A team of accelerator physicists at KEK has achieved effective head-on collisions of electrons and positrons while retaining the crossing angle. They accomplished this at the KEKB collider using new devices called "crab cavities". This success will pave the way to increasing KEKB’s luminosity – already the world’s highest – to an unprecedented level.
At KEKB, the electron and positron bunches cross at an angle of 22 mrad (1.3°). This non-zero crossing angle is one of the machine’s novel design features: it provides effective beam separation at the collision point without a high level of background in the detector. However, it is necessary to tilt the bunches of electrons and positrons so that they collide head-on while still crossing at an angle to boost the luminosity further.
To accomplish this goal, the team at KEKB built several "crab cavities", special superconducting radio-frequency cavities that tilt each bunch sideways – somewhat like the way a crab walks. The concept was first suggested by R Palmer almost 30 years ago for linear electron–positron colliders, and K Oide and K Yokoya proposed the use of crab cavities in storage rings around 10 years later (Oide and Yokoya 1989). This was followed by designs and prototype models of the crab cavity by K Akai as part of a collaboration between the KEK and Cornell laboratories around 1992. Detailed engineering and prototyping were then done at KEKB by K Hosoyama’s team, with the first full-size cavities being developed after a long struggle and installed in January 2007 (figure 1 and 2). Commissioning at KEKB started in February and continued until the end of June (Abe et al. 2007.)
The crab cavities achieved a tune shift at low currents, comparable to the record achieved at LEP-II at CERN (figure 3). Proportional to the luminosity divided by the product of beam currents, this is a measure of luminosity potential. The team was recently able to operate the machine at high beam currents (1300 mA in the low-energy positron beam and 700 mA in the high-energy electron beam) at a luminosity above 1034 cm–2 s–1.
These results from the first round of commissioning demonstrate the potential of the crab cavities, which according to simulations may eventually improve the luminosity by a factor of two. More commissioning runs and R&D will enable further increase in performance.
The KEKB collider operates at the Υ(4S) resonance and is used for studies of matter–antimatter asymmetry with beauty quarks and searches for new physics by the Belle experiment (CERN Courier June 2006 p22). For the future, a super B-Factory using crab cavities to achieve luminosities two orders of magnitude higher than existing accelerators is under discussion in Japan, and a competing proposal is also being discussed in Italy. Such a machine has the potential to discover physics beyond the Standard Model in rare decays. Crab cavities will also play a role in achieving high luminosity at other machines with a crossing angle, including the proposed International Linear Collider, upgrades of the LHC at CERN, and future synchrotron light sources.