The Higgs boson
The Higgs boson is the only Standard Model (SM) particle not yet fully established – follow this link for further details. However in July 2012, the ATLAS and CMS experiments at CERN announced the discovery of a new boson at a mass of roughly 125 GeV and which is compatible with the long-awaited Higgs boson.
The SM does not say how heavy the Higgs boson is. However, for a given Higgs mass the SM predicts how often it is produced in particle collisions and how it decays to the known particles. The following chart summarizes how often a 125-GeV Higgs decays to various final known particles.
Such a Higgs boson is very heavy, even heavier than the Z boson you have met before, and has a very short lifetime and, therefore, travels only a very tiny distance. There is therefore no chance to directly detect it in ATLAS or any other particle detector.
The Higgs boson can be produced at high-energy proton-proton collisions at the LHC. ATLAS and CMS have observed decays to (i) 2 Z bosons, leading to two pairs of charged leptons, to (ii) 2 photons, and to (iii) 2 W bosons, leading to 2 charged leptons and 2 neutrinos.
In this Z-path, you will have the chance to search for the Higgs decays (i) and (ii) above and partly repeat what the ATLAS physicists have done at CERN!
You may wonder why the CERN-discovery is about a “Higgs-like” particle, not “the” Higgs particle! The reason is purely scientific. Although decays to ZZ, W+W- and γγ have been observed, decays to leptons and quarks have not yet been confirmed, in particular decays to pairs of b-quarks and pairs of tau-leptons.
But why is it so that “rare” decays such as H→γγ(with 0.2% probability) are observed, but not the “frequent” decays H→bb̅ (with 57% probability)? The latter decay of the Higgs leads to 2 jets of particles that are very difficult to separate from the much more abundant production of normal jets, those allowed by the strongest of all interactions – the strong force.
If you are patient you will, hopefully in future Masterclasses, study such Higgs decays. For now, good luck with your search of the “Higgs-like” particles in decays to ZZ and γγ.
Isn’t it marvelous that just a few months after announcing a discovery you have the opportunity to have a close look at it yourself?
Follow this link in case you want to know more about the Higgs. In the Standard Model, to the Higgs field, which is responsible for particle masses, correspond the Higgs boson, which is a particle of spin 0. It is the latter that we have been searching for at the LHC!