The innermost shell of the nuclear onion

While the above experiments were searching for quarks, others were looking into the genealogy of the lepton family. In 1975, Martin L. Perl (1921-) and his team at the Stanford Linear Accelerator published the discovery of a sibling to the electron and the muon — a super-heavy electron. This particle, almost 3500 times heavier than the electron, was given the name Tau'. It was expected that each of the three leptons would have an accompanying neutrino. As of the end of the 20th century, this completed the list of the fundamental, indivisible building blocks of matter:

6 quarks ld) Is) Vb)

6 leptons

I Ve ) I nm ) I nt)

Building blocks of the

Present members of the

strongly interacting

Club of Immunity from the


Strong Force

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