV with the ATLAS detector

TL;DR

ATLAS performs a comprehensive, multi-channel Higgs boson search using 7 TeV pp data from 2011, combining decay channels H→γγ, ZZ, WW, ττ, and bb across ggF, VBF, and VH production. The analysis employs a unbinned extended likelihood with correlated nuisance parameters, constrained by auxiliary measurements and robust treatment of systematics, to set CLs exclusions over mH from ~110 to 600 GeV. The results exclude large mass regions at 95% CL and reveal a local excess near 126 GeV with a significance of about 2.9σ (global ~15% probability across the scanned mass range), consistent with a SM Higgs signal. This work demonstrates the power of a statistically rigorous, cross-channel combination and lays the groundwork for the eventual Higgs discovery by corroborating a SM-like Higgs around 126 GeV with multiple decay channels and production modes.

Abstract

A combined search for the Standard Model Higgs boson with the ATLAS detector at the LHC is presented. The datasets used correspond to integrated luminosities from 4.6 fb^-1 to 4.9 fb^-1 of proton-proton collisions collected at sqrt(s) = 7 TeV in 2011. The Higgs boson mass ranges of 111.4 GeV to 116.6 GeV, 119.4 GeV to 122.1 GeV, and 129.2 GeV to 541 GeV are excluded at the 95% confidence level, while the range 120 GeV to 560 GeV is expected to be excluded in the absence of a signal. An excess of events is observed at Higgs boson mass hypotheses around 126 GeV with a local significance of 2.9 standard deviations (sigma). The global probability for the background to produce an excess at least as significant anywhere in the entire explored Higgs boson mass range of 110-600 GeV is estimated to be ~15%, corresponding to a significance of approximately one sigma.

Figures (10)

• Figure 1: Invariant or transverse mass distributions for the selected candidate events, the total background and the signal expected in the following channels: (a) $H \rightarrow \gamma\gamma$, (b) $H \rightarrow ZZ^{(*)}\rightarrow \ell^+\ell^-\ell^+\ell^-$ in the entire mass range, (c) $H \rightarrow ZZ^{(*)}\rightarrow \ell^+\ell^-\ell^+\ell^-$ in the low mass range, (d) $H \rightarrow ZZ\rightarrow \ell^+\ell^-\nu\overline{\nu}$, (e) b-tagged selection and (f) untagged selection for $H \rightarrow ZZ\rightarrow \ell^+\ell^- q\overline{q}$, (g) $H \rightarrow WW^{(*)} \rightarrow \ell^+\nu\ell^-\overline{\nu}$+0-jets, (h) $H \rightarrow WW^{(*)} \rightarrow \ell^+\nu\ell^-\overline{\nu}$+1-jet, (i) $H \rightarrow WW^{(*)} \rightarrow \ell^+\nu\ell^-\overline{\nu}$+2-jets, (j) $H \rightarrow WW \rightarrow \ell\nu q\overline{q}'$+0-jets, (k) $H \rightarrow WW \rightarrow \ell\nu q\overline{q}'$+1-jet and (l) $H \rightarrow WW \rightarrow \ell\nu q\overline{q}'$+2-jets. The $H \rightarrow WW^{(*)} \rightarrow \ell^+\nu\ell^-\overline{\nu}$+2-jets distribution is shown before the final selection requirements are applied.
• Figure 2: Invariant or transverse mass distributions for the selected candidate events, the total background and the signal expected in the following channels: (a) $H\rightarrow \tau_{\rm lep}\tau_{\rm lep}$+0-jets, (b) $H\rightarrow \tau_{\rm lep}\tau_{\rm lep}$ 1-jet, (c) $H\rightarrow \tau_{\rm lep}\tau_{\rm lep}$+2-jets, (d) $H \rightarrow \tau_{\rm lep}\tau_{\rm had}$+0-jets and 1-jet, (e) $H \rightarrow \tau_{\rm lep}\tau_{\rm had}$+2-jets, (f) $H \rightarrow \tau_{\rm had} \tau_{\rm had}$. The $b\overline{b}$ invariant mass for (g) the $ZH \rightarrow \ell^+\ell^- b\bar{b}$, (h) the $WH \rightarrow \ell\nu b\bar{b}$ and (i) the $ZH \rightarrow \nu\overline{\nu} b\bar{b}$ channels. The vertical dashed lines illustrate the separation between the mass spectra of the subcategories in $p_{\rm T}^Z$, $p_{\rm T}^W$, and $E_{\rm T}^{\rm miss}$, respectively. The signal distributions are lightly shaded where they have been scaled by a factor of five or ten for illustration purposes.
• Figure 3: The observed (solid) and expected (dashed) 95% CL cross section upper limits for the individual search channels and the combination, normalized to the SM Higgs boson production cross section, as a function of the Higgs boson mass; (a) for the full Higgs boson mass hypotheses range and (b) in the low mass range. The expected limits are those for the background-only hypothesis i.e. in the absence of a Higgs boson signal.
• Figure 4: The observed (full line) and expected (dashed line) 95% CL combined upper limits on the SM Higgs boson production cross section divided by the SM expectation as a function of $m_{H}$, (a) in the full mass range considered in this analysis and (b) in the low mass range. The dotted curves show the median expected limit in the absence of a signal and the green and yellow bands indicate the corresponding $\pm 1\sigma$ and $\pm 2\sigma$ intervals.
• Figure 5: The value of the combined $CL_s$ for $\mu=1$ (testing the SM Higgs boson hypothesis) as a function of $m_{H}$, (a) in the full mass range of this analysis and (b) in the low mass range. The regions with $CL_s < \alpha$ are excluded at the ($1-\alpha$) CL.