TRISTAN - mission complete

The high energy physics mission of the TRISTAN electron-positron collider at the Japanese KEK Laboratory ended in May. TRISTAN was the first accelerator in Japan at the high energy frontier, and its success owes a great deal to help and encouragement from the world high energy physics community. Its success also marks the first step toward the KEKB project now underway and the subsequent Linear Collider scheme. TRISTAN began operation in November 1986 with a collision energy of 50 GeV, the world's highest electron-positron collision energy at that time. With the addition of superconducting radiofrequency cavities, the energy was continuously increased, reaching a maximum of 64 GeV in 1989. In this exploratory era, the three large detectors - AMY,TOPAZ and VENUS - together with the smaller SHIP group made a rapid survey of particle phenomena in this new energy range. The sixth ('top') quark was first on the list of wanted particles, but the three large groups concluded that there were no new quarks below 32 GeV. The CDF and DO Collaborations at Fermilab's Tevatron recently reported the top quark as being six times as heavy as TRISTAN'S physics reach. Although initial experimental results suggested that the event-shape distributions of multi-hadron events were broadly consistent with the production of the five known quarks, the production rate of hadrons, compared to muons, was seen to rise with energy. The increased energy reach of TRISTAN increased the visibility of the subtle virtual effects of the Z (the electrically neutral carrier of the weak force) produced through the interference of weak and electromagnetic interactions. The rise was found to be slightly larger than expected from five quarks and a Z mass of 92 or 93 GeV, the accepted value at that time. This hinted that the Z mass had to be smaller, as later verified when the SLC and LEP electron-positron colliders at SLAC (Stanford) and CERN respectively came into operation in 1989. Besides the top quark, ''standard'' types of new particles were systematically searched for, including fourthgeneration, supersymmetric and higgs particles. In addition, TRISTAN data definitively settled questions regarding some little understood sightings. Among them was the socalled ''MARK-J phenomenon'', an observation of excess multi-hadron production accompanied by muons in a direction isolated from the rest of the hadrons. This was reported by the MARK-J group at the PETRA electron-positron collider at DESY near the end of PETRA running, and was left for TRISTAN to investigate. Another case was a possible narrow two-photon resonance at around 58 GeV mass, suggested by L3 at LEP in 1992, which was subsequently ruled out by all three TRISTAN experiments and by the other LEP experiments. After LEP and SLC took over the high energy frontier, TRISTAN experiments turned toward deeper and broader studies of electroweak and quark-gluon (quantum chromodynamics - QCD) processes. The storage ring was operated at 58 GeV, high enough for the interference between the weak and electromagnetic interactions to be large, for the machine to operate stably with ample margin, and where maximum integrated luminosity could be provided to the experiments