„Antimatter search engine“ Belle II is now complete

Final piece inserted into detector in Japan before commissioning

Precision work: the international research team installs the vertex detector in the Belle II experiment. Credit: KEK

Normally this is where the captain sleeps: detector components on their way to Japan. Credit: DESY, Carsten Niebuhr

At the Japanese research centre KEK, the last component has been inserted into the Belle II particle detector, which has been completely upgraded over the last few years and is scheduled to start up in spring. The final piece, the vertex detector, consists of two independent parts that now complete the overall detector. Belle II is specifically designed to look for physical phenomena in the particle collisions at the upgraded SuperKEKB collider that extend beyond the previously explored realms of physics. It specialises in measuring rare particle decays, for example that of so-called beauty quarks, charm quarks or tauons. Scientists hope that this will help the to unravel the mysteries of dark matter and the imbalance between matter and antimatter in the universe, and to track down new phenomena.

The inner part of the vertex detector is a novel pixel detector developed and built by twelve institutes in Germany. This highly sensitive detector consists of two half-shells and is only about the size of a soda can. It has been extensively tested at DESY over the past few months. The two half-shells survived their long-distance flight from Germany to Japan carefully wrapped up and stored in the captain's sleeping cabin directly behind the cockpit.

Having safely arrived at KEK, the pixel detector was mounted on the beam pipe through which the fast particles of the SuperKEKB collider will travel and combined with the rest of the vertex detector, which was assembled in Japan. Everything is small and tight that close to the collision point of the particles, so wiring the detector is a big challenge. The inner beam pipe will be connected to the rest of the accelerator only after it has passed a big system test. This step is planned for mid-January.

Over the last years, the electron-positron collider SuperKEKB has been upgraded so that soon it will produce more particle collisions than ever before in the world. This means a lot of pressure for particle detectors in general, because they are exposed to high radiation levels and have to process many events at once.

However, it’s especially arduous for those parts of the detector that are located exactly around the point where particles collide. There’s a lot going on - in mere fractions of a second, particles collide, form and decay, then fly further into the outer shells of the detector. The vertex detector now installed sits right there and will be able to tell scientists with maximum precision exactly where the collision took place and when and where the particles formed during the collision decayed.

The researchers are especially interested in so-called B mesons, i.e. particles that contain a beauty quark or an anti-beauty quark. The way they decay could for example provide the key to the open question, why antimatter has disappeared from the universe and where to.

More about Bellle II: https://belle2.desy.de