Finding the Higgs boson through experiment would prove that the Higgs field exists, and this is one of the main goals of the CMS experiment.
If the Higgs boson is formed in an LHC collision, it will be relatively heavy and will decay very quickly to other particles. What it decays into, the telltale ‘signature’ it leaves behind, will depend on its mass. From previous LEP experiments, we know that the Higgs boson must be heavier than around 100GeV (where GeV is a unit of energy or mass used in particle physics – a proton has a mass of about 1GeV), but since the theory does not say exactly how massive it is, any experiment must look carefully at a range of energies.
Different signatures are more or less likely within the different energy ranges and CMS is designed to find the most easily identifiably signatures within each range.
If the Higgs boson is relatively light, below about 140 GeV, it is likely to be identified first by its decay into two photons, detected in the electromagnetic calorimeter. In the first image we see the detector head-on with the electromagnetic calorimeter (ECAL) in green. Green tracks show these photons, whilst the blue tracks show other particles emerging from the collision with the red marks showing hits in the hadron calorimeter (HCAL). The ECAL is able to tell the mass of the particle to better than 1% in this range.
The most distinctive signature of the Higgs in the range from around 150 GeV to 180 GeV is the decay to two W bosons, which then decay into two leptons and two neutrinos. Detecting leptons requires excellent performance not only from the ECAL but also from the muon chambers and tracking detectors. Neutrinos however are neutral, weakly interacting particles that the detector cannot stop or detect. But still their presence can be inferred. If we see particles from a single collision shoot out one side of the detector but not the other, we can add up the energies of all the emerging particles and deduce if some is “missing”. With a “hermetic” hadron calorimeter that totally surrounds the collision point and stops all detectable particles, we can infer that the “missing energy” must be due to an “invisible” particle, in this case, a neutrino.
Another possible signature, that is likely from low masses up to 600 GeV and above, is the decay into two Z bosons, which in turn decay into four leptons, particles like electrons or muons.
A typical event where each Z has produced two muons is shown below. In this zoomed-out image the muons chambers are marked in red and the electromagnetic calorimeter is the green ring near the centre.
Finally, if the Higgs boson is very heavy, with a mass in excess of 500 GeV, there are yet more ways in which it might decay that become useful in trying to detect it. One possibility is shown here, where the Higgs decays into two Z bosons, which in this example then decay into two electrons and two quarks. The electrons produce clean tracks whilst the quarks produce sprays of particles known as jets.
Supersymmetric models suggest that there are in fact five or more new Higgs bosons to be found. These would have a range of signatures with some of the more complicated signatures becoming more probable, for instance the decay to specific types of short-lived quarks, which we would see through the particle jets they produce.