I know the tau lepton has been predicted before it was discovered – unlike the muon. But how does our theory (SM/electroweak theory) predict the existence of a third lepton generation?
$\begingroup$
$\endgroup$
3
-
1$\begingroup$ As far as I remember, there wasn't a strong theoretical motivation. Just a handful of theorists all thinking "if there's two, why not three." Note that we also have theorists today who work on predicting the consequences of, and finding the excluded masses of, a fourth generation of electrons. This is in contrast to the top and bottom quarks, which were genuinely predicted as an explanation of CP violation. $\endgroup$– AXensenCommented Mar 1 at 6:53
-
$\begingroup$ It was speculatively anticipated, but not quite predicted. $\endgroup$– Cosmas ZachosCommented Mar 1 at 12:29
-
1$\begingroup$ Tsai starts his 1971 paper with "Since muons exist in nature for no apparent reason, it is possible that other heavy leptons may also exist in nature. If one discovers heavy leptons, one may be able to understand why muons exist ...". $\endgroup$– Cosmas ZachosCommented Mar 1 at 12:38
Add a comment
|
1 Answer
$\begingroup$
$\endgroup$
2
The discovery of the $\tau$ lepton by Martin L. Perl in 1975 came essentially as a surprise to the particle physics community, although there had been some previous speculations (see e.g. M.L. Perl's paper on the "prehistory" and the discovery
https://www.slac.stanford.edu/pubs/slacpubs/5750/slac-pub-5937.pdf
and Yung-Su Tsai's pioneering work "Correlations in $e^+ e^- \to \ell^+ \ell^-$", Phys. Rev. D4 (1971) 2821).
On the other hand, the discovery of the third charged lepton triggered strong efforts searching for the associated quarks of the third generation, leading to the discovery of the bottom quark by Leon M. Lederman in 1977 and of the top quark in 1995 at Fermilab. Remarkably, there was a stronger theoretical support for the existence of a third generation of quarks (than leptons), as Makoto Kobayashi and Toshide Maskawa had found an explanation of CP violation in the standard model requiring the existence of a third generation in the quark sector already in 1973.
-
1$\begingroup$ A theorist's nitpick; Bouchiat, Iliopoulos & Meyer, 1972 had already understood the fundamental requirement of quarks accompanying leptons in a generation, to cancel deadly anomalies, so the discovery of the b and the t were virtually necessary/predicted post τ! $\endgroup$ Commented Mar 1 at 14:46
-
1$\begingroup$ @CosmasZachos Thank you for pointing that out. Anomaly cancelation of the (local) gauge symmetries was indeed the most beautiful and convincing theoretical argument for "completing" the third generation after the discovery of the $\tau$. $\endgroup$– HyperonCommented Mar 1 at 14:55