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'''Тау''' (τ), наречена и '''тау лептон''', '''тау честичка''' или '''тауон''' — [[елементарна честичка]] слична на електронот, со негативен [[електричен полнеж]] и [[спин|спин од {{frac|1|2}}]]. Заедно со [[електрон]]от, [[мион]]от, и трите [[неутрино|неутрина]], е [[лептон]]. Како и сите други елементарни честички со половичен спин, тау честичката има своја [[античестичка]] со спротивен полнеж но еднаква [[маса]] и спин, што во случајот со тау честичката е '''антитау''' (или уште наречена и ''позитивно тау''). Тау честичките се обележуваат со {{SubatomicParticle|tau}} а антитау со {{subatomicParticle|antitau}}.
 
Tau leptons have a lifetime of {{val|2.9|e=-13|u=s}} and a [[mass]] of {{val|1776.82|u=MeV/c2}} (compared to {{val|105.7|u=MeV/c2}} for muons and {{val|0.511|u=MeV/c2}} 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|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).<ref>
{{cite journal
|author=D. Fargion, P.G. De Sanctis Lucentini, M. De Santis, M. Grossi
|year=2004
|title=Tau Air Showers from Earth
|journal=[[The Astrophysical Journal]]
|volume=613 |issue=2 |page=1285
|doi =10.1086/423124
|bibcode=2004ApJ...613.1285F
|arxiv=hep-ph/0305128
}}</ref>
 
As with the case of the other charged leptons, the tau has an associated [[tau neutrino]], denoted by {{SubatomicParticle|Tau neutrino}}.
 
== History ==
 
The tau was detected in a series of experiments between 1974 and 1977 by [[Martin Lewis Perl]] with his colleagues at the [[SLAC]]-[[Lawrence Berkeley National Laboratory|LBL]] group.<ref name="Perl1975">
{{cite journal
|last1=Perl|first1= M. L.
|title=Evidence for Anomalous Lepton Production in {{SubatomicParticle|Positron}}{{SubatomicParticle|Electron}} Annihilation
|journal=[[Physical Review Letters]]
|volume=35 |issue=22 |page=1489
|year=1975
|doi=10.1103/PhysRevLett.35.1489
|bibcode=1975PhRvL..35.1489P
|last2=Abrams
|first2=G.
|last3=Boyarski
|first3=A.
|last4=Breidenbach
|first4=M.
|last5=Briggs
|first5=D.
|last6=Bulos
|first6=F.
|last7=Chinowsky
|first7=W.
|last8=Dakin
|first8=J.
|last9=Feldman
|first9=G. |displayauthors=8
}}</ref> Their equipment consisted of [[SLAC]]'s then-new {{SubatomicParticle|positron}}–{{SubatomicParticle|electron}} colliding ring, called [[SPEAR]], and the [[Lawrence Berkeley National Laboratory|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''
 
:{{SubatomicParticle|positron}} + {{SubatomicParticle|electron}} &rarr; {{SubatomicParticle|electron+-}} + {{SubatomicParticle|muon-+}} + 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:
 
:{{SubatomicParticle|positron}} + {{SubatomicParticle|electron}} &rarr; {{SubatomicParticle|antitau}} + {{SubatomicParticle|tau}} &rarr; {{SubatomicParticle|electron+-}} + {{SubatomicParticle|muon-+}} + 4{{SubatomicParticle|neutrino}}
 
This was difficult to verify, because the energy to produce the {{SubatomicParticle|Antitau}}{{SubatomicParticle|Tau}} 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.<ref>
{{cite conference
|author=M.L. Perl
|title=Evidence for, and properties of, the new charged heavy lepton
|url=http://slac.stanford.edu/pubs/slacpubs/1750/slac-pub-1923.pdf
|booktitle=Proceedings of the XII Rencontre de Moriond
|editor=T. Thanh Van (ed.)
|volume= |page=
|year=1977
|doi=
|id=SLAC-PUB-1923
}}</ref>
 
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]].
 
==Tau decay==
[[File:Feynman diagram of decay of tau lepton.svg|thumb|right|260px|[[Feynman diagram]] of the common decays of the tau by emission of a [[W boson]].]]
The tau is the only lepton that can decay into [[hadron]]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 nuclear force|weak interaction]].<ref name="NCP">{{Cite journal
| last =Riazuddin
| first =
| authorlink =Riazuddin (physicist)
| coauthors =
| title =Non-standard interactions
| journal =NCP 5th Particle Physics Sypnoisis
| volume =1
| issue =1
| pages =1–25
| publisher =Riazuddin, Head of High-Energy Theory Group at National Center for Physics
| location =Islamabad,
| year =2009
| url =http://www.ncp.edu.pk/docs/snwm/Riazuddin_Non_Standartd_Interaction.pdf
| doi =
| id =
| accessdate = }}</ref>
 
The [[branching ratio]] of the dominant hadronic tau decays are:<ref name="PDG"/>
*25.52% for decay into a charged pion, a neutral pion, and a tau neutrino;
*10.83% for decay into a charged pion and a tau neutrino;
*9.30% 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;
*2.70% for decay into three charged pions (of which two have the same electrical charge), a neutral pion, and a tau neutrino;
*1.05% 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.
 
Since the [[lepton number|tauonic lepton number]] is conserved in weak decays, a [[tau neutrino]] is always created when a tau decays.<ref name="NCP"/>
 
The [[branching ratio]] of the common purely leptonic tau decays are:<ref name="PDG"/>
*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 ratios is a consequence of [[lepton universality]].
 
== Exotic atoms ==
The tau lepton is predicted to form [[exotic atom]]s like other charged subatomic particles. One of such, called '''tauonium''' by the analogy to [[muonium]], consists in antitauon and an electron: {{SubatomicParticle|antitau}}{{SubatomicParticle|Electron}}.<ref name="Brodsky"/>
 
Another one is an [[onium]] atom {{SubatomicParticle|antitau}}{{SubatomicParticle|tau}} 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 is important for [[quantum electrodynamics]].<ref name="Brodsky">
{{cite journal
| last=Brodsky | first=Stanley J.
| last2=Lebed |first2=Richard F.
| year=2009
| title=Production of the Smallest QED Atom: True Muonium (μ<sup>+</sup>μ<sup>−</sup>)
| journal=[[Physical Review Letters]]
| volume=102 | issue=21 | page=213401
| arxiv=0904.2225
| bibcode= 2009PhRvL.102u3401B
| doi=10.1103/PhysRevLett.102.213401
}}</ref>
 
==Поврзано==
Преземено од „https://mk.wikipedia.org/wiki/Тау-честичка“