News & Analysis Physics World  March 2018

Revamped particle accelerator set for first collisions

Ready to go An upgrade to the KEKB collider will increase the number of collision events by a factor of 40. (CC BY SA Belle II Collaboration )

First collisions at an upgrade to one of Japan’s premier particle-physics experiments are set to begin next month. Following six years of work, the SuperKEKB accelerator will start smashing its first electrons and positrons together, heralding a new era of particle physics at the KEK particle-physics lab in Tsukuba.

SuperKEKB is a ¥29bn ($370m) upgrade of the 3 km circumference KEKB collider, which consists of two circular accelerators – one carrying electrons and the other positrons. KEKB was shut down in 2011 for construction to start on SuperKEKB, which involved upgrading the beampipes to allow SuperKEKB to produce electrons with an energy of 7 GeV, while the positron beam has an energy of 4 GeV.

The current of the electron ring has also been ramped up from 1.2 A to 2.6 A, with the positron ring boosted from 1.6 A to 3.6 A to increase the number of collision events by a factor of 40 over KEKB. The beam “spot” will be just 50 nm high at the collision point. The upgraded accelerator is designed to pump out far more B mesons (around 50 billion pairs) as well as other particles such as D mesons and tau leptons that could shed further light on why the universe contains more matter than antimatter.

The original KEKB facility included a detector called Belle that allowed physicists to study the remnants of the particle collisions. It has also been upgraded, and the renamed Belle-II can handle the huge increase in the collision rate and survive the radiation damage caused by the flux. Key to this design is the inner vertex detector, which has four layers of conventional silicon strips as well as two layers of pixel detectors made from a depleted P-channel field effect transistor (DEPFET). This material, it is hoped, should make the detector better at pinpointing where particles decay.

“Belle II is a unique universal spectrometer with full capabilities for detecting charged particles, photons and neutrinos with high efficiency,” Belle II’s Thomas Browder from the University of Hawaii told Physics World. “This will allow unprecedented sensitivity to the full range of new physics in the ‘flavour sector’”.

Collisions will now run for six months to refine the accelerator and detector. The vertex detector will be installed in Belle II by January 2019, with the full physics programme starting a month after that.

Michael Banks