On 11 November 1974, elementary particle physics entered a new era, with simul taneous announcements from the east and west coasts of America th a t a new heavy particle with astonishingly small decay width had been observed in two quite independent experiments, of different types. Since th a t time we have all been living through one of the most exciting periods which our field of research has known. The possibility th a t there might exist new particles of some kind, and possibly of more than

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... one kind, was very much ' in the air ' during the preceding year or so (Iliopoulos 1974). A ttractive theoretical ideas had been pu t forward some years before (Weinberg 1967) suggesting th a t a finite gauge theory could be constructed for the weak interactions, which could achieve a unification of the weak and electro magnetic interactions, a goal long sought (Salam & W ard 1964). When a proof of this finiteness (renormalizability) was achieved by t 'Hooft (1971 a, b), physicists had for the first time calculable and meaningful theories unifying the weak and electro magnetic interactions, the analogue for the weak interactions to the photon for the electromagnetic field being very heavy vector bosons, both charged and neutral, whose direct detection still lies quite far in the future. However, not all such theories were necessarily finite. Further conditions had to be met, and a key feature of these is the situation concerning the neutral weak currents. (a) For the lepton weak currents and their interactions alone, the condition is simply th a t there must exist either neutral lepton currents or heavy leptons, beyond the electron and muon, or both. (b) For the hadronic weak currents and their interactions, the condition is th a t there must exist either neutral hadronic currents with ± 1, or additional hadronic particles, or both. I f the hadronic states are constructed from quarks, then the latter condition would require th a t there should exist further quarks beyond the quarks (u, d, s) underlying SU(3) symmetry. I t was clear that, if these neutral weak currents did not exist, then there was likely to be at least one new domain of physics still to be discovered, and there was much discussion how searches should be carried out most efficiently in order to detect these hypothetical objects, since the theories did not specify in any unique way what parameters or quantum numbers these objects should have. I t was a major step forward when the work of H asert et al. (1973) established the existence of lepton-hadron weak interactions due to the coupling of neutral leptonic and neutral hadronic weak currents, in their analysis and interpretation of events observed in the Gargamelle chamber exposed to the neutrino beams at CERN. This demonstrated the existence of neutral weak currents for both the [ 443 ] 16-3