Accelerator physicists at the SuperKEKB electron–positron accelerator in Tsukuba, Japan, are celebrating their December 2024 world-record luminosity of 5.1 × 1034 cm−2 s−1. At the same time, they are scratching their heads about how to reach their target luminosity, which is roughly an order of magnitude higher. Success has implications both for Belle II, the onsite experiment that studies B mesons and other particles, and for future electron–positron colliders.
The researchers’ two-pronged approach to increasing luminosity is conceptually simple: “First, we put in more particles, and then we squeeze the beam,” says Mika Masuzawa, a leading accelerator physicist at SuperKEKB. In practice, though, it’s anything but, she notes. The aim is to use powerful magnets to squeeze the beams to about 50 nm in the vertical dimension and create a so-called nanobeam. So far, they’ve gotten down to 260 nm. For comparison, conventional beam sizes are on the order of microns.
Higher luminosity means more particle collisions per unit time and thus faster data accumulation. From the “narrow perspective” of the Belle II experiment, “we want more luminosity to do more physics,” says Thomas Browder, a professor at the University of Hawaii at Manoa who represents US universities in the experimental collaboration. “From the perspective of future accelerator projects, they have to see that nanobeams are not a dead end.”
The promise of nanobeams
SuperKEKB consists of two 3-km rings, with 4 GeV positrons circling in one and 7 GeV electrons in the other. The resulting collision energy is a sweet spot that yields B-meson pairs, allowing for the study of their various decay pathways and products. Both the experiment and the accelerator were upgraded before starting up in 2019—Belle II is the follow-on to the Belle experiment, and SuperKEKB had an earlier, lower-luminosity incarnation as KEKB.
For its upgrade, Belle II was outfitted with more-sensitive detectors and with new software that makes the experiment more robust against beam-related background signals. “The only part we retained was the crystal calorimeter,” says Browder. It’s difficult to pinpoint the cost of the upgrade, he says, because many contributions were in kind, and they came from many countries. About 700 researchers from 123 institutions in 28 countries make up the Belle II collaboration.
The SuperKEKB upgrade was mainly to introduce nanobeams. The Japanese government footed the bill, ¥31.4 billion (now roughly $225 million). In addition to getting higher luminosity for the same amount of current, the nanobeam approach, if it works, will use less power for a given luminosity. That, notes Browder, is significant: Electricity costs have surged in Japan in recent years, starting with the 2011 Fukushima Daiichi nuclear disaster (see Physics Today, May 2011, page 18, and November 2011, page 20), and more recently because of the COVID-19 pandemic and the war in Ukraine. And the value of the yen has nose-dived. All those factors have led to a curbing of run times for SuperKEKB.
For the most part, says Browder, SuperKEKB can squeeze a single positron or electron beam. But when two squeezed beams interact, they blow up and grow several times larger in diameter.
Other concerns about the accelerator include low injection efficiency, according to CERN’s Frank Zimmermann, who in January chaired the annual international review meeting for SuperKEKB. “The injected beam is much larger than the design value,” he says, “which makes further squeezing at the collision point difficult.” Sudden beam loss, which aborts a run and can damage both accelerator components and the detector, is another ongoing problem at SuperKEKB.
Nanobeams are difficult, Browder says. “There are many unanticipated problems in the hardware, and there are new accelerator phenomena in the beam–beam interactions at nanoscales.” For now, SuperKEKB is the only particle accelerator that is actively working on nanobeams. “We are concerned about the progress,” says Belle II spokesperson Karim Trabelsi, a researcher at the CNRS in France, “but we think the accelerator team is on the right track.”
Cracks in the standard model
The Belle II team needs higher luminosity to increase the collision rate in order to spy rare events, infer the existence of dark-matter particles, and make precision measurements to glimpse deviations from theory. “We need much larger statistics than have previously been available,” says Trabelsi. “The idea is to have a huge amount of data on forbidden decays—decays not allowed by the standard model—which would be signs of new physics,” he says. “And Belle II can do a good job in the dark sector because of the clean positron–electron environment. We can study all the signatures.”
Peter Križan is a Belle II researcher based at the University of Ljubljana in Slovenia. He notes that the decay of a B meson to a kaon, a neutrino, and an antineutrino has been observed at Belle with higher-than-expected probability. “It’s super exciting. It’s a crack in the standard model,” he says. “But it’s not conclusive. We need more data.”
CERN’s Zimmermann and accelerator physicists from other facilities are troubleshooting with the SuperKEKB team. “We are trying to help them measure and correct their optics, simulate beam–beam effects, and compute beam losses around the ring,” says Zimmermann. With a new software package developed at CERN, he says, simulations can optimize collimator settings, for example. “In principle, with our model, we could help in many ways.”
Among the recommendations that Zimmermann’s review panel made in January are for the SuperKEKB team to explore shaping the incoming beam phase space by using nonlinear magnets. The panel also said that the team should continue investigating sudden beam loss “until one or more physical reasons and mechanisms have been found and verified beyond doubt.” Another recommendation is to “develop accelerator conditions” such that Belle II can restore the use of one of its key new detectors, which was turned off to protect it from sudden beam loss events.
The SuperKEKB accelerator team has cycled through various possible explanations for sudden beam loss. Accelerator physicist Masuzawa is confident that the team has identified the culprit: dust from a goopy vacuum sealant. “We cleaned the area, and the sudden beam losses almost disappeared,” she says. Zimmermann says that he is hopeful but not yet convinced that the sealant is the sole explanation.
Mastering nanobeams at SuperKEKB would also benefit future projects like the Future Circular Collider (FCC) that CERN envisions. The FCC would be about 90 km in circumference and, in its initial electron–positron incarnation, would operate at collision energies up to 365 GeV. A similar project in China, the Circular Electron Positron Collider, would also require nanobeams. (See Physics Today, September 2020, page 26, and “China plans a Higgs factory,” Physics Today online, 17 December 2018.)
“It’s better to understand the problems at SuperKEKB, but it’s unlikely that the FCC would have the same problems,” says Zimmermann. If SuperKEKB achieves a 50 nm vertical-beam height, that would be excellent, he adds, but if the collaboration doesn’t reach its nanobeam goals, “it doesn’t necessarily bode poorly for future machines, although it could be bad for public perception.”
Still, particle and accelerator physicists see nanobeams as a must for such future machines. Keeping power use in check, says Browder, would be necessary to limit electricity costs, prevent melting components, and maintain a reasonable carbon footprint.
