Discovery of the Higgs Boson

The following sections are included:

• Dedication
• Contents
• Preface

The major goal of the Large Hadron Collider is to probe the electroweak symmetry breaking mechanism and the generation of the elementary particle masses. In the Standard Model this mechanism leads to the existence of a scalar Higgs boson with unique properties. We review the physics of the Standard Model Higgs boson, discuss its main search channels at hadron colliders and the corresponding theoretical predictions. We also summarize the strategies to study its basic properties.

The Large Electron Positron (LEP) collider installed at CERN provided unprecedented possibilities for studying the properties of elementary particles during the years 1989-2000. The four detectors associated to the collider, run by the ALEPH, DELPHI, L3, and OPAL Collaborations, were based on the latest available technologies. The conjunction of high collision energies, precise instrumentation and data analysis techniques allowed the Standard Model (SM) of elementary particles to be tested at the level of quantum corrections. The search for new particles, in particular the long-sought Higgs boson, was one of the primary research subjects. During the twelve years of LEP, data samples of the highest quality and statistical weight were analysed. Concerning the search for the SM Higgs boson, the domain extending from zero mass to the kinematic limit imposed by the collider energy was scrutinised. The spirit of scientific competition gradually gave way to a collaborative effort, allowing the final results of LEP to be optimised. The methodology of Higgs boson searches is summarised in this paper together with the statistical methods adopted to combine the data of the four collaborations.

During Run II of the Tevatron collider, which took place from 2001 until 2011, the CDF and D0 detectors each collected approximately 10 fb ⁻¹ of collision data at a center-of-mass energy of . This dataset allowed for tests for the presence of the SM Higgs boson in the mass range 90-200 GeV in the production modes gg → H, W/ZH, vector-boson fusion, and H, with H decay modes H → , H → W⁺W⁻, H → τ⁺τ⁻, H → γγ, and H → ZZ. This chapter summarizes the search methods and the results of the Higgs boson search at the Tevatron. The increased sophistication of the analysis techniques as the collider run progressed is discussed, covering the strategies used over time to improve the sensitivity and breadth of the analyses. Using the full Tevatron data sample for both experiments, the combined Higgs search in all channels observes an excess consistent with the predicted SM Higgs boson signal with mass of 125 GeV, with a significance of 3.0 standard deviations above the background prediction.

The Large Hadron Collider is the most powerful particle accelerator ever built. It has allowed the discovery of a Higgs boson with mass near 125 GeV in 2012 by the ATLAS and CMS experiments. This chapter provides first an overview of the main characteristics of this collider, as well as a short description of the two general purpose experiments, ATLAS and CMS, which discovered in 2012 a Higgs boson with mass close to 125 GeV. This is followed by a summary of the main aspects of particle identification and reconstruction by these two detectors, together with a short presentation of the main analysis tools used to extract the LHC results of the Higgs boson(s) searches and measurements.

In their searches for the Standard Model (SM) Higgs boson in the mass range 115-1000 GeV, both ATLAS and CMS Collaborations observed a narrow four-lepton resonance with a mass near 125 GeV with local significances in excess of 5?. In the combination of the ATLAS and CMS H → ZZ → 4ℓ measurements, the mass of the observed boson was found to be 125.15 ± 0.37 (stat) ± 0.15 (syst) GeV. The event rates attributed to the signal and the studied differential cross sections were compatible with the SM Higgs boson hypothesis. Kinematic properties of leptons in signal candidate events agreed with those expected for a state with spinparity quantum numbers of the SM Higgs boson (J^P = 0⁺) and strongly disfavoured states with alternative quantum numbers or 0⁺ states with non-SM-like tensor structures of their couplings to Z bosons. The yield and kinematic properties of events in the high four-lepton mass region allowed one to probe off-shell production of the discovered boson and set model-dependent upper limits on its natural width.

The diphoton decay channel contributed strongly to the discovery of the new particle in the summer of 2012, and has since then played an important role in the measurement of its properties. This chapter describes the reconstruction of this final state in the CMS and ATLAS detectors, the strategies employed for the Higgs search and property measurements, and the results obtained with the data collected at = 7 and 8 TeV.

After analysing the full LHC Run 1 data set, the ATLAS and CMS experiments both observed an excess of events above the expected background in the H → W⁺ W⁻ → ℓ⁺υℓ⁻υ channel (ℓ = e, µ) consistent with the expectations from the SM Higgs boson with a mass of 125 GeV. The probability to observe an excess equal to or larger than the one seen, under the background-only hypothesis, corresponded to a significance of 6.1 standard deviations for ATLAS (5.8 standard deviations expected) and 4.3 standard deviations for CMS (5.8 standard deviations expected). The observed σ/σ_SM value for m_H = 125 GeV was for ATLAS and 0.72 ± 0.20 for CMS. The 0⁺ spin-parity hypothesis of the SM was favoured by the data over a narrow resonance with J^P = 2⁺ or J^P = 0⁻ decaying into W⁺ W⁻. The results were consistent between the two experiments and provided strong evidence for a SM-like Higgs boson decaying into a W boson pair.

The Higgs boson discovery in 2012 was based primarily on the analyses of Higgs boson decays to vector bosons. In the Standard Model (SM) the fermions acquire their masses via Yukawa couplings to the Higgs field, and therefore it was of fundamental importance to also confirm Higgs boson decays to fermions. In spite of significant experimental challenges, this was eventually achieved in the analyses of ττ final states. Using the full LHC Run 1 data set, ATLAS and CMS established H → ττ signals with local significances above three standard deviations and with properties consistent with SM predictions. These results provided the first evidence for a direct coupling of the 125 GeV Higgs boson to fermions.

In Run 1 at the LHC, the search for the Higgs boson decay into by the ATLAS and CMS experiments has resulted in the observation of a small excess of events consistent with the expectation from the production of a Standard Model Higgs boson with a mass of ≈ 125 GeV. The significance of this excess is below the 3σ level, compatible with the expected sensitivity. This chapter summarizes the essential elements of these challenging analyses: the production processes studied, the techniques used in the analyses, and the prospects for the observation of H → in Run 2 of the LHC.

Among proton-proton collision events at the LHC where two Wbosons are produced, the region defined by large masses of their system is sensitive to the presence of SM-like Higgs boson resonances. The final state with one Wboson decaying leptonically and the other one hadronically features both the purity granted by the presence of a lepton, and the large cross section due to the hadronic branching ratio of the Wboson. The ATLAS and CMS analyses of this final state performed with the LHC data acquired at 7TeV and 8 TeV centre-of-mass energy are presented, in the frame of Standard Model (SM) Higgs boson searches: besides excluding high mass resonances beyond the one found at a mass of around 125 GeV, these studies pave the way for the investigation of the electroweak symmetry breaking that will be carried on in the forthcoming runs of the LHC.

A Standard Model Higgs boson with a mass larger than 200 GeV decays almost exclusively to a pair of weak bosons. This chapter presents the search for such a high mass Higgs boson in the H → ZZ ℓℓqq and H → ZZ ℓℓvv final states performed by the ATLAS and CMS experiments. The search in these final states was instrumental in excluding a large range of Higgs boson masses beyond 200 GeV with the first few fb⁻¹ of integrated luminosity delivered by the LHC. This helped to narrow down the window for the Standard Model Higgs boson to masses around 100 GeV.

The combination of different Higgs boson searches was instrumental for the discovery of the Higgs boson in summer 2012. After the discovery the ATLAS and CMS collaborations have used the LHC Run 1 data to determine the properties of the Higgs boson. The mass of the Higgs boson is 125.09 ± 0.21(stat.) ± 0.11(syst.) GeV. Measurements of kinematic properties of Higgs boson production and decay as well as measurements of signal yields and the coupling strengths to other particles show very good consistency with the predictions of the Standard Model.

About fifty years after the conjecture by Englert & Brout, Higgs, and Guralnik, Hagen & Kibble, we now have good experimental proof for the existence of the mechanism that breaks the electroweak symmetry and generates the masses of the known elementary particles. Since the initial observation of a resonance with an observed mass near 125 GeV, a series of detailed measurements investigating the nature of this particle have been performed by the ATLAS and CMS collaborations. The measurements in Ref. [4] combine the individual results from ATLAS and CMS as discussed in Chapter 12 and provide a more statistically precise profile of this particle…

The following sections are included:

• Introduction
• Limits
• Significance of an excess of events
• Extracting signal model parameters
• References

The following sections are included:

• Introduction
• Historical development
• Common MVA methods
• Multivariate regression
• Systematic uncertainty
• Validation
• Optimization
• References

The following sections are included:

• Acknowledgements
• About the Authors