SUMMER CONFERENCES: Heavy on flavour

Afocus of attention at the major international high energy physics conferences this summer in Brussels and in Beijing was the latest batch of precision information from major experiments at electronpositron colliders - the four big detectors at CERN's LEP storage ring and the SLD experiment at the SLC linear collider at SLAC (Stanford). These experiments study the decay of the Z particle - the electrically neutral carrier of the weak nuclear force - produced when the colliding electron and positron beams are tuned to the Z resonance. This precision data is a stringent test of the six-quark Standard Model, and as the weight of evidence builds up, physicists look hard for any cracks in the theoretical foundations. In 1994, the LEP experiments almost doubled their accumulated score of Z particles (an integrated luminosity of 64.5 inverse picobarns in 1994 compared with 93.5 in the previous 4 years). In addition to the increased mass of data, improved precision came from better determinations of key parameters (beam energy, luminosity, electromagnetic coupling strength,....). SLD Z data has more than doubled over the past year. SLC also provides spin oriented (polarized) beams and the machine's polarization level has improved from 63 to 77%. The intercorrelation of the different parameters of the six-quark Standard Model was also boosted this year by the discovery of the sixth ('top') quark at Fermilab's Tevatron proton-antiproton collider (April/May, page 1). In the electron-positron sector, although the LEP experiments provide the mass of the data, the SLC's polarized beams mean that the delicate asymmetries seen in SLD provide the most precise single measurement of the vital electroweak mixing parameter. Last year, it was difficult to reconcile these SLD asymmetry results from those from LEP, and some people were whispering about possible nonconformist physics effects, but with a year's additional data, the gap between the two sets of results has narrowed. To correlate contributions from the different experiments, LEP Working Groups take results from all four - Aleph, Delphi, L3 and Opal - to furnish a useful pan-LEP result. Precision information comes from analysing the different decay channels of the Z - giving various kinds of lepton pairs and different patterns of quark flavour. In particular the asymmetries of lepton pairs (electrons, muons or taus) or quarkantiquark pairs reflect the interference of different weak effects. In this precision work, the spin orientation (polarization) of the tau lepton provides a useful additional handle. With the decays of the Z into heavy quarks providing especially useful information, a special working group pulls together results in this sector from the four LEP experiments and from SLD at the SLC. The Standard Model predicts exactly the Z decays into different quark-antiquark pairs. Each of the four LEP experiments has so far seen about half a million examples of Zs decaying into B particles containing the fifth (b) quark. Combining these with results from SLD, the fraction of Z decays into B pairs, compared to decays into all quark flavours, is slightly too high for Standard Model comfort - 22.19% compared with an expected 21.56%. At the current level of LEP/ SLC precision, this disagreement of about half a percent is practically a yawning gap. Already some physicists are talking about a possible breakdown of the Standard Model foundations, which have so far remained firm for more than a decade