Ready for the Bs

29 April 1999


In February, physicists at SLAC, Stanford, successfully finished commissioning the PEP-II B-factory. A few weeks later, engineers and technicians rolled the 1200 ton BaBar detector into position at the interaction point of this dual-ring electron­positron collider. With the insertion of its vertex detector and final checkout scheduled, everything seems ready for the long-anticipated start of the first physics run.

During this final commissioning phase, a team of physicists led by John Seeman gradually nudged the 3.1 GeV positron beam up to very high intensities ­ several times exceeding a current of 1 A. To do so they managed to circulate more than 1500 positron bunches simultaneously, countering the deleterious effects of multibunch instabilities using a sophisticated fast-feedback control system.

At one point the current reached 1.2 A, which is thought to be the highest current ever achieved with an antimatter beam. Its lifetime is fairly short ­ typically less than 1 h ­ but continued “scrubbing” of the ring’s vacuum chambers by circulating the positron beam should eventually resolve this limitation.

Bringing the positron beam into collision with the 9.0 GeV electron beam, the commissioning team achieved a peak luminosity of 5.2 x 1032/sq. cm/s in late February. This is more than 17% of the design value ­ an encouraging feat at this early stage in the life of the new collider.

Design intensities have already been achieved in single-bunch operation. “There are no single-bunch issues remaining on PEP-II, ” remarked Seeman. “It is all about multibunch issues now.” A remaining area of concern is the high background levels encountered during commissioning, which are factors of 5-10 greater than what was anticipated in early simulations. A group of BaBar physicists led by Tom Mattison and Witold Kozanecki has been studying these backgrounds in detail, with an eye to reducing them or coping with them during the experimental runs. Better collimation of the beams well upstream on either side of the detector is one likely solution.

Physicists from the BaBar collaboration commissioned their detector ­ minus its vertex detector and most of its differential Cherenkov detector ­ on cosmic rays in late 1998 and early 1999. The on-line software package has come along smartly in recent months, enabling the reconstruction of cosmic-ray events.

One continuing problem is an unfortunate delay in obtaining enough quartz bars of sufficiently high quality for use in the Cherenkov detector, which begins the first run with only five (out of twelve) bars in place. Thus an idea proposed more than a decade ago by Lawrence Berkeley Lab’s Piermaria Oddone ­ that an asymmetric electron­ positron collider would prove a superb facility for studying CP violation in the B meson system ­ is about to become reality at SLAC (and across the Pacific at KEKB at the Japanese KEK lab too).

It has been a long, twisting and demanding road, deftly navigated by PEP-II project director Jonathan Dorfan, BaBar spokesman David Hitlin and many others. In the months to come, high-energy physics should begin reaping the benefits of all of the diverse efforts that have gone into this imaginative, challenging project.

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