Three months into run 4 and the PEP II accelerator, the electron-positron collider at SLAC, is performing beautifully. Recent modifications of PEP II’s hardware and operations have allowed it to maintain more intense beams, and it looks to be on course to reach the ambitious goal for run 4: 100 inverse femtobarns. If this goal is reached, the data sample from the first three runs of the BaBar detector will be almost doubled by July 2004. The detector recorded some 125 million BBbar pairs between October 1999 and July 2003, but the physicists are eager for more.
New equipment is one key to the improvements. An eighth radiofrequency (RF) cavity has been added to the accelerator, allowing more particles to be stored in the ring. Another improvement was to solve the problem of unwanted electrons in the positron ring, which are kicked loose from the beam pipe by synchrotron radiation. Their effect is to diffuse the tightly packed positron beam, thus lowering the chance of collisions with the electron beam in the detector. So technicians spent a number of gruelling weeks in a hot tunnel, winding narrow wire tape around every accessible part of the beam pipe in the positron ring. The windings created a solenoid magnet that traps the slower electrons and keeps them out of the positrons’ way. Maintenance has been another important ingredient. Over the summer, a vacuum leak in the interaction region was quickly repaired by the Mechanical Fabrication Department, and the Accelerator Maintenance RF group overhauled the entire RF system.
New ways of operating the accelerator have also started to pay off. Previously, PEP II operated with two empty buckets following each filled one. In autumn 2003 the pattern was changed: strings of buckets in which every other one is filled, alternate with shorter strings of empty buckets. Each change to the spacing between bunches affects the beams’ behaviour and the new pattern has opened up empty slots to which more particles can eventually be added.
A new approach to keeping the rings full was adopted at the beginning of December. As the beams collide their intensity gradually declines, and previously it was necessary to “top off” the beams by injecting new particles every 50 minutes or so. During the 5 or 10 minutes required for injection, the detector had to be shut off to avoid the risk of radiation damage. Now a new “trickle injection” scheme in the positron ring adds small pulses of particles as soon as the buckets begin to be depleted, maintaining the beam at full brightness around the clock. This approach has a double data payoff: the collision rate does not fall off and, as the detector is desensitized for much less time, it can record up to 20% more events.
