Tau (particle)

<h2 id="history">History</h2>
The search for tau started in 1960 at <a href="/facts/CERN/y1gwRchm">CERN</a> by the Bologna–CERN–Frascati (BCF) group led by <a href="/facts/Antonino_Zichichi/IGN1QL8s">Antonino Zichichi</a>. 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 the <a href="/facts/ADONE/EzF6Vo5q">ADONE</a> facility in 1969 once its accelerator became operational; however, the accelerator he used did not have enough energy to search for the tau particle.<a class="footnote-ref" id="fnref:2" href="#fn:2">2</a><a class="footnote-ref" id="fnref:3" href="#fn:3">3</a><a class="footnote-ref" id="fnref:4" href="#fn:4">4</a>
The tau was independently anticipated in a 1971 article by <a href="/facts/Yung-su_Tsai/2zqmArRN">Yung-su Tsai</a>.<a class="footnote-ref" id="fnref:5" href="#fn:5">5</a> Providing the theory for this discovery, the tau was detected in a series of experiments between 1974 and 1977 by <a href="/facts/Martin_Lewis_Perl/g1BeBJ5z">Martin Lewis Perl</a> with his and Tsai's colleagues at the <a href="/facts/Stanford_Linear_Accelerator_Center/CzX2afas">Stanford Linear Accelerator Center</a> (SLAC) and <a href="/facts/Lawrence_Berkeley_National_Laboratory/tvjPSwC9">Lawrence Berkeley National Laboratory</a> (LBL) group.<a class="footnote-ref" id="fnref:6" href="#fn:6">6</a> Their equipment consisted of <a href="/facts/SLAC/CzX2afas">SLAC</a>'s then-new electron–positron colliding ring, called <a href="/facts/SPEAR/CAs47lgJ">SPEAR</a>, and the LBL magnetic detector. They could detect and distinguish between leptons, hadrons, and <a href="/facts/Photon/mFNhAEzA">photons</a>. They did not detect the tau directly, but rather discovered anomalous events:

<blockquote>"We have discovered 64 events of the form
e+ + e− → e± + μ∓ + at least two undetected particles
for which we have no conventional explanation."</blockquote>
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∓ + 4ν 
This was difficult to verify, because the energy to produce the τ+τ− pair is similar to the threshold for <a href="/facts/D_meson/o6pW8BLc">D meson</a> production. The mass and spin of the tau were subsequently established by work done at <a href="/facts/DESY/rpquB3uR">DESY</a>-Hamburg with the Double Arm Spectrometer (DASP), and at SLAC-Stanford with the <a href="/facts/SPEAR/CAs47lgJ">SPEAR</a> Direct Electron Counter (DELCO),
The symbol τ was derived from the Greek τρίτον (triton, meaning "third" in English), since it was the third charged lepton discovered.<a class="footnote-ref" id="fnref:7" href="#fn:7">7</a>
Martin Lewis Perl shared the 1995 <a href="/facts/Nobel_Prize_in_Physics/o6w1nCbr">Nobel Prize in Physics</a> with <a href="/facts/Frederick_Reines/sdWH4hNg">Frederick Reines</a>. The latter was awarded his share of the prize for the experimental discovery of the <a href="/facts/Electron_neutrino/e5hQPWIQ">electron neutrino</a>.

<h2 id="tau-decay">Tau decay</h2>
 
The tau is the only lepton with enough mass to decay into <a href="/facts/Hadron/C0sd1zty">hadrons</a>. Like the leptonic decay modes of the tau, the hadronic decay is through the <a href="/facts/Weak_nuclear_force/f6upmKMs">weak interaction</a>.<a class="footnote-ref" id="fnref:8" href="#fn:8">8</a><a class="footnote-ref" id="fnref:9" href="#fn:9">9</a>
The <a href="/facts/Branching_fraction/QrbbS9PI">branching fractions</a> of the dominant hadronic tau decays are:<a class="footnote-ref" id="fnref:10" href="#fn:10">10</a>

<ul><li>25.49% for decay into a charged <a href="/facts/Pion/x5HamzG3">pion</a>, a neutral pion, and a tau neutrino;</li>
<li>10.82% for decay into a charged pion and a tau neutrino;</li>
<li>9.26% for decay into a charged pion, two neutral pions, and a tau neutrino;</li>
<li>8.99% for decay into three charged pions (of which two have the same electrical charge) and a tau neutrino;</li>
<li>2.74% for decay into three charged pions (of which two have the same electrical charge), a neutral pion, and a tau neutrino;</li>
<li>1.04% for decay into three neutral pions, a charged pion, and a tau neutrino.</li></ul>
In total, the tau lepton will decay hadronically approximately 64.79% of the time.
The <a href="/facts/Branching_fraction/QrbbS9PI">branching fractions</a> of the common purely leptonic tau decays are:<a class="footnote-ref" id="fnref:11" href="#fn:11">11</a>

<ul><li>17.82% for decay into a tau neutrino, electron and electron antineutrino;</li>
<li>17.39% for decay into a tau neutrino, muon, and muon antineutrino.</li></ul>
The similarity of values of the two branching fractions is a consequence of <a href="/facts/Lepton_universality/X4a6MGop">lepton universality</a>.

<h2 id="exotic-atoms">Exotic atoms</h2>
The tau lepton is predicted to form <a href="/facts/Exotic_atom/FrPmnXvk">exotic atoms</a> like other charged subatomic particles. One of such consists of an antitau and an electron: τ+e−, called tauonium.
Another one is an <a href="/facts/Onium/85uteEFR">onium</a> atom τ+τ− called ditauonium or true tauonium, which is a challenge to detect due to the difficulty to form it from two (opposite-sign) short-lived tau leptons.<a class="footnote-ref" id="fnref:12" href="#fn:12">12</a>
Its experimental detection would be an interesting test of <a href="/facts/Quantum_electrodynamics/zPteAr6G">quantum electrodynamics</a>.<a class="footnote-ref" id="fnref:13" href="#fn:13">13</a>

<h2 id="see-also">See also</h2>
<ul><li><a href="/facts/Flavour_(particle_physics)/RR7rsuTD">Flavour (particle physics)</a></li>
<li><a href="/facts/Generation_(particle_physics)/hAbgvUfV">Generation (particle physics)</a></li>
<li><a href="/facts/Koide_formula/m9ceao9r">Koide formula</a></li>
<li><a href="/facts/Lepton/X4a6MGop">Lepton</a></li></ul>
<h2 id="footnotes">Footnotes</h2>

<h2 id="external-links">External links</h2>

Look up tau, tau lepton, or tauon in Wiktionary, the free dictionary.

<ul><li><a href="https://www.nobelprize.org/prizes/physics/1995/summary/">"Nobel Prize in Physics 1995"</a>. nobelprize.org.</li>
<li><a href="https://www.symmetrymagazine.org/article/octobernovember-2004/tau-lepton">"Perl's logbook showing tau discovery"</a>. symmetrymag.org. November 2004.</li>
<li><a href="http://www.slac.stanford.edu/pubs/slacreports/reports01/ssi92-034.pdf">"A Tale of Three Papers"</a> (PDF). slac.stanford.edu. — gives the covers of the three original papers announcing the discovery.</li></ul>

<h2 id="references">References</h2>

<ol>
<li id="fn:1">Fargion, D.; de Sanctis Lucentini, P.G.; de Santis, M.; Grossi, M. (2004). "Tau air showers from Earth". The Astrophysical Journal. 613 (2): 1285–1301. arXiv:hep-ph/0305128. Bibcode:2004ApJ...613.1285F. doi:10.1086/423124. S2CID 119379401. <a href="/wiki/The_Astrophysical_Journal" target="_blank">/wiki/The_Astrophysical_Journal</a> <a href="#fnref:1" class="footnote-back-ref">↩</a></li>
<li id="fn:2">Zichichi, A. (1996). "Foundations of sequential heavy lepton searches" (PDF). In Newman, H.B.; Ypsilantis, T. (eds.). History of Original Ideas and Basic Discoveries in Particle Physics. NATO ASI Series (Series B: Physics). Vol. 352. Boston, MA: Springer. pp. 227–275. <a href="https://cds.cern.ch/record/268975/files/laa-94-027.pdf" target="_blank">https://cds.cern.ch/record/268975/files/laa-94-027.pdf</a> <a href="#fnref:2" class="footnote-back-ref">↩</a></li>
<li id="fn:3">Hooft, G. 't (1996). In search of the ultimate building blocks. Cambridge; New York, NY, USA: Cambridge University Press. p. 111. ISBN 978-0-521-55083-3. <a href="978-0-521-55083-3" target="_blank">978-0-521-55083-3</a> <a href="#fnref:3" class="footnote-back-ref">↩</a></li>
<li id="fn:4">Wu, C. S.; Barnabei, O., eds. (1998). The origin of the third family: in honour of A. Zichichi on the XXX anniversary of the proposal to search for the Third Lepton at Adone. World Scientific series in 20th century physics. Singapore; River Edge, N.J: World Scientific. ISBN 978-981-02-3163-7. <a href="978-981-02-3163-7" target="_blank">978-981-02-3163-7</a> <a href="#fnref:4" class="footnote-back-ref">↩</a></li>
<li id="fn:5">Tsai, Yung-Su (1 November 1971). "Decay correlations of heavy leptons in e+ + e− → ℓ+ + ℓ−". Physical Review D. 4 (9): 2821. Bibcode:1971PhRvD...4.2821T. doi:10.1103/PhysRevD.4.2821. <a href="/wiki/Yung-su_Tsai" target="_blank">/wiki/Yung-su_Tsai</a> <a href="#fnref:5" class="footnote-back-ref">↩</a></li>
<li id="fn:6">Perl, M.L.; Abrams, G.; Boyarski, A.; Breidenbach, M.; Briggs, D.; Bulos, F.; Chinowsky, W.; Dakin, J.; Feldman, G. (1975). "Evidence for anomalous lepton production in e+e− annihilation". Physical Review Letters. 35 (22): 1489. Bibcode:1975PhRvL..35.1489P. doi:10.1103/PhysRevLett.35.1489. <a href="/wiki/Martin_Breidenbach" target="_blank">/wiki/Martin_Breidenbach</a> <a href="#fnref:6" class="footnote-back-ref">↩</a></li>
<li id="fn:7">Perl, M.L. (6–18 March 1977). "Evidence for, and properties of, the new charged heavy lepton" (PDF). In Van, T. Thanh; Orsay, R.M.I.E.M. (eds.). Proceedings of the XII Rencontre de Moriond. XII Rencontre de Moriond. Flaine, France (published April 1977). SLAC-PUB-1923. Retrieved 25 March 2021. <a href="http://slac.stanford.edu/pubs/slacpubs/1750/slac-pub-1923.pdf" target="_blank">http://slac.stanford.edu/pubs/slacpubs/1750/slac-pub-1923.pdf</a> <a href="#fnref:7" class="footnote-back-ref">↩</a></li>
<li id="fn:8">Riazuddin (2009). "Non-standard interactions" (PDF). NCP 5th Particle Physics Sypnoisis. 1 (1): 1–25. <a href="/wiki/Riazuddin_(physicist)" target="_blank">/wiki/Riazuddin_(physicist)</a> <a href="#fnref:8" class="footnote-back-ref">↩</a></li>
<li id="fn:9">Since the tauonic lepton number is conserved in weak decays, a tau neutrino is always created when a tau decays.[12] <a href="/wiki/Lepton_number" target="_blank">/wiki/Lepton_number</a> <a href="#fnref:9" class="footnote-back-ref">↩</a></li>
<li id="fn:10">Tanabashi, M.; et al. (Particle Data Group) (2018). "Review of Particle Physics". Physical Review D. 98 (3): 030001. Bibcode:2018PhRvD..98c0001T. doi:10.1103/PhysRevD.98.030001. hdl:10044/1/68623. <a href="http://pdglive.lbl.gov/Particle.action?node=S035" target="_blank">http://pdglive.lbl.gov/Particle.action?node=S035</a> <a href="#fnref:10" class="footnote-back-ref">↩</a></li>
<li id="fn:11">Tanabashi, M.; et al. (Particle Data Group) (2018). "Review of Particle Physics". Physical Review D. 98 (3): 030001. Bibcode:2018PhRvD..98c0001T. doi:10.1103/PhysRevD.98.030001. hdl:10044/1/68623. <a href="http://pdglive.lbl.gov/Particle.action?node=S035" target="_blank">http://pdglive.lbl.gov/Particle.action?node=S035</a> <a href="#fnref:11" class="footnote-back-ref">↩</a></li>
<li id="fn:12">d'Enterria, David; Perez-Ramos, Redamy; Shao, Hua-Sheng (2022). "Ditauonium spectroscopy". European Physical Journal C. 82 (10): 923. arXiv:2204.07269. Bibcode:2022EPJC...82..923D. doi:10.1140/epjc/s10052-022-10831-x. S2CID 248218441. <a href="/wiki/European_Physical_Journal_C" target="_blank">/wiki/European_Physical_Journal_C</a> <a href="#fnref:12" class="footnote-back-ref">↩</a></li>
<li id="fn:13">d'Enterria, David; Shao, Hua-Sheng (2023). "Prospects for ditauonium discovery at colliders". Physics Letters B. 842: 137960. arXiv:2302.07365. Bibcode:2023PhLB..84237960D. doi:10.1016/j.physletb.2023.137960. <a href="/wiki/Physics_Letters_B" target="_blank">/wiki/Physics_Letters_B</a> <a href="#fnref:13" class="footnote-back-ref">↩</a></li>
</ol>

Tau (particle) open-in-new

Tau (particle)