Martin Perl and his colleagues detected peculiar events occurring in SPEAR. From the scene of collision an electron and a heavy electron (the well-known muon) carrying opposite electric charges were ejected at the same moment without any other detectable particles coming out. No conventional process, involving conventional particles, could account for such events.
The particle called U was grotesquely weighty for an electron. Theorists had been asking one another for many years why the muon, a heavy electron of 105 mass-energy units, was two hundred times heavier than the ordinary electron (0.5 units). They had no answer even to that. And the U particle was estimated to be 1800 mass-energy units, twice as heavy as the hydrogen atom!
Doubts also overtook Stanford's heavy lepton, the U particle. There were suggestions, notably from the DORIS experiments in Hamburg, that it was an illusion - perhaps a misinterpretation of the decay of the charmed particles. At the time of writing these doubts have not been resolved and they illustrate again the difficulties and tensions of high-energy physics. A fair summary of the situation may be that there is no very compelling evidence so far for nature deploying more than four types of quarks and four members of the electron family.
But in the second edition Calder wrote:
The particle called U was grotesquely weighty for an electron.
Theorists had been asking one another for many years why the muon, a
heavy electron of 105 mass-energy units, was two hundred times heavier
than the ordinary electron (0.5 units). They had no answer even to
that. And the U particle was estimated to be 1800 mass-energy units,
twice as heavy as the hydrogen atom! Experiments with DORIS in Hamburg confirmed the discovery."
(Martin Perl, "The Discovery of the Tau Lepton", in L. Hoddeson et al., The Rise of the Standard Model, Cambridge University Press 1997.)
