On 22 June, the SuperKEKB accelerator at the KEK laboratory in Tsukuba, Japan set a new world record for peak luminosity, reaching 3.1 × 1034 cm–2 s–1 in the Belle II detector. Until last year, the luminosity record stood at 2.1 × 1034 cm–2 s–1, shared by the former KEKB accelerator and the LHC. In the summer of 2020, however, SuperKEKB/Belle II surpassed this value with a peak luminosity of 2.4 × 1034 cm–2 s–1.
In physics operation since 2019, SuperKEKB is an innovative nanobeam, asymmetric-energy accelerator complex that collides 7 GeV electrons with 4 GeV positrons, sitting mostly on or near the ϒ(4S) resonance. It uses a large crossing angle and strong focusing at the interaction point (β*y = 1 mm), and has implemented a crab-waist scheme to stabilise beam–beam blowup using carefully tuned sextupole magnets on either side of the interaction point. These innovations have enabled the SuperKEKB team to attain record luminosities with rather modest beam currents: 0.8 A in the low-energy positron ring and 0.7 A in the high-energy electron ring – a product of beam currents 3.5 times smaller than were used at KEKB when its record luminosity was achieved.
SuperKEKB/Belle II is also reaching super-B-factory-level performance in integrated luminosity, achieving the highest values collected in a day (1.96 fb–1), in a week (12 fb–1) and in a month (40 fb–1). These are about 40% higher than the old records of KEKB/ Belle and about twice the level of SLAC’s PEP-II/BaBar, which completed operations more than a decade ago.
SuperKEKB team is making impressive progress towards an eventual target luminosity of 6.5 × 1035 cm–2 s–1Tom Browder
“Despite the challenges brought by the COVID-19 pandemic and necessary social-distancing protocols, the SuperKEKB team is making impressive progress towards an eventual target luminosity of 6.5 × 1035 cm–2 s–1,” says Belle II physicist Tom Browder of the University of Hawai’i. “The improving performance of SuperKEKB should enable Belle II to collect a large data sample to clarify the intriguing and potentially ground-breaking anomalies in the flavour sector, constrain the dark sector, and search for new physics.”