This paper presents the search for the Standard Model Higgs boson in proton–proton collisions at the Large Hadron Collider using the ATLAS detector. The search combines datasets from 2011 at a centre-of-mass energy of 7 TeV (integrated luminosity of 4.8 fb⁻¹) and 2012 at 8 TeV (integrated luminosity of 5.8 fb⁻¹). Clear evidence is presented for the production of a new neutral boson with a measured mass of 126.0 ± 0.4 (stat) ± 0.4 (sys) GeV. The discovery achieves a local significance of 5.9 standard deviations, corresponding to a background fluctuation probability of 1.7 × 10⁻⁹, which is compatible with the Standard Model Higgs boson.
Key Takeaways
Observed a new neutral boson with a measured mass of 126.0 ± 0.4 (stat) ± 0.4 (sys) GeV.
Achieved a local statistical significance of 5.9 standard deviations (σ), equivalent to a background fluctuation probability of 1.7 × 10⁻⁹.
Analyzed datasets consisting of proton-proton collision integrated luminosities of approximately 4.8 fb⁻¹ at √s = 7 TeV and 5.8 fb⁻¹ to 5.9 fb⁻¹ at √s = 8 TeV.
Constructed the primary discovery using three main decay channels: H → ZZ(*) → 4l, H → γγ, and H → WW(*) → eνμν.
Excluded the Standard Model Higgs boson at 95% confidence level in the mass range of 111–559 GeV, except for a narrow region of 122–131 GeV.
Learning Objectives
Understand the experimental methods and particle decay channels used by the ATLAS collaboration to detect a Higgs-like boson.
Explain the significance of the 5.9σ local statistical threshold required for establishing a new particle discovery in high-energy physics.
Analyze how datasets from different run periods (7 TeV and 8 TeV) and luminosities are combined to enhance statistical sensitivity.
Describe the cylindrical geometry and multi-component layout of the ATLAS detector at CERN.
Glossary
Higgs boson
A neutral scalar elementary particle in the Standard Model representing the excitation of the Higgs field, which gives mass to other elementary particles.
Standard Model
The theoretical framework of particle physics describing high-energy particle interactions and fundamental forces.
Luminosity
A measure of the number of potential particle collisions per unit area and time in a particle accelerator.
Standard Deviation (sigma)
A statistical measure used in physics to quantify the significance of an observed excess of events above the expected background.
Pseudorapidity
A spatial coordinate parameter used in collider physics describing the angle of a particle relative to the beam axis.
Branching Ratio
The fraction of a particle's total decay rate that proceeds through a specific decay mode or channel.
Timeline
2012 (31 July)The discovery manuscript is officially received by the journal Physics Letters B.
2012 (8 August)The manuscript is received in revised form.
2012 (11 August)The paper is accepted for publication in Physics Letters B.
2012 (14 August)The publication is made available online to the public.
Mind Map
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ATLAS Higgs Boson Discovery
Decay Channels
H to ZZ to 4l
H to gamma gamma
H to WW to evmv
ATLAS Detector Components
Inner Tracker and Solenoid
Electromagnetic and Hadronic Calorimeters
Muon Spectrometer System
Discovery of a New Particle
Statistical and physical markers of the 126 GeV neutral boson
mass
126.0 GeV
Measured Boson Mass
sigma
5.9σ
Local Statistical Significance
probability
1.7 x 10^-9
Background Fluctuation Probability
luminosity
10.6 fb^-1
Total Integrated Luminosity Combined
Electroweak Symmetry Breaking
The experimental confirmation of this mechanism explains the origin of mass for elementary particles.
ZZ and diphoton Channels
These two high-resolution decay channels provided the cleanest resonant mass peaks in the search.
Spin-1 Disfavored
The observation of the particle decaying to two photons disfavours the spin-1 hypothesis under the Landau-Yang theorem.
Flashcards
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Quiz
Frequently Asked Questions
How does pile-up affect particle reconstruction at the LHC, and how did ATLAS mitigate it?
Pile-up refers to multiple proton-proton collisions occurring in the same or neighbouring bunch crossings due to high beam intensity. It can deteriorate lepton isolation and missing transverse momentum resolutions. ATLAS mitigated this by developing improved reconstruction, identification, and isolation criteria for electrons and photons to remain robust under increased pile-up.
Why are the ZZ(*) and diphoton channels considered high-resolution channels?
The H → ZZ(*) → 4l and H → γγ channels allow for the full reconstruction of the invariant mass of the final-state particles (four leptons or two photons). Because the detector has excellent tracking and electromagnetic calorimetry resolution, these channels produce very sharp, narrow resonance peaks above the continuous background.
What does '5.9 sigma significance' mean in the context of this discovery?
A local significance of 5.9 standard deviations means that the probability of the background physics processes alone fluctuating to produce an excess as large or larger than the observed signal is extremely small (approximately 1.7 × 10⁻⁹). This far exceeds the traditional 5.0 sigma threshold required in particle physics to declare a formal discovery rather than a statistical fluke.
Is the newly discovered particle definitively the Standard Model Higgs Boson?
While the measurements of its mass, neutral charge, and decay to vector bosons are fully compatible with the Standard Model Higgs boson, the paper notes that more data are needed to assess its nature and coupling properties in precise detail.
References
Observation of a new particle in the search for the Standard Model Higgs boson with the ATLAS detector at the LHC - doi.org/10.1016/j.physletb.2012.08.020