Limits on the production of the Standard Model Higgs Boson in pp collisions at sqrt(s) =7 TeV with the ATLAS detector

TL;DR

This ATLAS study combines multiple Higgs decay channels using 2010 LHC data at 7 TeV to search for the Standard Model Higgs across 110–600 GeV. It employs a profile-likelihood framework with power-constrained limits and also tests a heavy fourth-generation scenario, using extensive MC simulations and data-driven background estimates across γγ, WW, and ZZ channels. No significant excess is observed; the paper sets cross-section limits relative to the SM and excludes a fourth generation Higgs in the 140–185 GeV range, providing the strongest limits for mh above ~250 GeV with the available data. The analysis demonstrates the power of combining diverse channels to maximize sensitivity with limited luminosity early in LHC operations.

Abstract

A search for the Standard Model Higgs boson at the Large Hadron Collider (LHC) running at a centre-of-mass energy of 7 TeV is reported, based on a total integrated luminosity of up to 40 pb^-1 collected by the ATLAS detector in 2010. Several Higgs boson decay channels: H -> γγ, H -> ZZ (*) -> llll, H -> ZZ -> llνν, H -> ZZ -> llqq, H -> W W (*) -> lνlν and H -> W W -> lνqq (l is e, μ) are combined in a mass range from 110 GeV to 600 GeV. The highest sensitivity is achieved in the mass range between 160 GeV and 170 GeV, where the expected 95% CL exclusion sensitivity is at Higgs boson production cross sections 2.3 times the Standard Model prediction. Upper limits on the cross section for its production are determined. Models with a fourth generation of heavy leptons and quarks with Standard Model-like couplings to the Higgs boson are also investigated and are excluded for a Higgs boson mass in the range from 140 GeV to 185 GeV.

Figures (12)

• Figure 1: The cross section multiplied by decay branching ratios for Standard Model Higgs boson production in $pp$ collisions at a 7 TeV centre-of-mass energy as a function of massLHCHiggsCrossSectionWorkingGroup:2011ti. All production modes are summed, and only final states considered in this paper are shown. Two bands are shown for each curve; the inner represents the QCD scale uncertainty and the outer also includes the $\alpha_s$ and PDF uncertainty.
• Figure 2: Distribution of the di-photon invariant mass for the 99 events from data passing all event selection criteria in the $H \rightarrow \gamma\gamma$ search and for the Monte Carlo prediction. The overall uncertainty on the expected total yield is illustrated by the yellow band. The uncertainty due to the reducible background is also shown (dark yellow band). The predictions for the main components of the background (di-photon, photon-jet, jet-jet and Drell-Yan) are also illustrated.
• Figure 3: Distributions of the transverse mass $m_{\mathrm{T}}$ in the 0-jet channel (a) and 1-jet channel (b) for the $H \rightarrow WW^{(*)} \rightarrow \ell\nu\ell\nu$ search after all selections except the transverse mass cut for the combined $e\mu$, $ee$ and $\mu\mu$ channels. The error bars reflect Poisson asymmetric errors. A Higgs boson signal is shown for $m_H=170$ GeV. The selections applied for $m_H=170$ GeV are indicated by the two vertical dotted lines.
• Figure 4: Distributions of the invariant mass $m_{\ell\nu qq}$ for the $H \rightarrow WW \rightarrow \ell\nu qq$ search after the application of all selection criteria and the W-mass constrained fit. The background fit is shown as a continuous line. In (a) no extra jets are allowed and in (b) one additional jet is required. The Higgs boson signal is shown for $m_H=400$ GeV and the expected yield is scaled up by a factor of 30 for illustration purposes.
• Figure 5: Distribution of $m_{\ell\ell\ell\ell}$ in the $H \rightarrow ZZ^{(*)}\rightarrow \ell\ell\ell\ell$ search before applying the lepton impact parameter and isolation requirements which remove the two candidates. The error bars reflect Poisson asymmetric errors.