The SuperKEKB will produce 50-times more collisions than previous generations of experiments, and provide more data to investigate the CP violation problem.
A report published by the US Particle Physics Project Prioritisation Panel in 2008 set out three frontiers on which discoveries in fundamental physics would be made: cosmic, energy and intensity. On the cosmic frontier, world-spanning networks of ground-based telescopes, satellites and interplanetary probes have been mapping the universe. The leading experiment making discoveries on the energy frontier is the Large Hadron Collider (LHC), a giant particle smasher in Europe. However, matters have been quiet on the intensity frontier since 2010, when one of its foremost experiments shut down for upgrades.
That experiment, the KEKB, cleared an important milestone on February 10, signalling its readiness for more rigorous test runs before opening for business in early 2017. The development was announced on March 2 by an international collaboration of scientists at Japan’s High-energy Accelerator Research Organisation (KEK), which manages the machine.
While the LHC makes a living by accelerating protons to extremely high energies and smashing trains of them head on, the KEKB accelerates electrons and anti-electrons to lower energies but in relatively much higher quantities. As a result, it’s able to realise extremely rare phenomena more often and allows detectors to probe them in more detail. The precision studies will be used to complement results from the LHC, according to Jim Libby, an experimental particle physicist at IIT-Madras.
The February-10 milestone was called ‘first turns’. It involved KEKB – or SuperKEKB as it’s now being called – storing beams of electrons and anti-electrons by keeping them revolving in separate rings. “This is an essential first step in showing that the key components of this new accelerator are working fine,” says Libby. After this, the collaboration at KEK will “accelerate the beams to the design energy, focus the beams at the collision point, then increase the current and focusing so that the design parameters are met.” For a particle accelerator, chiefs among these parameters are the luminosity and beam energy.
The luminosity is the number of collisions produced by an accelerator. Evidently, this is the number that defines how intense SuperKEKB can be, and what it will be able to achieve from the edge of the intensity frontier it occupies. According to Libby, it will be targeting 1036 per sq. centimetre per second and 1043 per sq. centimetre overall – both “at least 50 times higher than the previous generation of experiments”. To compare, the upgraded LHC (at the head of the energy frontier) aspires to achieve an instantaneous luminosity of 7.7 x 1034 only by 2022.
As for beam energy: KEKB will be able to accelerate electrons to 7 GeV and anti-electrons to 4 GeV, as against the 8 GeV and 3.5 GeV it achieved earlier. The difference in energies has to do with what happens at the moment of collision itself. Being anti-particles of each other, an electron and an anti-electron will mutually annihilate themselves and the energy will be released in the form of a ‘virtual’ photon. Some of these virtual photons will decay in a series of steps to form particles called B mesons – composed of a bottom antiquark and a charm, down, up or strange quark (Quarks are the building blocks of protons and neutrons. They come in six types organised into three generations: top/bottom, up/down and charm/strange).
The difference in energies will impel the B mesons to move after being produced and allow physicists to measure how their properties change over time. “This opens up a window on phenomena that are a function of time, in particular the difference in behaviour between beauty quarks and beauty antiquarks, which is a matter-antimatter asymmetry,” according to Libby.
B mesons are interesting because they have a curious property of decaying less often than their antimatter counterparts. Understanding why really this happens would give physicists crucial insights into why the universe contains more matter than antimatter today although they were created in equal amounts in the beginning.
The particle has already had a brush with fame in the past: A part of the answer has to do with the B meson violating a fundamental principle of nature called charge conjugation parity (CP) symmetry. In 2008, one-fourth of the Nobel Prize for physics was awarded to Makoto Kobayashi, a professor emeritus at KEK, for his role in proving that for CP violations to occur, three generations of quarks must exist. Collecting more data to further investigate this problem was one of the motivations to upgrade KEKB to SuperKEKB.
And by helping answer such questions, SuperKEKB will chart territories of research lying squarely outside of what existing theories are able to explain. Then again, as an accelerator it only produces the exotic particles, leaving it up to a detector to analyse them and glean meaningful results. At KEK, that job belongs to Belle II, an enhanced version of the Belle detector that functioned until 2010. Libby says it “will be completed and installed for operation at the end of 2017”. Once that’s in place, KEKB will be back to take charge of the intensity frontier.