KEK’s B-factory upgrade promises a 40-fold increase in luminosity.
What is dark matter? Why is there more matter than antimatter in the universe? These are some of the most fundamental puzzles in modern particle physics. The clues to the answers might reside in the physics of the symmetry breaking of electroweak forces at tera-electron-volt scale. Projects at the energy frontier, such as CERN’s LHC, are complementary to the approach of flavour physics, where measurements of rare processes at lower energies can be sensitive to new types of interaction. SuperKEKB will provide a promising path to such new physics by enabling detailed studies of the decay processes of heavy quarks and leptons.
From 1998 to 2010, KEK, the Japanese High-Energy Accelerator Research Organization, operated its B factory, KEKB – an asymmetric electron–positron collider, 3 km in circumference – and achieved the world’s highest luminosity of 2.11 × 1034 cm–2 s–1, more than double the design value. There, the Belle experiment precisely analysed the characteristics of pairs of B and B mesons produced in the collisions and confirmed the effect of CP-violation as indicated by the theory of Makoto Kobayashi and Toshihide Maskawa, who both received the Nobel prize in physics in 2008.
KEK has played a central role in flavour physics not only with the record-breaking KEKB but also with the world’s first long-baseline neutrino oscillation experiment, KEK-to-Kamioka (K2K). Now, because large facilities are increasingly required for experiments in particle physics, a new style of international competition and co-operation has become necessary between institutes around the world. Together with the Japan Proton Accelerator Complex, which is a high-intensity proton facility built jointly with the Japan Atomic Energy Agency, KEK is continuing to play a leading role as one of the international focal points in flavour physics. As an international collaboration, SuperKEKB is open to the world and currently more than 400 physicists from 60 institutions from 17 countries and regions are working together to upgrade the Belle experiment to Belle II.
Forty times more collisions
SuperKEKB is essentially an upgrade project to increase the luminosity to 8 × 1035 cm–2 s–1, or 40 times greater than at KEKB. Engineering a higher luminosity in a collider involves both increasing the beam current and reducing the beam size at the interaction point. The original approach for the upgrade was to increase the beam current and the beam–beam parameter – the “high current option”.
In March 2009 the SuperKEKB design changed course based on ideas from SuperB, using a large crossing-angle at the interaction point and beams squeezed to nanometre-scale to increase luminosity – the “nano-beam option”. The scheme has the advantage of reaching 8 × 1035 cm–2 s–1 with only the double the current, but that is not to say that there are no other challenges.
The high current (3.6 A in the 4 GeV positron ring, 2.6 A for the 7 GeV electron ring) will lead to “electron cloud” effects in the positron ring when synchrotron radiation hits the walls of the beam pipe. To counteract such phenomena, SuperKEKB will use a special vacuum chamber for the positron ring with two small “antechambers”, one on the outside of the main beam pipe and one on the inner side. One suppresses the formation of electron clouds and the other contains vacuum pumps. In addition, the chambers between magnets will be wrapped with solenoid coils to reduce the effect of secondary electrons. Test antechambers developed at KEK from a prototype produced in collaboration with a team at the Budker Institute of Nuclear Physics have already been tried out in sections of the KEKB ring.
The new design will be based on a larger crossing-angle (4.8°), requiring a redesign of the final-focus with eight higher-gradient quadrupole magnets placed nearer the interaction region to squeeze the beam to nanometre scale. These are unusually small superconducting quadrupole magnets, whose inner diameters are only 4–8 cm, or one-sixth the diameter of the KEKB quadrupoles. These magnets require current controls to protect them from quenches because the current density in the superconductor can go beyond 2000 A/mm2 and on quenching the temperature will rise to more than 1000 K within around 50 ms. In addition, the accelerator design demands much smaller fabrication errors to ensure a quadrupole field quality of a few 10–4.
SuperKEKB will provide a promising path to new physics via detailed studies of the decay processes of heavy quarks and leptons.
Therefore, while SuperKEKB will be based on KEKB, various parts of the accelerator will be replacements. Others will be new. A new positron source and a flux concentrator will help in generating a high-current positron beam and a new damping ring (135 m in circumference) will be added to reduce the emittance of the positron beam. Moreover, as well as changing the beam pipe in the main positron ring for one with “antechambers” as described above, the dipole magnets in this ring will be replaced. A new RF system will also help in accelerating high-current beam.
Because the SuperKEKB accelerator will produce electron–positron collisions at a much higher rate, the detector will also need to be upgraded. The aim is to accumulate an integrated luminosity of 50 ab–1 by 2021, which is 50 times more data than the previous Belle detector acquired. Thus the Belle II detector will also be an upgrade. The data-acquisition system will be redesigned with a network of optical fibres. Trigger electronics will be replaced with a new system. A pixel detector will be added for better resolution of particle tracking and a silicon vertex detector will cover a larger solid angle. A central tracking chamber, a time-of-propagation chamber and an aerogel ring-imaging Cherenkov detector are also being newly built.
The first beam of SuperKEKB is expected in 2014 and the physics run will start in 2015. Ultimately, Belle II should collect 40 times more B-meson samples per second than its predecessor – roughly 800 BB pairs per second. This will allow the Belle II collaboration to examine the effects of unknown particles in a higher energy region in the search for clues to new physics. Belle II at SuperKEKB will form one of the international focal points for particle physics in the coming decade.