Antimatter Matters: Making Sense of the Matter–Antimatter Asymmetry at Japan’s Belle-II Facility


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The Belle II Physics Book

E. Kou, P. Urquijo et al.
Prog. Theor. Exp. Phys. 2019, 123C01 (2019).

Scientists embark on a quest to unravel the mysteries behind matter–antimatter asymmetry in our universe with the new Belle-II experiment at the SuperKEKB accelerator in Japan.

The Standard Model of physics, while extremely successful, cannot answer several fundamental questions about nature, such as the observed imbalance of matter over antimatter in our universe, better known as matter–antimatter asymmetry.

Thus, in search of new physics beyond the Standard Model, while also making more precise measurements of known phenomena, the new Belle-II experiment was designed in 2016 at Japan’s SuperKEKB accelerator. Belle-II is successor to the Belle experiment, which saw a collaboration of over 400 international physicists and engineers, at that very accelerator.

Just why is there so much more matter than antimatter in our universe?

The Belle experiment attempted to answer this question by measuring a type of charge parity (or CP) symmetry violation—which is a necessary condition for matter–antimatter asymmetry—predicted for B-meson decays by the Kobayashi-Maskawa theory. But this effect was too small to explain the observed dominance of matter. It was necessary to measure CP symmetry violations caused by new particles and mechanisms.

But, the best way to clearly identify the effects of new particles is to investigate decay modes that are suppressed in the Standard Model and occur rarely. This is where the Belle-II experiment comes in. It includes the powerful SuperKEKB accelerator and Belle-II detector that scientists need to generate and detect a large number of B-meson decays for reliable data.

Compared to Belle, Belle-II collects data from the SuperKEKB accelerator at a 40 times higher “luminosity” or, equivalently, collision rate, allowing for a 40 times higher event rate. In addition, its improved detector efficiency enhances the sensitivity by a whole order of magnitude compared to the current measurements, and by two orders of magnitudes in very “clean search” measurements.

The Belle-II experiment is, thus, an ambitious attempt to identify new physics phenomena that will hopefully bring us closer to understanding how our universe managed to get rid of antimatter, allowing matter and life to flourish and lead, eventually, to us.

The Belle II Physics Book

E. Kou, P. Urquijo et al.
Prog. Theor. Exp. Phys. 2019, 123C01 (2019).

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