Tau (particle)

Not to be confused with theτ⁺particle of theτ–θ puzzle,which is now identified as akaon.

Tau

s
Composition	Elementary particle
Statistics	Fermionic
Family	Lepton
Generation	Third
Interactions	Gravity,electromagnetic,weak
Symbol	τ−
Antiparticle	Antitau ( τ+ )
Discovered	Martin Lewis Perlet al.(1975)
Mass	3.16754(21)×10−27kg‍ 1776.86(12)MeV/c2‍
Mean lifetime	2.903(5)×10−13s
Electric charge	−1e
Color charge	None
Spin	⁠1/2⁠ħ
Weak isospin	LH:−⁠1/2⁠,RH:0
Weak hypercharge	LH:−1,RH:−2

Thetau(τ), also called thetau lepton,tau particle,tauonortau electron,is anelementary particlesimilar to the electron, with negativeelectric chargeand aspin of⁠1/2⁠.Like theelectron,themuon,and the threeneutrinos,the tau is alepton,and like all elementary particles with half-integer spin, the tau has a correspondingantiparticleof opposite charge but equalmassand spin. In the tau's case, this is the "antitau" (also called thepositive tau). Tau particles are denoted by the symbol
τ⁻
and the antitaus by
τ⁺
.

Tau leptons have a lifetime of2.9×10⁻¹³sand amassof1776.9 MeV/c²(compared to105.66 MeV/c²for muons and0.511 MeV/c²for electrons). Since their interactions are very similar to those of the electron, a tau can be thought of as amuchheavier version of the electron. Because of their greater mass, tau particles do not emit as muchbremsstrahlung (braking radiation)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 (abovepetaelectronvoltenergies), whentime dilationextends its otherwise very short path-length.

As with the case of the other charged leptons, the tau has an associatedtau neutrino,denoted by
ν
_τ.

History

The search for tau started in 1960 atCERNby the Bologna-CERN-Frascati (BCF) group led byAntonino Zichichi.Zichichi came up with the idea of a new sequential heavy lepton, now called tau, and invented a method of search. He performed the experiment at theADONEfacility 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 byYung-su Tsai.Providing the theory for this discovery, the tau was detected in a series of experiments between 1974 and 1977 byMartin Lewis Perlwith his and Tsai's colleagues at theStanford Linear Accelerator Center(SLAC) andLawrence Berkeley National Laboratory(LBL) group.Their equipment consisted ofSLAC's then-new electron–positron colliding ring, calledSPEAR,and the LBL magnetic detector. They could detect and distinguish between leptons, hadrons, andphotons.They did not detect the tau directly, but rather discovered anomalous events:

"We have discovered 64 events of the form

e⁺
+
e⁻
→
e^±
+
μ^∓
+ 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:

e⁺
+
e⁻
→
τ⁺
+
τ⁻
→
e^±
+
μ^∓
+ 4
ν

This was difficult to verify, because the energy to produce the
τ⁺

τ⁻
pair is similar to the threshold forD mesonproduction. The mass and spin of the tau were subsequently established by work done atDESY-Hamburg with the Double Arm Spectrometer (DASP), and at SLAC-Stanford with theSPEARDirect Electron Counter (DELCO),

The symbolτwas derived from the Greekτρίτον(triton,meaning "third" in English), since it was the third charged lepton discovered.

Martin Lewis Perl shared the 1995Nobel Prize in PhysicswithFrederick Reines.The latter was awarded his share of the prize for the experimental discovery of theneutrino.

Tau decay

Feynman diagramof the decays of the tau by emission of anoff-shellW boson

The tau is the only lepton that can decay intohadrons– the masses of other leptons are too small. Like the leptonic decay modes of the tau, the hadronic decay is through theweak interaction.

Thebranching fractionsof the dominant hadronic tau decays are:

• 25.49% for decay into a chargedpion,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.

Thebranching fractionsof 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 oflepton universality.

Exotic atoms

The tau lepton is predicted to formexotic atomslike other charged subatomic particles. One of such consists of an antitau and an electron:
τ⁺

e⁻
,calledtauonium.

Another one is anoniumatom
τ⁺

τ⁻
calledditauoniumortrue tauonium,which is a challenge to detect due to the difficulty to form it from two (opposite-sign) short-lived tau leptons. Its experimental detection would be an interesting test ofquantum electrodynamics.

See also

• Flavour (particle physics)
• Generation (particle physics)
• Koide formula
• Lepton

External links

• "Nobel Prize in Physics 1995".nobelprize.org.
• "Perl's logbook showing tau discovery".symmetrymag.org.
• "A Tale of Three Papers"(PDF).slac.stanford.edu.— gives the covers of the three original papers announcing the discovery.