25 Days of Particles: Day 14

Tau

Classification: lepton, fermion
Fundamental: yes
Family: third
Mass: 1777 MeV
Interactions: Electromagnetic (charge -1), Weak, Gravity
Spin: 1/2
Lifetime: 2.9e-13 s

Next up in our tour of the particle physics world is the tau lepton. This is the third family's charged lepton, the correspondent to the electron and muon. The tau is also the most massive particle we've met to date. The tau lepton all by its fundamental self is about as heavy as a helium atom.

It's a fact of nature that objects tend to fall into the lowest energy state they can get to. In particle physics, this takes the form of heavy stuff decaying into light stuff. Heavy particles tend to decay really quickly, and if they don't, it normally means the decay is forbidden in someway. Tau leptons decay quickly enough that they are almost never seen in a particle detector. When particle physicists want to study taus, they need to search for their decay products. This was how the tau lepton was discovered in the first place. Physicists at SLAC (the Stanford Linear Accelerator Center) observed events containing an electron, a muon with the opposite charge from the electron, and at least two invisible particles in their electron-positron collisions. They knew there needed to be more than one invisible particle to make conservation of energy and momentum work out. It took several years to establish exactly what the physicists were seeing, since the decays of charmed mesons can produce a similar signature at a similar energy. While the tau was first observed in the mid-1970s, the Nobel prize in physics for its discovery was awarded to Martin Perl in 1995.

It's short lifetime distinguishes the tau from the other leptons for all experimental purposes. We never see it directly, and the tau lepton is the only charged lepton heavy enough to decay into quarks. Those quarks form jets, and it tends to be difficult to figure out what process a jet comes from. For these reasons, the tau tends to be more of an object to be searched for than a tool that can be used to discover new particles.