A joint Fermilab/SLAC publication

Record luminosity collisions due to "crab" crossings

05/11/09

When searching for rare particle events, brighter beams are better, and so the quest for higher luminosities is a key goal for accelerator and collider designers. At the KEK laboratory in Tsukuba, Japan, a recently installed upgrade to the KEKB electron-positron collider has set new luminosity records, showing the value of the new technology and putting the facility on an upgrade path to the SuperKEKB collider.

For many years, the KEKB/Belle collider and experiment in Japan and the PEP-II/BaBar combination at SLAC National Accelerator Laboratory, developed in parallel to explore the physics of B particles, revealing details of subtle asymmetries in nature among many other accomplishments. The BaBar experiment finished data collection last year but Belle continues.

Although the experiments were very similar, one key difference involves how the beams of electron and positrons interact. At PEP-II, collisions happen head-on inside the BaBar detector, which increases luminosity and has a few other advantages. The KEKB collider makes the beams cross at a small angle--22 milliradians, or about 1.3 degrees--which means that the beams separate cleanly after collision, leading to lower background noise for the detector and data analysis to eliminate. However, the angled crossing causes luminosity to suffer because the cigar-shaped particle bunches cross at an angle, meaning less chance for the individual electrons and positrons to annihilate in a burst of particle-creating energy.

Crab cavities tilt the electron and positron bunches so that they collide head-on at the interaction point. (Image: KEK)

Crab cavities tilt the electron and positron bunches so that they collide head-on at the interaction point. (Image: KEK)

KEK's innovation was to develop a pair of "crab cavities", which rotate the cigar-shaped bunches of particles so that they pass through each other aligned head-on. This means greater chances of collision and a higher luminosity. The idea was first proposed in 1989 and the first prototypes tested in 1992. After 15 years of development and improvement, KEK installed the crab cavities in 2007. The recent addition of some extra "sextupole" magnets to better steer the particles that don't have quite the same energy as the rest of the bunch gave the machine the boost in luminosity it needed to set a new world record, which is twice what KEKB was originally designed to achieve.

The combination of these new technologies, along with an upgraded collider and detector will form the SuperKEKB facility, designed to achieve 40 times current luminosities and opening physicists' eyes to super-rare particle events which could reveal new types of physics, never before observed.

Read the press release from KEK.

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