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Combined results of searches for the standard model Higgs boson in pp collisions at sqrt(s) = 7 TeV

CMS Collaboration

arXiv:1202.1488v1hep-ex

TL;DR

Whether the SM Higgs boson exists remained unresolved after earlier searches yielded null results. This paper combines searches across five decay modes and excludes the boson at 95% CL from 127–600 GeV, while observing a 3.1σ local excess at 124 GeV.

  • Problem

    The search addresses whether the SM Higgs boson exists, a key prediction of the proposed mechanism for electroweak symmetry breaking.

    Source: §1 · p. 3

  • Method

    The paper combines CMS searches in five Higgs decay modes in pp collisions at 7 TeV, using 4.6–4.8 fb−1 over 110–600 GeV.

    Source: §1 · p. 3, §5 · p. 13

  • Results

    127–600 GeV at 95% CL and 129–525 GeV at 99% CL are excluded; the 124 GeV excess has 3.1σ local significance.

    Source: §4 · pp. 9, 11

  • Takeaways & Limitations

    The excess has global significance of 1.5σ across 110–600 GeV and 2.1σ across 110–145 GeV.

    Source: §4 · p. 11

    Production cross sections and decay branching fractions, including their uncertainties, are taken from Ref. and derived from Refs. [18–59].

    Source: §1 · p. 3

Abstract

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Combined results are reported from searches for the standard model Higgs boson in proton-proton collisions at sqrt(s)=7 TeV in five Higgs boson decay modes: gamma pair, b-quark pair, tau lepton pair, W pair, and Z pair. The explored Higgs boson mass range is 110-600 GeV. The analysed data correspond to an integrated luminosity of 4.6-4.8 inverse femtobarns. The expected excluded mass range in the absence of the standard model Higgs boson is 118-543 GeV at 95% CL. The observed results exclude the standard model Higgs boson in the mass range 127-600 GeV at 95% CL, and in the mass range 129-525 GeV at 99% CL. An excess of events above the expected standard model background is observed at the low end of the explored mass range making the observed limits weaker than expected in the absence of a signal. The largest excess, with a local significance of 3.1 sigma, is observed for a Higgs boson mass hypothesis of 124 GeV. The global significance of observing an excess with a local significance greater than 3.1 sigma anywhere in the search range 110-600 (110-145) GeV is estimated to be 1.5 sigma (2.1 sigma). More data are required to ascertain the origin of this excess.

1 Introduction

This introduction presents a CMS combination of Higgs boson searches in 7 TeV proton-proton collisions, using 2010–2011 data and covering Higgs masses from 110 to 600 GeV. It outlines the decay channels, production mechanisms, and mass-dependent sensitivity that shape the searches.

1 Introduction

  • Previous searches had found no Higgs signal, with direct-search exclusions and precision electroweak measurements constraining the allowed mass range.The passage reports LEP mH > 114.4 GeV, Tevatron mH ∉ (162–166) GeV [11], ATLAS exclusions at 145–206, 214–224, and 340–450 GeV [12] [13] [14], and an indirect upper constraint of 158 GeV.Source: §1 · p. 3
  • CMS combines Higgs boson searches in 7 TeV proton-proton collisions, using 4.6–4.8 fb−1 of 2010–2011 data across the 110–600 GeV mass range.Source: §1 · p. 3
  • 20–0.3 pb: the total Higgs production cross section varies across the explored mass range, with gluon-gluon fusion largest, followed by VBF, WH/ZH, and ttH.Production cross sections and branching fractions, including their uncertainties, are taken from Ref. and derived from Refs. [18–59].Source: §1 · p. 3
  • The combination uses five decay modes, with γγ, ττ, and bb searches limited to masses below 150 GeV, while WW and ZZ searches span the full range.The ZZ channels include 4ℓ, 2ℓ2ν, 2ℓ2q, and 2ℓ2τ final states; WW is followed by two lepton-neutrino decays.Source: §1 · p. 3
  • Sensitivity is best in γγ below 120 GeV, WW from 120–200 GeV, and ZZ→4ℓ or ZZ→2ℓ2ν at higher masses.The γγ and ZZ→4ℓ channels provide especially good mass resolution; low-mass bb and ττ sensitivity is reduced by large backgrounds.Source: §1 · pp. 3–4

2 Search channels

Eight Higgs boson searches are combined, spanning distinct decay signatures and event categories; Fig. 1 compares their expected exclusion sensitivity across the explored mass ranges.

H →γγ

  • The H →γγ search seeks a narrow diphoton-mass peak, separating VBF-like events from others and subdividing the latter by photon location and shower compactness.The background is estimated by fitting the observed diphoton mass distribution in data.Source: §2 · p. 4

H →ττ

  • The H →ττ search uses eµ, eτh, and µτh signatures, each split into VBF, one-high-ET-jet, and low/no-jet categories to seek a broad mass excess.The irreducible Z →ττ mass distribution is derived using Z →µµ data with reconstructed muons replaced by simulated τ-decay particles; reducible backgrounds use data control samples.Source: §2 · pp. 4–5

Combined search channels

  • Eight searches are combined; Fig. 1 shows median expected 95% CL σ/σSM limits, with values below unity indicating expected exclusion, while Table 1 summarizes search characteristics.The figure covers mass ranges of 110–600 and 110–145 GeV; jagged limits reflect differing selection criteria across mass sub-ranges.Source: §2 · p. 4, Table 1 · p. 4, Figure 1 · p. 5

H →bb

  • The H →bb search targets W/Z-associated production, uses b-tagged dijets and an MVA classifier, and estimates major W/Z+jets and top backgrounds from data control samples.W →eν/µν and Z →ee/µµ/νν decays are considered; WZ, ZZ with Z →bb, and single-top backgrounds are estimated from simulation.Source: §2 · p. 5

H →WW(∗) →2ℓ2ν

  • The H →WW(∗) →2ℓ2ν search categorizes opposite-charge dilepton events by jet count and flavour, using mass-trained MVA classifiers and mostly data-derived background rates.The two-jet category exploits VBF topology; non-resonant WW dominates zero-jet backgrounds, while top production is important with one or two jets.Source: §2 · pp. 5–6

H →ZZ(∗) →4ℓ

  • The H →ZZ(∗) →4ℓ search looks for a four-lepton mass peak, analyzing 4e, 4µ, and 2e2µ separately; simulated non-resonant ZZ and data-based reducible backgrounds are used.The subchannels differ in mass resolution and backgrounds from jets misidentified as leptons.Source: §2 · p. 6

H →ZZ →2ℓ2ν

  • The H →ZZ →2ℓ2ν search selects an on-shell dilepton Z plus large missing transverse energy and seeks a broad excess in transverse invariant mass.Non-resonant ZZ and WZ backgrounds come from simulation; other backgrounds are estimated from data control samples.Source: §2 · p. 6

H →ZZ channels

  • The H →ZZ(∗) →2ℓ2q and H →ZZ →2ℓ2τ searches use b-tagged dilepton-dijet states or dilepton-ditau final states to seek mass peaks or broad excesses, respectively.The 2ℓ2q analysis defines six states by jet b-tags and estimates background from data control regions; 2ℓ2τ defines eight final subchannels, with non-resonant ZZ from simulation and misidentified-τ backgrounds from data.Source: §2 · p. 6

3 Combination methodology

The analysis combines 43 exclusive search sub-channels while accounting for correlated statistical and systematic uncertainties, and repeats the combination across 183 Higgs boson mass hypotheses from 110–600 GeV.

3 Combination methodology

  • The combination uses event counts or binned or unbinned discriminant distributions from 43 sub-channels, incorporating 156–222 sources of correlated systematic uncertainty across 183 mass hypotheses.Uncertainties cover theoretical predictions, detector response, and statistical measurements; the mass scan uses 0.5 GeV steps at lower masses and 20 GeV steps at large masses.Source: §3 · pp. 6–7

3.1 General framework

  • A signal-strength modifier μ scales the expected Higgs yield, while nuisance parameters model systematic uncertainties and enter a likelihood combining channel data with constraint probabilities.Test-statistic distributions under background-only and signal-plus-background hypotheses are obtained from pseudo-datasets generated using likelihood-maximizing nuisance parameters.Source: §3.1 · p. 7

3.2 Quantifying an excess

  • The excess statistic q0 compares the best-fit signal-plus-background likelihood with the background-only likelihood; a positive q0 indicates a signal-like excess.The local p-value measures the background-fluctuation probability at a given mass, while the global probability concerns the maximum excess across a mass range and accounts for correlations between analyses.Source: §3.2 · pp. 7–8

3.3 Quantifying the absence of a signal

  • Exclusion limits use a one-sided test statistic qμ and the modified frequentist CLs construction to test a hypothesized Higgs signal rate.The SM Higgs boson is excluded at confidence level 1 − α when CLs ≤ α for μ = 1; the 95% upper limit on μ is set where CLs = 0.05.Source: §3.3 · pp. 8–9

4 Results

The combined searches exclude the standard model Higgs boson at 95% CL for masses 127–600 GeV and at 99% CL for 129–525 GeV. A low-mass excess weakens the observed limits, with the largest local excess at 124 GeV but lower global significance across the search ranges.

Exclusion limits

  • 127–600 GeV is excluded at 95% CL, and 129–525 GeV at 99% CL, for the standard model Higgs boson.The observed CLs values below the corresponding thresholds define these excluded ranges.Source: §4 · p. 9, Figure 2 · p. 10

Low-mass excesses

  • In 122–124 GeV, observed results lie above the signal-plus-background expectation, and CLs degenerates to unity for the background-only hypothesis.Separate low-mass combinations show excesses in both the γγ + ZZ →4ℓ channels and the bb + ττ + WW channels; the former is more localized, while the latter has little mass dependence.Source: §4 · pp. 9, 11, Figure 5 · p. 12

Expected and observed limits

  • 118–543 GeV is the median expected 95% CL exclusion range without a signal; low-mass excesses weaken observed limits, while high-mass limits are somewhat stronger than expected.Observed limits are generally within the expected 68% or 95% bands, consistent with statistical fluctuations.Source: §4 · p. 9, Figure 3 · p. 10

Decay-channel contributions

  • For masses beyond 200 GeV, H →ZZ drives the limits; H →WW largely defines them from 125–200 GeV, while H →γγ dominates sensitivity below 120 GeV.Figure 4 compares observed 95% CL upper limits across eight individual decay channels and their combination; these limits use the asymptotic CLs formula.Source: §4 · pp. 9, 11, Figure 4 · p. 11

Excess significance

  • 3.1σ is the local significance of the minimum p-value at approximately 124 GeV; global significances are 1.5σ over 110–600 GeV and 2.1σ over 110–145 GeV.The low-mass p-value scan shows a broad offset from channels with poor mass resolution and localized features at 119.5 GeV, associated with three ZZ →4ℓ events, and 124 GeV, arising mostly from γγ.Source: §4 · p. 11, Figure 6 · p. 12

Best-fit signal strength

  • 117–126 GeV is the range in which observed best-fit signal strengths are within 1σ of unity.The fit does not impose a nonnegative signal-strength constraint, so a negative best-fit value indicates an observation below the background-only expectation.Source: §4 · pp. 11, 13, Figure 6 · p. 12

5 Conclusions … Purdue University Calumet, Hammond, USA

Combined searches across five Higgs boson decay modes excluded the standard model Higgs boson over broad mass ranges, while a low-mass excess remained inconclusive and requires more data.

5 Conclusions

  • The searches combined proton-proton collision data at √s = 7 TeV in five decay modes: γγ, bb, ττ, WW, and ZZ.Source: §5 · p. 13
  • Observed exclusions span 127–600 GeV at 95% CL and 129–525 GeV at 99% CL.The observed 95% CL exclusion was weaker than expected because of an excess at the low end of the explored mass range.Source: §5 · p. 13
  • The expected excluded mass range was 118–543 GeV at 95% CL in the absence of the standard model Higgs boson.Source: §5 · p. 13
  • The largest excess had a local significance of 3.1σ at a Higgs boson mass hypothesis of 124 GeV.Source: §5 · p. 13
  • The global significance for an excess with local significance ≥3.1σ was 1.5σ over 110–600 GeV and 2.1σ over 110–145 GeV.More data are required to ascertain the origin of the excess.Source: §5 · p. 13
  • The explored Higgs boson mass range was 110–600 GeV, using data corresponding to an integrated luminosity of 4.6–4.8 fb−1.Source: §5 · p. 13
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