Tau (particle)

The tau is a fundamental building block of our universe. Scientists call it an elementary particle. It belongs to a specific family of particles known as leptons. This family also includes the electron and the muon. The tau is very similar to an electron in many ways. It carries a negative electric charge and has a spin of 1/2. However, the tau is much more massive than its smaller cousins. Because it is so heavy, it behaves in unique ways during physical interactions.

To understand the tau, we must look at its mass and lifetime. The tau has a mass of 1776.86 MeV/c². This is much larger than the muon, which is 105.66 MeV/c². It is also much heavier than the electron, which is only 0.511 MeV/c². Because of this high mass, the tau has a very short lifetime of about 2.9 × 10⁻¹³ seconds. Its heavy nature also changes how it moves through matter. When particles slow down, they release energy called bremsstrahlung, or braking radiation. Because the tau is so heavy, it does not emit as much of this radiation as an electron does. This makes the tau potentially very penetrating at ultra-high energies above one petaelectronvolt.

Because the tau is so heavy, it can decay in many different ways. This process is called decay, and it happens through the weak interaction. The tau is the only lepton with enough mass to decay into hadrons. Hadrons are a different class of particles. About 64.79% of the time, a tau undergoes hadronic decay. One common way is decaying into a charged pion, a neutral pion, and a tau neutrino. This specific path happens 25.49% of the time. Other hadronic paths include decaying into a charged pion and a tau neutrino, which occurs 10.82% of the time. It can also decay into three charged pions, which happens 8.99% of the time.

The tau can also decay into lighter leptons instead of hadrons. These are called purely leptonic decays. It turns into an electron and an electron antineutrino about 17.82% of the time. It turns into a muon and a muon antineutrino about 17.39% of the time. Both paths also produce a tau neutrino. The fact that these two percentages are so similar is a result of a principle called lepton universality. This principle suggests that the weak interaction treats all leptons in a similar way.

The history of the tau is a story of careful searching. The hunt began in 1960 at CERN. A scientist named Antonino Zichichi led the Bologna–CERN–Frascati group. He proposed the idea of a new, heavy sequential lepton. In 1969, he used the ADONE accelerator to look for it. However, the accelerator did not have enough energy to find the particle. In 1971, Yung-su Tsai independently predicted the particle in a scientific article. The discovery finally came through experiments conducted between 1974 and 1977.

Martin Lewis Perl led the team that finally detected the tau. He worked with colleagues at the Stanford Linear Accelerator Center (SLAC) and the Lawrence Berkeley National Laboratory (LBL). They used a machine called SPEAR, which was an electron–positron colliding ring. They also used the LBL magnetic detector. The team did not see the tau directly at first. Instead, they saw anomalous events where energy and momentum were not conserved. This suggested that at least two undetected particles were created. They eventually realized these were pairs of taus. For this discovery, Martin Lewis Perl shared the 1995 Nobel Prize in Physics with Frederick Reines.

The name tau comes from the Greek letter τ, which means "third." This is because the tau was the third charged lepton ever discovered. Scientists also study how the tau might interact with its own antiparticle. An antitau is a particle with the same mass but an opposite charge. These two could theoretically form an exotic atom called tauonium. Other predicted exotic atoms include those involving the tau and other leptons. As of 2022, these exotic atoms have not been observed. Finding them would provide an important test for the theory of quantum electrodynamics.