The Physics
Take a look at the fundamental physics that support and guide the experimental data analysis searches -and discoveries- relative to Higgs candidates, predicted by the Standard Model (SM) of Particle Physics.
Brief introduction to the Higgs Boson
The Standard Model of particle physics is a theory that describes the known matter in terms of its elementary constituents and their interactions. It is a widely proven and very successful theory in modern physics.
The Standard Model (SM) of particle physics postulates the existence of a complex scalar doublet with a vacuum expectation value, which spontaneously breaks the electroweak symmetry, gives masses to all the massive elementary particles in the theory, and gives rise to a physical scalar known as the Higgs boson. The Higgs boson is a fundamental particle, first observed by ATLAS and CMS in 2012, although theorised in the 1960s. The Higgs boson is the carrier particle for the Higgs field, a field present throughout our universe, which gives particles their mass. The more a particle interacts with the Higgs field, the higher its mass.
An illustration of the “Mexican hat” shape of the Higgs field potential is presented below:
Higgs boson production
Standard Model production of the Higgs boson at the LHC is dominated by the gluon fusion process (ggF), followed by the vector-boson fusion process (VBF). Associated production also have sizeable contributions, with a W or Z boson (VH) or a pair of top quarks (qqH).
The representative Feynman diagrams for the production processes are shown below:
The figure below shows the Standard Model Higgs boson production cross sections as a function of the centre-of-mass energy. If the cross-section value (left axis) is multiplied by the luminosity of the dataset to be analysed, that is effectively how many Higgs bosons are expected to be produced (for different LHC energies).
Quantum chromodynamics (QCD) and Electroweak (EW) models are used to predict the production cross sections. Next-to-leading order(NLO) and next-to-next-to leading order (NNLO) calculations are carried. High order corrections are required to achieve the desired precision for these predictions.
Currently, the SM Higgs boson mass has been measured to be 125.09 ± 0.24 GeV by combining ATLAS and CMS measurements.
Higgs boson decay
According to the Standard Model (SM), the Higgs boson can decay into pairs of fermions or bosons. The Higgs boson mass is not predicted by the SM, but once measured the production cross sections and branching ratios can be precisely calculated.
The Standard Model Higgs boson decay branching ratios and total width are shown in the figure below [PDG]. You can see that the decay modes change depending on the mass of the Higgs boson. The figure represents how likely the Higgs boson is going to decay into a certain particle, or group of particles, depending on its mass.
The following table displays the branching ratios and the relative uncertainty for a Standard Model Higgs boson with a mass of 125 GeV.
The decay mode with the highest branching ratio (BR) is the decay to hadrons, with around 70%, which is not easy to detect due to multijet QCD backgrounds. A large fraction of the leptonic decays are to a pair of neutrinos, with around 20%, which are difficult to detect since the neutrinos hardly interact with matter. The decay to pairs of electrons, muons and tau-leptons have a BR of about 10% of the total. In fact, the tau life time is very short, 3x10-13s, so it can be reconstructed only from its decay products. The efficiency of reconstructing tau-leptons is much lower than that of electrons and muons.
The Experimental Searches
Click on each of the plots to go into deeper documentation for each analysis
| $$H\rightarrow W^+W^-$$ | $$H\rightarrow ZZ$$ | $$H\rightarrow \gamma\gamma$$ |
|---|---|---|
 |
 |
 |
| As described in the “Brief introduction to the Higgs Boson”, the H → WW decay branching ratio for the Higgs boson with a mass of 125 GeV is predicted to be 0.214 in the SM, and corresponds to the second-largest branching fraction after the dominant H → bb decay mode. The predicted Higgs-boson production cross sections via the dominant gluon–gluon fusion (ggF) and vector-boson fusion (VBF) times H → WW branching fraction are 10.4 pb and 0.81 pb for ggF and VBF, respectively. Reducing the numerous backgrounds contributing to this channel and accurately estimating the remainder is a major challenge in this analysis. | The search for the Higgs through the decay H → ZZ → 4l, where l = e or μ, represents the so called “golden channel” and leads to a narrow four-lepton invariant-mass peak on top a relatively smooth and small background, largely due to the excellent momentum resolution of the ATLAS detector. The Higgs-boson decay branching ratio to the four-lepton final state for the Higgs boson mass of 125 GeV is predicted to be 0.0124% in the SM, and the expected cross section times branching ratio for the process H → ZZ → 4l is 2.9 fb at 13 TeV. Hence, based on an integrated luminosity of the current ATLAS Open Data set of 10/fb, one expects 29 events to have been produced in the final state. | The H → yy decay mode provides a very clear and distinctive signature of two isolated and highly energetic photons, and is one of the main channels studied at the LHC. Despite the small branching ratio, a reasonably large signal yield can be obtained thanks to the high photon reconstruction and identification efficiency at the ATLAS experiment. Furthermore, due to the excellent photon energy resolution of the ATLAS calorimeter, the signal manifests itself as a narrow peak in the diphoton invariant mass spectrum on top of a smoothly falling irreducible background from QCD production of two photons. |