Upgraded B factory set to pursue new physics

Nearly two decades ago, the study of B mesons confirmed the largest source of charge–parity violation predicted by the standard model of particle physics. Now scientists are welcoming the next-generation B factory in the hopes it will reveal exotic physics beyond the standard model.

Last month the Belle II detector at the SuperKEKB accelerator in Tsukuba, Japan, recorded the facility’s first collisions of electrons and positrons. When SuperKEKB reaches its designed performance level, it will achieve the highest luminosity of any collider, yielding about 8 x 10³⁵ collisions per second in each square centimeter of scattering cross section. Over a decade, researchers should be able to examine the decays of up to 50 billion pairs of B–anti-B mesons, 50 times as many as the yield of the original Belle program, which ran from 1999 to 2010.

“There’s a lot of excitement about B physics,” says Belle spokesperson Tom Browder of the University of Hawaii at Manoa. In the coming years, Belle researchers will probe previously uncovered hints of tension with the standard model and attain precision measurements that could reveal more promising avenues in the unrelenting hunt for new physics.

The B-factory era began shortly before the new millennium to probe CP violation. Physicists had already observed slight differences in the decays of K and anti-K mesons, which contain a strange quark or antiquark. B mesons, which consist of a bottom quark or antiquark paired with a lighter partner, were more enticing targets because they are more massive than K mesons and were thus expected to violate CP symmetry more strongly (see Physics Today, May 2001, page 17 ).

Both Belle and the BaBar experiment, which was located at SLAC’s PEP-II accelerator, began data collection in 1999. Within two years both collaborations had attained convincing evidence of CP violation in B-meson decays, a discovery that netted theorists Makoto Kobayashi and Toshihide Maskawa a share of the 2008 Nobel Prize in Physics (see Physics Today, December 2008, page 16 ). BaBar’s experimental run ended in 2008.

As part of a $400 million overhaul funded by the Japanese government, SuperKEKB received a host of upgrades and new equipment, including a 3-km-circumference vacuum chamber to help in the preparation of the positrons and a superconducting magnet system to focus the particle beams. The accelerator smashes a 7 GeV beam of electrons into a 4 GeV beam of positrons, a collision that produces enough energy to form the γ(4S) particle, which promptly decays into a pair of B and anti-B mesons. The asymmetry in beam energies ensures that the generated mesons have significant momentum as they pass through the detector, enabling researchers to track the particles in the roughly 2 picoseconds between their production and decay. The Belle II detector includes new components that will improve both the tracking of the mesons and the identification of the decay products that follow.

The upgrades will enable physicists to dissect symmetry-violating decays more carefully and to determine whether there are any more that go beyond the scope of Kobayashi and Maskawa’s 1973 prediction. Such investigations could help physicists chip away at the burning questions of why and how matter outdueled antimatter just after the Big Bang.

B factories offer insights into more than just differences in the decays of matter and antimatter. As a B meson decays via the weak interaction, it can be influenced by virtual particles that are many times heavier than the mass–energy of the meson. As a result, scientists at the 11 GeV SuperKEKB facility can search for indirect evidence of particles that are even more massive than the ones physicists hope to produce directly at the 14 TeV Large Hadron Collider (LHC).

Belle scientists can look forward to exploring several intriguing hints left unresolved by the first generation of B factories. For example, Belle, BaBar, and the LHCb experiment at CERN have attained 4-sigma-level evidence that a particular B-meson decay (into a D meson, a tau, and a tau neutrino) occurs more frequently than the standard model predicts. One possible explanation is that the decay is influenced by a charged Higgs boson. In addition, LHCb data hint that the decay of a positively charged B meson into a K meson and a pair of leptons favors the production of electrons over muons—a disparity prohibited by the standard-model principle of lepton universality. If confirmed, that behavior could signal the influence of a heavy counterpart of the Higgs boson or the Z boson.

Researchers at SuperKEKB will be able to compare their results with those from LHCb, which has been tracking B decays amid the shrapnel of proton–proton collisions since 2010. With such different approaches to B-meson production, the two experiments complement each other nicely, Browder says. LHCb researchers have the advantage of higher energy and more B decays to analyze; the cost is having to sift through scores of unrelated particles that aren’t created in the relatively clean electron–positron collisions at SuperKEKB.

The new facility will also have something to say about strong interactions. In 2003 Belle scientists discovered X(3872), a perplexing particle that doesn’t seem to qualify as a meson or a baryon. B factories have since discovered a slew of similar particles, some of which behave as molecules of multiple mesons, others as cohesive tetraquarks (see the article by Steve Olsen, Physics Today, September 2014, page 56 ).

Belle scientists hope to begin probing all those areas of interest in February 2019, the expected start of the first full physics run. Until then, the researchers are focused on a commissioning run that goes through July and on further upgrades of Belle II’s innards in the late summer and fall.

Browder notes that SuperKEKB is the first particle collider to debut since the LHC a decade ago. The Belle collaboration of 750 researchers from 25 countries hopes the facility makes just as large an impact on the field—and perhaps leads to the discovery of more exotic physics than its far more energetic and expensive counterpart.