The ideas of spontaneous symmetry breaking were introduced into particle physics from condense matter physics. Imagine a Mexican-hat potential, as depicted in the graph, such that its minimum is not at (0,0). The ball will inevitably settle at a lower energy level. As a result, the local symmetry is spontaneously broken. In the context of the Standard Model Lagrangian this mechanism leads to spontaneous breaking of the electro-weak symmetry and the generation of at least one scalar boson.
Now the Standard Model Lagrangian respects gauge invariance. The graph below summarizes interactions between particles in the Standard Model, including the presence of a Higgs boson. The lines that start and end on the same particle indicate self-interactions.
When it comes to defining the experimental strategy for searches of the Higgs boson several theoretical inputs are necessary: the Higgs boson width, decay products and production mechanisms. The CMS and ATLAS detectors have been designed to a significant degree according to these theoretical inputs and within the context of inclusive analyses. Therefore the properties and features of these decays play a central role. The probability of a particle to decay into other particles depends on the couplings between these particles and it is expressed in the form of partial decay widths. The sum of all partial decay widths yield the total width of the particle. The plots below display the branching of the Standard Model Higgs boson to known particles as a function of the mass.
The Table given below gives the values of the branching fractions of the Standard Model Higgs boson with a mass of 125 GeV to known particles. The errors correspond to current level of theory uncertainties and are expressed in terms of fractional deviations in percent. The total width is given in MeV.
The Standard Model Higgs boson is produced at the LHC via several mechanisms. These are determined by the way the Higgs boson couples to SM particles. The figure below displays the LO diagrams Feynmann diagrams of the leading production mechanisms in proton-proton collisions. Shown are four main production mechanisms (from left to right): gluon-gluon fusion, vector boson fusion, associated production with weak bosons (Z,W) and associated production with top quarks.
The plot below gives the cross sections in pb for the production of the Standard Model Higgs boson in proton-proton collisions. Results are given for center of mass energy of 8 TeV. The bands correspond to the current level of theoretical uncertainty in the calculation.
On July 4th 2012 the ATLAS and CMS collaboration provided first strong evidence of a new particle consistent with a consistent with a scalar boson with a mass about 125 GeV. Analysis of data taken after have further confirmed this finding. Analyses by the ATLAS and CMS collaborations indicate that the newly observed particle is consistent with the hypothesis of the scalar boson in the SM within the accuracy provided by the available data.
With the accuracy provided by the data analyzed so far by the ATLAS and CMS collaborations the observed new boson seems consistent with the Higgs boson in the Standard Model. However, a lot of work in coming years will go into verifying if indeed this is true. This entails the following endeavors:
Our group currently has sustained efforts in the following decay channels:
Our group is also involved in contributing to the phenomenology and preparation for the exploration of the new boson in future accelerators: e-p and e+e-.
Join us in the exploration of the new boson!