With a peak luminosity that now exceeds 1.3 x 1034/cm2/s, KEKB, the KEK B-factory, is delivering more than 1 fb-1 per day to the Belle experiment. This peak luminosity is equivalent to the production of 14 B-meson – anti-B-meson pairs every second, and Belle is now accumulating approximately one million B pairs every day.
KEKB, which consists of an 8 GeV electron ring and a 3.5 GeV positron ring, started operation in 1999. Since then the performance of the facility has been steadily improved by increasing the currents of the electron and positron beams that are stored in the rings, and by using solenoidal coils wound over the entire positron ring to suppress the photoelectron cloud that was previously producing an instability. Since January this year the machine has been operating in a “continuous injection mode”, where beam particle losses are compensated by injecting beam from the linac injector without interrupting data taking at Belle. This new mode of operation has successfully enabled KEKB to deliver 30% more integrated luminosity to Belle, and led to the new record of 1 fb-1 per day. Belle has already accumulated a total of more than 260 fb-1 since the beginning of the experiment.
Thanks to this huge data rate, Belle reported charge-parity (CP) violation in the B-meson system in 2001, at the same time as the BaBar experiment at SLAC, and has continued to improve the precision of sin2φ1 (sin2ß), the fundamental CP violation parameter of the Standard Model. In addition, last year Belle measured a value for the CP violation parameter in B → φKS decay that differs from the Standard Model prediction by 3.4 standard deviations. A more precise measurement based on more data will be reported this summer. Since a deviation from the Standard Model prediction for this parameter would be an unambiguous indication of new physics, Belle’s new result is eagerly awaited by the particle-physics community.
