Reconstructing Invisible Short-lived Particles

Many of the most interesting particles produced in the collisions are extremely short-lived, living for a fraction of a billionth of a second or much less (as short as 10-24 sec). These particles decay very close to the collision point, making it impossible to observe them directly in the particle detectors.

One such example is the Z0 particle, a neutral particle, 91 times heavier than the proton (1 GeV in the used units) which sometimes decays to a pair of electron e- and anti-electron e+ (positron), or muon μ- and anti-muon μ+. By measuring the direction and momentum of the produced particles it is possible to reconstruct the "invariant" mass of the invisible particle Z0 which should be close to 91 GeV.

The Z0 particle is one of the "carriers" of the weak force. By using the buttons insert and delete track one can introduce the selected tracks->decay products on the lower table which calculates automatically the invariant mass of two or four particle combinations (one way to discover the Higgs particle). The table also shows the ETMiss of the event.