The High Energy Accelerator Research Organisation (KEK) in Japan is getting ready to launch the Belle-II experiment, a massive collaboration of 700 scientists from across the globe. At Belle-II, highly intense electron-positron beams will be made to collide and a huge number of B-mesons (a boson containing the B, or beauty, quark) produced. Building a detector to observe the resultant decay products is a challenging task and that is one area where Indians have contributed significantly.
Indians have been involved in the preceding experiment, Belle, for decades now, however, with Belle-II, their engagement is deeper. “Initially we did not have the chance to build the detector, and this is the second step – to work with the inner part of the detector, where the resolution has to be high. We are happy we did it and are now among four important groups in the world that can build such detectors,” says Tariq Aziz of Tata Institute of Fundamental Research, Mumbai, who led the effort along with Gagan Mohanty.
Crucial folding
Indians built the fourth layer of the six-layer silicon vertex detector and developing the analysis and theory. The highly miniaturised sensor engineering and the “origami chip-on sensor” design of the readout chip, which improves the signal to noise ratio, are novel and highly complex aspects.
The strips from one side of the silicon microstrip sensors are first connected to a flexible electrical circuit, which is turn is connected to readout chips. “We fold over the flexible circuit such that the strips of the other side of the sensor can be connected to the readout chip. This ‘folding over’ enables us to place the readout chips as close as possible to the strips reducing the noise,” explains Prof. Mohanty in an email to this correspondent.
Belle-II is some fifty times more sensitive than its predecessor Belle. The SVD detector is meant to measure the charged particles passing through it to an accuracy of 15-20 microns. Compare this with the average thickness of human hair, which is 100 microns. Such a precise position measurement significantly enhances the physics potential of the Belle-II experiment.
Physicists’ goal
This experiment has the same aim as the LHCb experiment at CERN — to study the decay of the short-lived B-mesons, and unearth clues to “new physics”. If these experiments are successful in their endeavour, they will cause a massive rethink of particle physics as we know it today. The two setups are complementary. “But the idea in both cases is to search for new physics and discover it,” says Rahul Sinha of The Institute of Mathematical Sciences, Chennai, who is leading the theoretical studies. Both will seek evidence that can significantly enlarge the picture of particle physics painted by the Standard Model.
For nearly fifty years, the world of elementary particles has been best described by the Standard Model. This also provides a unified description of all the forces in the universe except gravitation. It accounts for various particles and how they get their masses with the help of the Higgs boson. However, now many questions remain which could be helped by Belle-II.
The group at IMSc focuses on decays in which the beauty quark within a meson changes to a different flavour of quark known as the strange quark. These processes are very rare according to the Standard model, but can possibly be detected at Belle-II and LHCb.
