36 metre chain of superconducting magnets under test at the German DESY Laboratory in for the HERA - collider now under construction.

KEK Eventful year

Just one year ago, the three-kilo­ metre TRISTAN ring at the Japa­ nese KEK Laboratory saw its first electron-positron collisions. In a few months of actual running time it has gone on to achieve its basic design aims and to supply impor­ tant physics. While the Stanford Linear Collider struggles to achieve its hairsbreadth colliding beams and with CERN's LEP machine still two years away from switchon, TRISTAN has the high energy elec­ tron-positron stage to itself. Earlier this year, TRISTAN sup­ plied 25 and 26 GeV colliding beams and all three major experi­ at the German DESY Laboratory and mounted in cryostats at BBC- ments - VENUS, AMY and TOPAZ in Hamburg in data collected sev­ Mannheim) and two quadrupoles - collected good data samples eral years ago (see September (made at Saclay). They are installed (see September issue, page 25). issue, page 37). As the first TRIS­ in a tunnel-shaped hall and One of today's major particle TAN analyses got underway this mounted with an inclination of one physics preoccupations is the year, there was a suggestion that per cent corresponding to the larg­ search for the long-awaited sixth these unexpected single muons est slope of the HERA tunnel. The ('top') . While other experi­ were still there (see October issue, system has its own 900 watt he­ ments have inferred that it is heav­ page 1), but now the combined lium refrigeration plant and is pro­ ier than about 45 GeV, confirma­ weight of all three experiments is vided with full quench protection tion comes from direct scan of pushing these signals towards and complete vacuum equipment, electron-positron annihilation. oblivion in the statistical junkyard. as they will be installed for HERA. Thus the three TRISTAN exper­ Nobody ever saw a corresponding The magnet string has already iments looked closely at hadron effect with anyway. been cycled several times between production from annihilations at In October, TRISTAN started a room temperature and liquid helium these newly available collision en­ new physics run at 27.5 GeV per temperatures and has been kept ergies, but saw no sign of any beam. cold for extended periods. It has increased activity. In addition, the been quenched more than 20 times shape of the hadron production at magnetic fields between 1.8 patterns is in accord with a five and 5.8 tesla, the latter corre­ quark picture. Thus the DESY sponding to a proton beam energy was out of TRISTAN'S reach. How­ Magnet string tests of 1010 GeV (the HERA design ever next year the radiofrequency value is 820 GeV). In these tests accelerating power will be boosted, A 36 metre chain of superconduct­ the current was limited by one of and a higher energy region will be ing magnets built for the HERA the prototype dipole-magnets, carefully scanned. electron-proton collider has been which was made using an old type Physicists had been bewildered under test since April at DESY in of cable. by the isolated muons accompan­ Hamburg. The total heat leak is in good ied by a broad spread of hadrons The magnet string is composed agreement with the values pre­ reported by some experiments at of three full-sized prototype dipole viously measured for the individual the PETRA electron-positron ring magnets (with coils made at DESY magnets and no décrémentai effect

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20 CERN Courier, December 1987 resulted from the inclination. No problems were found during the temperature cycling and the pro­ pagation of the cold front through the inclined magnet string agrees with the calculated values. N An excellent vacuum was 5MeV ARGUS achieved in the beampipe, kept at 25 liquid helium temperature. The helium leak rate into the 36 metre 20 pipe was very small and the quench protection system works reliably. 15 Shunt diodes kept at liquid he­ lium temperature take over the 10 current as soon as a magnet quenches. The resistive voltage is 5 detected for each pair of coils in a sensitive bridge circuit and 0 quench-heaters are fired in order 5.24 5.25 5.26 5.27 5.28 5.29 5.30 to increase the quench propagation velocity in the magnet. Thus, in 2 principle, a quench is limited to the m[GeV/c ] involved coils only. However for values of the magnetic field above 4.2 tesla neighbouring magnets are also affected. The reasons for The B meson signal reconstructed from vided input by studying the spec­ this quench propagation at higher decays producing a proton and an trum of emitted in the de­ antiproton together with one or two pions currents are being investigated. seen by the ARGUS experiment at the cay of B mesons, providing an The reliability and easy operation German DESY Laboratory. upper limit for the ratio of the rel­ of the system demonstrated that evant CKM matrix elements, how­ it is ready for installation in the ever no significant evidence was HERA tunnel. Series production of found for the charmless decay of all required components is well a b quark to a u quark. underway and the schedule is still Beauty without charm At the recent Hamburg - in good shape. Symposium (see Septem­ All the beauty in a beauty (b) quark ber issue, page 4), the ARGUS does not live for ever. Eventually group working at the DORIS elec­ it will decay through the weak nu­ tron-positron collider at the German clear force, transforming into a DESY Laboratory in Hamburg re­ HERA injection charm (c) quark or possibly an 'up' ported the first evidence for the (u) quark. However the relevant charmless b/u transition from the In November, a 7 GeV elec­ transition rates cannot be predicted rare decays of a positively charged tron beam was successfully by the Standard Model of today's B meson into a proton, an antipro­ injected into the first com­ physics. They depend on funda­ ton and a positive pion, and a neu­ pleted portion of the HERA mental parameters - elements of tral B meson into a proton, an anti- electron-proton collider at the Cabibbo-Kobayashi-Maskawa proton and a charged pion pair. DESY in Hamburg, passing (CKM) matrix - which so far can The (preliminary) measured branch­ through the initial 130 metres only be measured from experi­ ing ratios are (3.7 ± 1.3 ± 1.4) of the 6.3 km ring. ments. x 10~4and (6.0 ±2.0 ± 2.2) x Many experimental groups pro­ 10~4 respectively.

CERN Courier, December 1987 21