Shortly after the dedication of the PEP-II B-Factory at the Stanford Linear Accelerator Centre (SLAC) on 26 October, a team led by John Seeman resumed the task of commissioning the new electronpositron collider.
The 9.0 GeV electron ring and the 3.1 GeV positron ring both turned on quickly, successfully storing beams before the end of the month. Collisions between the two beams, first achieved on 23 July occurred again on 10 November with 11 bunches in each ring. This time significant luminosity was observed, at about 3 x 1030 per cm2 per second – but still a factor of 1000 below the design goal.
Further commissioning included attempts to store much higher currents in each beam and focus them better at the interaction point, while improving their lifetimes. With the extensive progress already achieved on the electron ring, attention shifted to the positron beam, which had a lifetime of about 30 minutes in early November and slowly improved during the run. Radiation scrubbing of the vacuum in this ring permitted the stored current to reach 415 millliamps in a total of 291 bunches by run’s end. The current per bunch now exceeds the design goal of 2.1 amperes in 1658 bunches.
By run’s end a luminosity of 3 x 1031 per cm2 per second was measured with 261 bunches circulating in each beam. But calculations based on the beam sizes and currents indicate that the true value could be 2 to 4 times higher. Interaction region group leaders Stan Ecklund and Michael Sullivan are studying the reasons for the apparent discrepancy. Several ring parameters (e.g. tunes and emittances) were varied to maximize the luminosity, giving beam-beam tune-shift limits of about 0.01 to 0.02.
Physicists led by Tom Mattison of SLAC and Witold Kozanecki of Saclay monitored the backgrounds in both rings during this run. These backgrounds believed to be largely due to beamgas interactions are 5 to 10 times higher than anticipated in the PEP-II conceptual design report. Although the BaBar detector will be able to handle such backgrounds, they are still a cause for concern. Work continues to understand these backgrounds and reduce them. Additional beam collimators are being installed before the next commissioning run.
The current schedule calls for a final commissioning run from mid-January to mid-February followed by installation of the BaBar detector at the interaction point. If all goes well, physicists in this collaboration can expect to begin taking data on 1 May. “We are very pleased with this progress, ” said Seeman, “but we must keep a firm eye on our goals.”
