Animated Z --> tau+ + tau- decay

This is what a typical Z --> tau+ + tau- decay might look like. An electron coming in from the right collides with an anti-electron, a positron, coming in from the left. They annihilate and produce a Z particle, which exists for just a fleeting moment before decaying into one positively and one negatively charged tau particle.

The tau is different from the electron and the muon in that it has a very short lifetime, about 10-15s. This actually means that the decay typically takes place inside the beam pipe and that we never detect the tau particle itself inside the detector. Instead we will have to identify it by looking at what it decays into. This makes the tau particle the most difficult to identify.

The tau particle can decay into many different things. However, the decay typically results in one or a few visible particles. There is always at least one neutrino among the decay products carrying of a part of the energy and momentum. Therefore, the visible particles from the two tau particles decaying may not be back-to-back as is the case when the Z decays into e+e- or mu+mu-.

Since the tau can decay into many different things, there is a chance that we will the two taus decaying in different ways. This is the most important way for you to identify tau particles. For instance, in the example above one tau decays into a muon and two neutrinos and the other into three charged hadrons, which interact in the hadon calorimeter, plus neutrinos.

To summarize, if you see an event where there is an electron or a muon on one side and on the other side you see something different, maybe be a muon or an electron or two or three hadrons, and if the decay products on the two sides are not back-to-back, you can suspect that the Z particle has decayed into tau particles.

If all particles were detected, the total energy measured by the detector would ideally be the same as the collision energy, that is about 91 GeV. However, in events where the Z decays into tau particles the total energy measured is typically well below 91 GeV due to the undetected neutrinos that are produced in the subsequent tau particle decays.