There are three flavors of charged leptons: electrons, muons and taus. Experimentally, tau leptons are very different from electrons and muons, in that they decay before they pass through the detector. When they decay, they can either turn into
- An electron or muon, plus a neutrino and anti-neutrino; in this case they are registered as an electron or muon by the detector, since the neutrinos cannot be detected (except indirectly through an imbalance in the momentum of what is observed). Such taus cannot be identified as such.
- A single charged hadron, possibly with neutral hadrons; in this case they form what looks like a very narrow jet with a single track in the tracking detector. This type of tau decay has the lowest fake background, since it looks like a very unusual jet, and so this is the best decay mode for identifying a tau.
- Three charged hadrons of total charge +1 or -1, possibly with neutral hadrons and a neutrino; this also looks like a very narrow jet but now with three tracks. This is still unusual as jets go, but not as unusual as the previous case, so the fake background from odd-looking jets is larger.
Fake taus are common, especially for low-energy taus, so looking for isolated taus (and anti-taus) is harder than looking for isolated electrons and muons.