A generic particle detector consists of four layers:
Long-lived particles are identified according to the energy deposited in these four layers.
A photon is detected as energy in the electromagnetic calorimeter, with little energy in the hadronic calorimeter, and no track pointing at the calorimeter cell. Isolation cuts are used to reduce the backgrounds, mainly from pi-zero decays.
An electron is detected as energy in the electromagnetic calorimeter, with little energy in the hadronic calorimeter, with an isolated track pointing at the calorimeter cell. Electron isolation cuts are used to reduce backgrounds, mainly from a charged pion track pointing at a calorimeter cell with a photon from a pi-zero decay.
A muon leaves little energy in the calorimeters, has a track, and travels all the way to the muon-detection system outside the calorimeters.
Jets are the most common objects in hadronic collider physics. Any colored particle can hadronize into a jet. Crudely, jets are defined to be a collection of particles as detected by the calorimeters that are either spatially clustered (in azimuth and pseudorapidity) or clustered via some invariant measure (such as a pairwise invariant mass).
The detector can imperfectly distinguish jets from b quarks (and to a lesser extent charm), from other jets because they leave a displaced vertex. A great deal more about jets and b-tagging can be found here.
Tau leptons decay about one third of the time to either an electron or a muon plus neutrinos. In this case, they cannot be distinguished from electrons or muons and appear in the detector as objects of electron and muon type.
The most common hadronic decays of the tau are to a neutrino plus
In the first two cases a single charged track, distinguished from an electron by its energy deposition in the hadronic calorimeter, is the result — a “1-prong” tau. Any hadronic or electromagnetic energy is clustered in a very narrow cone surrounding the charged track. In the third case, three tracks result — a “3-prong” tau. Thus, what appears in the detector is a very narrow jet, with invariant mass no greater than 2 GeV, and with 1 or 3 tracks. Such an object is unlikely to be an ordinary QCD jet, though fake taus do occur.
“Missing transverse energy” is the magnitude of the missing transverse momentum in an event. Energy conservation cannot be used in a hadronic collider, because so much energy is carried off in unmeasurable particles — remnants of the shattered initial protons — inside or very near the beampipe. For the same reason, momentum conservation along the beampipe cannot be used. However, momentum conservation transverse to the beampipe should work. A failure of momentum conservation in the transverse plane suggests the presence of a neutrino, or neutrinos, or new undetectable objects that have carried off momentum invisibly. Alternately, it could come from a mismeasurement of the energy of a jet, which is a common occurrence or a particle sneaking through a crack in the detector structure.
See the particle identification page for information on how PGS does object identification.