The tau (), also called the tau lepton, tau particle, tauon or tau electron, is an
elementary particle
In particle physics, an elementary particle or fundamental particle is a subatomic particle that is not composed of other particles. Particles currently thought to be elementary include electrons, the fundamental fermions ( quarks, leptons, an ...
similar to the electron, with negative
electric charge
Electric charge is the physical property of matter that causes charged matter to experience a force when placed in an electromagnetic field. Electric charge can be ''positive'' or ''negative'' (commonly carried by protons and electrons respe ...
and a
spin of . Like the
electron
The electron ( or ) is a subatomic particle with a negative one elementary electric charge. Electrons belong to the first generation of the lepton particle family,
and are generally thought to be elementary particles because they have no kn ...
, the
muon
A muon ( ; from the Greek letter mu (μ) used to represent it) is an elementary particle similar to the electron, with an electric charge of −1 '' e'' and a spin of , but with a much greater mass. It is classified as a lepton. As wi ...
, and the three
neutrino
A neutrino ( ; denoted by the Greek letter ) is a fermion (an elementary particle with spin of ) that interacts only via the weak interaction and gravity. The neutrino is so named because it is electrically neutral and because its rest mass ...
s, the tau is a
lepton
In particle physics, a lepton is an elementary particle of half-integer spin ( spin ) that does not undergo strong interactions. Two main classes of leptons exist: charged leptons (also known as the electron-like leptons or muons), and neutr ...
, and like all elementary particles with half-integer spin, the tau has a corresponding
antiparticle
In particle physics, every type of particle is associated with an antiparticle with the same mass but with opposite physical charges (such as electric charge). For example, the antiparticle of the electron is the positron (also known as an antie ...
of opposite charge but equal
mass
Mass is an intrinsic property of a body. It was traditionally believed to be related to the quantity of matter in a physical body, until the discovery of the atom and particle physics. It was found that different atoms and different elementar ...
and spin. In the tau's case, this is the "antitau" (also called the ''positive tau''). Tau particles are denoted by the symbol and the antitaus by .
Tau leptons have a lifetime of and a
mass
Mass is an intrinsic property of a body. It was traditionally believed to be related to the quantity of matter in a physical body, until the discovery of the atom and particle physics. It was found that different atoms and different elementar ...
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'' (), from "to brake" and "radiation"; i.e., "braking radiation" or "deceleration radiation", is electromagnetic radiation produced by the deceleration of a charged particle when deflected by another charged particle, typicall ...
as electrons; consequently they are potentially much more highly penetrating than electrons.
Because of its short lifetime, the range of the tau is mainly set by its decay length, which is too small for bremsstrahlung to be noticeable. Its penetrating power appears only at ultra-high velocity and energy (above
petaelectronvolt
In physics, an electronvolt (symbol eV, also written electron-volt and electron volt) is the measure of an amount of kinetic energy gained by a single electron accelerating from rest through an electric potential difference of one volt in vacuum. ...
energies), when
time dilation
In physics and relativity, time dilation is the difference in the elapsed time as measured by two clocks. It is either due to a relative velocity between them ( special relativistic "kinetic" time dilation) or to a difference in gravitational ...
extends its otherwise very short path-length.
As with the case of the other charged leptons, the tau has an associated
tau neutrino, denoted by .
History
The search for tau started in 1960 at CERN by the Bologna-CERN-Frascati (BCF) group led by
Antonino Zichichi Antonino may refer to:
* Antonino (name), a given name and a surname (including a list of people with the name)
* Antonino, Kansas, an unincorporated community in Ellis County, Kansas, United States
See also
* Antoniano (disambiguation)
* Anto ...
. Zichichi came up with an idea of a new sequential heavy lepton, now called tau, and invented a method of search. He performed the experiment at the
ADONE ADONE (''big AdA'') was a high-energy (beam energy 1.5 GeV, center-of-mass energy 3 GeV) particle collider. It collided electrons with their antiparticles, positrons. It was 105 meters in circumference. It was operated from 1969 to 1993, by t ...
facility in 1969 once its accelerator became operational; however, the accelerator he used did not have enough energy to search for the tau particle.
The tau was independently anticipated in a 1971 article by
Yung-su Tsai
Yung-su Tsai (born 1 February 1930 in Yuli, Hualien, Taiwan) is a Taiwan-born American theoretical particle physicist who was a professor at Stanford University and was noted for his work at the SLAC National Accelerator Laboratory and specifically ...
. Providing the theory for this discovery, the tau was detected in a series of experiments between 1974 and 1977 by
Martin Lewis Perl
Martin Lewis Perl (June 24, 1927 – September 30, 2014) was an American chemical engineer and physicist who won the Nobel Prize in Physics in 1995 for his discovery of the tau lepton.
Life and career
Perl was born in New York City, New York. Hi ...
with his and Tsai's colleagues at the
Stanford Linear Accelerator Center
SLAC National Accelerator Laboratory, originally named the Stanford Linear Accelerator Center,
is a United States Department of Energy National Laboratory operated by Stanford University under the programmatic direction of the U.S. Departme ...
(SLAC) and
Lawrence Berkeley National Laboratory
Lawrence Berkeley National Laboratory (LBNL), commonly referred to as the Berkeley Lab, is a United States Department of Energy National Labs, United States national laboratory that is owned by, and conducts scientific research on behalf of, t ...
(LBL) group.
[
] Their equipment consisted of
SLAC
SLAC National Accelerator Laboratory, originally named the Stanford Linear Accelerator Center,
is a United States Department of Energy National Laboratory operated by Stanford University under the programmatic direction of the U.S. Departm ...
's then-new electron–positron colliding ring, called
SPEAR
A spear is a pole weapon consisting of a shaft, usually of wood, with a pointed head. The head may be simply the sharpened end of the shaft itself, as is the case with fire hardened spears, or it may be made of a more durable material fasten ...
, and the LBL magnetic detector. They could detect and distinguish between leptons, hadrons, and
photon
A photon () is an elementary particle that is a quantum of the electromagnetic field, including electromagnetic radiation such as light and radio waves, and the force carrier for the electromagnetic force. Photons are massless, so they always ...
s. They did not detect the tau directly, but rather discovered anomalous events:
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
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 (Ds) were called "F mesons" prior to 1986.
Overview
The D mesons were discovere ...
production. The mass and spin of the tau was subsequently established by work done at
DESY
The Deutsches Elektronen-Synchrotron (English ''German Electron Synchrotron''), commonly referred to by the abbreviation DESY, is a national research center in Germany. It operates particle accelerators used to investigate the structure of matt ...
-Hamburg with the Double Arm Spectrometer (DASP), and at SLAC-Stanford with the
SPEAR
A spear is a pole weapon consisting of a shaft, usually of wood, with a pointed head. The head may be simply the sharpened end of the shaft itself, as is the case with fire hardened spears, or it may be made of a more durable material fasten ...
Direct Electron Counter (DELCO),
The symbol was derived from the Greek (''triton'', meaning "third" in English), since it was the third charged lepton discovered.
[
]
Martin Lewis Perl shared the 1995
Nobel Prize in Physics
)
, image = Nobel Prize.png
, alt = A golden medallion with an embossed image of a bearded man facing left in profile. To the left of the man is the text "ALFR•" then "NOBEL", and on the right, the text (smaller) "NAT•" then " ...
with
Frederick Reines
Frederick Reines ( ; March 16, 1918 – August 26, 1998) 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. He may be the only scientist in ...
. The latter was awarded his share of the prize for experimental discovery of the
neutrino
A neutrino ( ; denoted by the Greek letter ) is a fermion (an elementary particle with spin of ) that interacts only via the weak interaction and gravity. The neutrino is so named because it is electrically neutral and because its rest mass ...
.
Tau decay
![Feynman diagram of decay of tau lepton](https://upload.wikimedia.org/wikipedia/commons/1/11/Feynman_diagram_of_decay_of_tau_lepton.svg)
The tau is the only lepton that can decay into
hadron
In particle physics, a hadron (; grc, ἁδρός, hadrós; "stout, thick") is a composite subatomic particle made of two or more quarks held together by the strong interaction. They are analogous to molecules that are held together by the ele ...
s – the masses of other leptons are too small. Like the leptonic decay modes of the tau, the hadronic decay is through the
weak interaction
In nuclear physics and particle physics, the weak interaction, which is also often called the weak force or weak nuclear force, is one of the four known fundamental interactions, with the others being electromagnetism, the strong interaction, ...
.
[
]
The
branching fraction
In particle physics and nuclear physics, the branching fraction (or branching ratio) for a decay is the fraction of particles which decay by an individual decay mode or with respect to the total number of particles which decay. It applies to eithe ...
s of the dominant hadronic tau decays are:
* 25.49% for decay into a charged
pion
In particle physics, a pion (or a pi meson, denoted with the Greek letter pi: ) is any of three subatomic particles: , , and . Each pion consists of a quark and an antiquark and is therefore a meson. Pions are the lightest mesons and, more gene ...
, a neutral pion, and a tau neutrino;
* 10.82% for decay into a charged pion and a tau neutrino;
* 9.26% for decay into a charged pion, two neutral pions, and a tau neutrino;
* 8.99% for decay into three charged pions (of which two have the same electrical charge) and a tau neutrino;
* 2.74% for decay into three charged pions (of which two have the same electrical charge), a neutral pion, and a tau neutrino;
* 1.04% for decay into three neutral pions, a charged pion, and a tau neutrino.
In total, the tau lepton will decay hadronically approximately 64.79% of the time.
The
branching fraction
In particle physics and nuclear physics, the branching fraction (or branching ratio) for a decay is the fraction of particles which decay by an individual decay mode or with respect to the total number of particles which decay. It applies to eithe ...
s of the common purely leptonic tau decays are:
* 17.82% for decay into a tau neutrino, electron and electron antineutrino;
* 17.39% for decay into a tau neutrino, muon, and muon antineutrino.
The similarity of values of the two branching fractions is a consequence of
lepton universality.
Exotic atoms
The tau lepton is predicted to form
exotic atom
An exotic atom is an otherwise normal atom in which one or more sub-atomic particles have been replaced by other particles of the same charge. For example, electrons may be replaced by other negatively charged particles such as muons (muonic atoms) ...
s like other charged subatomic particles. One of such, consists of an antitauon and an electron: .
Another one is an
onium
An onium (plural: onia) is a bound state of a particle and its antiparticle. These states are usually named by adding the suffix ''-onium'' to the name of one of the constituent particles (replacing an ''-on'' suffix when present), with one except ...
atom called ''true tauonium'' and is difficult to detect due to tau's extremely short lifetime at low (non-relativistic) energies needed to form this atom. Its detection would be an important test of
quantum electrodynamics
In particle physics, quantum electrodynamics (QED) is the relativistic quantum field theory of electrodynamics. In essence, it describes how light and matter interact and is the first theory where full agreement between quantum mechanics and spec ...
, if possible.
[
]
See also
*
Flavour (particle physics)
In particle physics, flavour or flavor refers to the ''species'' of an elementary particle. The Standard Model counts six flavours of quarks and six flavours of leptons. They are conventionally parameterized with ''flavour quantum numbers'' t ...
*
Generation (particle physics)
In particle physics, a generation or family is a division of the elementary particles. Between generations, particles differ by their flavour quantum number and mass, but their electric and strong interactions are identical.
There are three gene ...
*
Koide formula
The Koide formula is an unexplained Empirical relationship, empirical equation discovered by Yoshio Koide in 1981. In its original form, it relates the masses of the three charged leptons; later authors have extended the relation to neutrinos, qua ...
*
Lepton
In particle physics, a lepton is an elementary particle of half-integer spin ( spin ) that does not undergo strong interactions. Two main classes of leptons exist: charged leptons (also known as the electron-like leptons or muons), and neutr ...
Footnotes
References
External links
*
*
* — gives the covers of the three original papers announcing the discovery.
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Elementary particles
Leptons