The Belle II detector records and analyses particle collisions produced by SuperKEKB.

The Belle II detector records and analyses particle collisions produced by SuperKEKB. (Photo: Shota Takahashi/KEK)

Belle II measures first particle collisions

The particle physics community has been waiting for this moment for a long time: On 26 April 2018 0:38, GMT+09:00 at KEK in Tsukuba, Japan matter and anti-matter particles collided for the first time in the new SuperKEKB accelerator. News of this success came from the detector, too: The Belle II instrument, which is also a new development, “saw” and recorded the particle tracks produced in the collision. Scientists hope the experiment will help them understand why the original balance of matter/anti-matter in the universe changed, so that the present universe contains almost exclusively matter.

What is the key to solving the matter/anti-matter mystery? Scientists are trying to find it in the decay patterns of short-lived particles, from B-mesons in particular, where a slight excess of matter can be observed.

B-mesons are pairs of quarks with a particular characteristic: One of the two quarks is a beauty (B-) quark or the corresponding antiparticle. B-mesons are produced when electrons and positrons (anti-electrons) collide and annihilate each other.

Search for special decays

SuperKEKB accelerates electrons and positrons circulating in opposite directions before they collide in the Belle II detector. Belle II records and analyzes the collision events. “The particle tracks must be measured very precisely if we are to detect decays that deviate from the norm,” explains Hans-Günther Moser from the Max Planck Institute for Physics (MPP). “This task falls to a high-sensitivity pixel detector, which is sited directly at the collision point in the center of Belle II.”

Eight years ago, upgrade measures began on the KEK accelerator and Belle detector in Tsukuba/Japan. The objective of this major project is to increase the yield of B-mesons by a factor of 40: During the next ten years, the SuperKEKB and Belle II duo is expected to produce and evaluate around 50 billion B-mesons. This enormous increase in data also enhances the chance of finding the sought-after decay pattern.

A ring accelerator on the home stretch

A crucial innovative feature of the SuperKEKB is a newly-designed positron ring and a complex system of superconducting magnets that keep the particle bunches on track. The new Belle II detector, whose functions are perfectly matched to the facility, will be commissioned at the same time as the upgraded accelerator.

A few weeks ago, one electron and one positron beam were fed in. Since then, scientists and technical staff have been working to align the particle beams to the collision inside the Belle II detector. Additionally, instruments built at the MPP are currently being used for measuring background signals interfering with future analyses. After this test phase, the final components, including the pixel detector in whose development the MPP has played a crucial role, will be installed and calibrated. The plan is for the scientific program to then start early next year.


Dr. Hans-Günther Moser
Max Planck Institute for Physics
Chair Institutional Board Belle II
+49 89 32354-248