Tau lepton
Encyclopedia
The tau also called the tau lepton, tau particle or tauon, is an elementary particle
similar to the electron, with negative electric charge
and a spin
of . Together with the electron
, the muon
, and the three neutrino
s, it is classified as a lepton
. Like all elementary particles, the tau has a corresponding antiparticle
of opposite charge but equal mass
and spin: the antitau (also called the positive tau). Tau particles are denoted by and the antitau by .
Tau leptons have a lifetime of and a mass
of (compared to for muons and for electrons). Since their interactions are very similar to those of the electron, a tau can be thought of as a much heavier version of the electron. Because of their greater mass, tau particles do not emit as much bremsstrahlung radiation
as electrons; consequently they are potentially highly penetrating, much more so than electrons. However, because of their short lifetime, the range of the tau is mainly set by their decay length, which is too small for bremsstrahlung to be noticeable: their penetrating power appears only at ultra high energy (above PeV
energies).
As with the case of the other charged leptons, the tau has an associated tau neutrino. Tau neutrinos are denoted by .
with his colleagues at the SLAC-LBL
group. Their equipment consisted of SLAC's then-new – colliding ring, called SPEAR
, and the LBL
magnetic detector. They could detect and distinguish between leptons, hadrons and photon
s. They did not detect the tau directly, but rather discovered anomalous events:
"We have discovered 64 events of the form
for which we have no conventional explanation."
The need for at least two undetected particles was shown by the inability to conserve energy and momentum with only one. However, no other muons, electrons, photons, or hadrons were detected. It was proposed that this event was the production and subsequent decay of a new particle pair:
This was difficult to verify, because the energy to produce the pair is similar to the threshold for D meson
production. Work done at DESY
-Hamburg, and with the Direct Electron Counter (DELCO) at SPEAR
, subsequently established the mass and spin of the tau.
The symbol τ was derived from the Greek τρίτον (triton, meaning "third" in English), since it was the third charged lepton discovered.
Martin Perl shared the 1995 Nobel Prize in Physics
with Frederick Reines
. The latter was awarded his share of the prize for experimental discovery of the neutrino
.
s—the other leptons do not have the necessary mass. Like the other decay modes of the tau, the hadronic decay is through the weak interaction.
Since the tauonic lepton number
is conserved in weak decays, a tau neutrino is created when a tau decays to a muon or an electron.
The branching ratio
of the common purely leptonic tau decays are:
Elementary particle
In particle physics, an elementary particle or fundamental particle is a particle not known to have substructure; that is, it is not known to be made up of smaller particles. If an elementary particle truly has no substructure, then it is one of the basic building blocks of the universe from which...
similar to the electron, with negative electric charge
Electric charge
Electric charge is a physical property of matter that causes it to experience a force when near other electrically charged matter. Electric charge comes in two types, called positive and negative. Two positively charged substances, or objects, experience a mutual repulsive force, as do two...
and a spin
Spin (physics)
In quantum mechanics and particle physics, spin is a fundamental characteristic property of elementary particles, composite particles , and atomic nuclei.It is worth noting that the intrinsic property of subatomic particles called spin and discussed in this article, is related in some small ways,...
of . Together with the electron
Electron
The electron is a subatomic particle with a negative elementary electric charge. It has no known components or substructure; in other words, it is generally thought to be an elementary particle. An electron has a mass that is approximately 1/1836 that of the proton...
, the muon
Muon
The muon |mu]] used to represent it) is an elementary particle similar to the electron, with a unitary negative electric charge and a spin of ½. Together with the electron, the tau, and the three neutrinos, it is classified as a lepton...
, and the three neutrino
Neutrino
A neutrino is an electrically neutral, weakly interacting elementary subatomic particle with a half-integer spin, chirality and a disputed but small non-zero mass. It is able to pass through ordinary matter almost unaffected...
s, it is classified as a lepton
Lepton
A lepton is an elementary particle and a fundamental constituent of matter. The best known of all leptons is the electron which governs nearly all of chemistry as it is found in atoms and is directly tied to all chemical properties. Two main classes of leptons exist: charged leptons , and neutral...
. Like all elementary particles, the tau has a corresponding antiparticle
Antiparticle
Corresponding to most kinds of particles, there is an associated antiparticle with the same mass and opposite electric charge. For example, the antiparticle of the electron is the positively charged antielectron, or positron, which is produced naturally in certain types of radioactive decay.The...
of opposite charge but equal mass
Mass
Mass can be defined as a quantitive measure of the resistance an object has to change in its velocity.In physics, mass commonly refers to any of the following three properties of matter, which have been shown experimentally to be equivalent:...
and spin: the antitau (also called the positive tau). Tau particles are denoted by and the antitau by .
Tau leptons have a lifetime of and a mass
Mass
Mass can be defined as a quantitive measure of the resistance an object has to change in its velocity.In physics, mass commonly refers to any of the following three properties of matter, which have been shown experimentally to be equivalent:...
of (compared to for muons and for electrons). Since their interactions are very similar to those of the electron, a tau can be thought of as a much heavier version of the electron. Because of their greater mass, tau particles do not emit as much bremsstrahlung radiation
Bremsstrahlung
Bremsstrahlung is electromagnetic radiation produced by the deceleration of a charged particle when deflected by another charged particle, typically an electron by an atomic nucleus. The moving particle loses kinetic energy, which is converted into a photon because energy is conserved. The term is...
as electrons; consequently they are potentially highly penetrating, much more so than electrons. However, because of their short lifetime, the range of the tau is mainly set by their decay length, which is too small for bremsstrahlung to be noticeable: their penetrating power appears only at ultra high energy (above PeV
PEV
PEV may refer to:*Evangelical People's Party of Switzerland*Plug-in electric vehicle*Position-effect variegation *Provincial episcopal visitor*Prince Edward Viaduct, a bridge in Toronto, Canada...
energies).
As with the case of the other charged leptons, the tau has an associated tau neutrino. Tau neutrinos are denoted by .
History
The tau was detected in a series of experiments between 1974 and 1977 by Martin Lewis PerlMartin Lewis Perl
Martin Lewis Perl is an American physicist, who won the Nobel Prize in Physics in 1995 for his discovery of the tau lepton.His parents were Jewish emigrants to the US from the Polish area of Russia....
with his colleagues at the SLAC-LBL
Lawrence Berkeley National Laboratory
The Lawrence Berkeley National Laboratory , is a U.S. Department of Energy national laboratory conducting unclassified scientific research. It is located on the grounds of the University of California, Berkeley, in the Berkeley Hills above the central campus...
group. Their equipment consisted of SLAC's then-new – colliding ring, called SPEAR
SPEAR
SPEAR was a collider at the SLAC National Accelerator Laboratory. It began running in 1972, colliding electrons and positrons with an energy of...
, and the LBL
Lawrence Berkeley National Laboratory
The Lawrence Berkeley National Laboratory , is a U.S. Department of Energy national laboratory conducting unclassified scientific research. It is located on the grounds of the University of California, Berkeley, in the Berkeley Hills above the central campus...
magnetic detector. They could detect and distinguish between leptons, hadrons and photon
Photon
In physics, a photon is an elementary particle, the quantum of the electromagnetic interaction and the basic unit of light and all other forms of electromagnetic radiation. It is also the force carrier for the electromagnetic force...
s. They did not detect the tau directly, but rather discovered anomalous events:
"We have discovered 64 events of the form
- + → + + at least two undetected particles
for which we have no conventional explanation."
The need for at least two undetected particles was shown by the inability to conserve energy and momentum with only one. However, no other muons, electrons, photons, or hadrons were detected. It was proposed that this event was the production and subsequent decay of a new particle pair:
- + → + → + + 4
This was difficult to verify, because the energy to produce the pair is similar to the threshold for D meson
D meson
The D mesons are the lightest particle containing charm quarks. They are often studied to gain knowledge on the weak interaction. The strange D mesons were called the "F mesons" prior to 1986.-Overview:...
production. Work done at DESY
DESY
The DESY is the biggest German research center for particle physics, with sites in Hamburg and Zeuthen....
-Hamburg, and with the Direct Electron Counter (DELCO) at SPEAR
SPEAR
SPEAR was a collider at the SLAC National Accelerator Laboratory. It began running in 1972, colliding electrons and positrons with an energy of...
, subsequently established the mass and spin of the tau.
The symbol τ was derived from the Greek τρίτον (triton, meaning "third" in English), since it was the third charged lepton discovered.
Martin Perl shared the 1995 Nobel Prize in Physics
Nobel Prize in Physics
The Nobel Prize in Physics is awarded once a year by the Royal Swedish Academy of Sciences. It is one of the five Nobel Prizes established by the will of Alfred Nobel in 1895 and awarded since 1901; the others are the Nobel Prize in Chemistry, Nobel Prize in Literature, Nobel Peace Prize, and...
with Frederick Reines
Frederick Reines
Frederick Reines was an American physicist. He was awarded the 1995 Nobel Prize in Physics for his co-detection of the neutrino with Clyde Cowan in the neutrino experiment, and may be the only scientist in history "so intimately associated with the discovery of an elementary particle and the...
. The latter was awarded his share of the prize for experimental discovery of the neutrino
Neutrino
A neutrino is an electrically neutral, weakly interacting elementary subatomic particle with a half-integer spin, chirality and a disputed but small non-zero mass. It is able to pass through ordinary matter almost unaffected...
.
Tau decay
The tau is the only lepton that can decay into hadronHadron
In particle physics, a hadron is a composite particle made of quarks held together by the strong force...
s—the other leptons do not have the necessary mass. Like the other decay modes of the tau, the hadronic decay is through the weak interaction.
Since the tauonic lepton number
Lepton number
In particle physics, the lepton number is the number of leptons minus the number of antileptons.In equation form,so all leptons have assigned a value of +1, antileptons −1, and non-leptonic particles 0...
is conserved in weak decays, a tau neutrino is created when a tau decays to a muon or an electron.
The branching ratio
Branching ratio
In particle physics and nuclear physics, the branching fraction for a decay is the fraction of particles which decay by an individual decay mode with respect to the total number of particles which decay. It is equal to the ratio of the partial decay constant to the overall decay constant...
of the common purely leptonic tau decays are:
- 17.85% for decay into a tau neutrino, electron and electron antineutrino;
- 17.36% for decay into a tau neutrino, muon and muon antineutrino.
External links
- Nobel Prize in Physics 1995
- Perl's logbook showing tau discovery
- A Tale of Three Papers gives the covers of the three original papers announcing the discovery.