The Belle experiment at the KEKB asymmetric-energy electron-positron B-meson factory recently completed a successful first year of operation. The KEKB luminosity, which was about 1031 cm-2s-1 at start-up in June 1999, reached 2 × 1033 by the end of July 2000. This was achieved with only a fraction of the design values for beam currents in each ring, so the prospects for ultimately achieving the design goal of luminosity of 1034 seem good.
The beam currents were limited by the excessive heating of some accelerator components. These are being replaced by more robust versions during the current shutdown. In addition, four more superconducting radiofrequency accelerating cavities are being installed in the high-energy electron ring. With these improvements there should be no technical problems to limit the beam currents below design values.
The luminosity was also limited by an instability that causes the low-energy positron vertical beam size to grow at high-beam currents. This beam blow-up problem may be caused by photoelectrons produced by synchrotron radiation X-rays hitting the walls of the vacuum chamber and attracted to the positively charged positron beam bunches.
This problem is also being dealt with. Wire coils being wound round the positron beampipe will provide a weak solenoid field that will curl photoelectrons back into the vacuum chamber wall soon after they are produced. Simulations indicate that these will be effective at suppressing photoelectron interactions with the beam. Happily, many of the novel design features of KEKB appear to be verified. Beam-beam limits to the luminosity have not yet been observed and no deleterious effects caused by the finite beam-crossing angle are evident. This crossing angle scheme, in addition to KEKB’s ability to produce high luminosity at relatively low total beam currents, reduces beam-induced background radiation levels in the Belle detector to reasonably comfortable levels.
At the ICHEP 2000 meeting in Osaka (September p5), the Belle group reported preliminary results based on most of its total data sample of more than 6 million B meson particle-antiparticle pairs. It submitted 17 papers, including a number of new results. A clear signal was reported for the interesting B decay into phi and a negative kaon, in which a b quark gives three strange quarks (a gluonic “penguin” process). First observations of charged and neutral B meson decays to J/psi and K1(1270) were reported. These modes may provide new possibilities for future CP violation studies. Belle’s high-quality particle identification system was exploited to make almost background-free first measurements of Cabibbo-suppressed B decays into D* and a kaon.
Other results included competitive measurements of D0 and Ds lifetimes, the neutral B mixing parameter, exclusive B meson semileptonic decay modes, angular correlations in B decays into J/psi and K* and the best limit so far for the rare decay B to rho and a photon. This latter result is interesting because it provides an important constraint on the elusive Vts element of the Kobayashi-Maskawa matrix.
A number of rare decay measurements are dominated by backgrounds from continuum quark-antiquark production processes. Belle did not accumulate much off-resonance running, so new analysis techniques were developed that enable B meson decays to be separated from these continuum events. These techniques enabled Belle to report measurements at interesting levels of precision for rare B decays into a kaon and a pion, into a K* and a photon, and the inclusive photon radiative penguin process.
In a plenary session talk, Belle spokesperson Hiroaki Aihara of Tokyo reported Belle’s first results on the relevant CP-violating parameter as 0.45 + 0.44 – 0.45. Although Belle’s current data sample is only about half that of BaBar’s at SLAC, Belle managed to get a competitive measurement by including many CP eigenstate decay channels, including the important but experimentally challenging J/psi and long-lived kaon B decay.
Although the precision of the Belle measurement is not yet sufficient to make a definitive statement about the Kobayashi-Maskawa theory, the results are important in that they demonstrate that Belle is capable of doing the measurements that it was designed to do.
Belle’s research activities are not confined to B-mesons. Three of the Osaka papers deal with tau lepton physics: two on searches for CP violations in the lepton sector and one on a search for lepton-flavour-violating tau decays. In addition, Belle’s particle identification capabilities are being exploited to make interesting measurements of the two-photon production of kaon pairs. Although all of the results are preliminary – some of them are based on data that were collected only a few days prior to the start of the conference – and there is still lots of work to be done before they can be published, they demonstrate that all components of the Belle detector and the data analysis software are functioning at near design levels. More important is that they demonstrate that the Belle collaboration is up to the job at hand and able to produce new and interesting results in a timely manner.
