SuperKEKB: back on track – better than ever
Particle accelerator SuperKEKB in Japan starts commissioning phase
After five years of upgrade work, the particle accelerator SuperKEKB at the Japanese research centre KEK has taken up operation again. In the first days of March 2016, the first stable beams of electrons and positrons were turning in the 3-kilometre-long ring. This is an important step towards producing particle collisions inside the similarly refurbished detector Belle II, which is still under construction by an international collaboration and will start operation in 2017. In the unprecedented large number of collisions of electrons and their anti-particles, physicists want to produce large numbers of B and D mesons as well as tau leptons. By studying very rare processes they hope to find new physics beyond the standard model of particle physics as well as an answer to the question why the universe consists largely of matter even though equal amounts of matter and antimatter should have been produced in the Big Bang.
In 2001, the Belle experiment at the KEKB accelerator and the competing BaBar experiment at the PEP-II accelerator in Stanford both succeeded in detecting an asymmetry in matter and antimatter, the so called violation of CP symmetry, in a rare decay of B mesons. That success led to the Nobel Prize in Physics being awarded to the Japanese theoretical physicists Makoto Kobayashi and Toshihide Maskawa in 2008. However, the amount of observed CP asymmetry is far too small to explain the dominance of matter in the universe. The upgraded SuperKEKB is expected to produce 40-50 times more collision data than its predecessor. This higher luminosity, which is a measure of the number of collisions, will be achieved using particularly powerful focusing magnets and other new components inside the accelerator.
These collisions are to be observed with the detector Belle II, which is currently being constructed at SuperKEKB by an international collaboration of almost 700 physicists from 23 countries from Asia, Europe and North America. Belle II will be equipped with a number of innovative components. European institutes are heavily involved in the construction of the innermost vertex detector, consisting of a novel silicon pixel detector with 8 million pixels and a read-out frequency of 50 kHz, surrounded by a four-layer double-sided silicon strip detector. A special feature of this experiment and the SuperKEKB accelerator is the asymmetry in the energy of the counter-running particles, colliding electrons at 7 GeV and positrons at 4 GeV energy. The resulting kinetic energy of the centre-of-mass system leads to a time dilation, which increases the observed flight path of very short-living B mesons in the laboratory system. The decay of the B mesons can be determined with a very high spatial resolution with the help of the vertex detector.
The construction of Belle II is planned to be completed in the next two years. With a well-defined initial state of the particles in these experiments, decays can be reconstructed and identified entirely, even if they contain non-detectable particles such as neutrinos. This makes Belle II an ideal and important counterpart to the LHCb experiment at the Large Hadron Collider at CERN.
