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01/17/2020 - "Beauty"-ful Physics

by Adam Hadhazy

a photo of a large facitility with a building-sized detector being builtBelle II detector for SuperKEKB electron-positron particle accelerator under construction at KEK (The High Energy Accelerator Research Organization) in Tsukuba, Japan. Photographed on Open House day by Kestral, wikimedia commons.

Who says scientists aren't creative types? A bit of whimsy surely inspired the names of the six elementary particles classified as quarks. Their monikers: up, down, charm, strange, top, and bottom. The last of those particles goes by another droll appellation: the "beauty" quark. And true to its name, the beauty quark has been a darling of researchers for the insights it is offering into fundamental physics.

Continuing in that naming vein is the Belle II experiment. (Hint: think of a certain, famous, yellow-dress-wearing character from Beauty and the Beast.) In 2017, engineers installed Belle II at the SuperKEKB particle collider in Tsukuba, Japan (less than 50 miles from Tokyo). They started putting it through its setup paces in Spring 2018 and data collection swung into gear in 2019, with an expected run until about 2027. Researchers from the Kavli Institute for the Physics and Mathematics of the Universe (Kavli IPMU) at the University of Tokyo are playing a critical role in Belle II, having designed and built a detector instrument at the heart of the experiment.

Takeo Higuchi, an associate professor at Kavli IPMU working on Belle II, describes the experiment as a focused effort to "vividly identify new physics." In this regard, the overarching mission of the experiment is to help answer big questions in physics about why our universe is composed just so. Inexplicably, matter—the stuff that makes up you, me, and the tree—utterly dominates over its counterpart, dubbed antimatter. Matter and antimatter are the opposites of each other, such that when the substances meet, they mutually annihilate in a shower of energy. Trouble is, according to the otherwise well-supported model of our universe's origins, the Big Bang theory, matter and antimatter should have sprung forth in equal quantities during the first few moments of cosmic existence. Clearly this was not the case, as we matterous beings are here to cogitate on the unexplained primordial imbalance.

Beyond antimatter, Belle II will also expand the hunt for new sorts of particles hypothetically constituting still another kind of matter, called dark matter. Thought to outnumber normal matter particles by about six to one, dark matter is proposed as a sort of glue that holds together galaxies, exerting gravity and preventing them from otherwise flinging all their stars into intergalactic space.

Belle II will forge ahead in these inquiries by smashing together electrons—everyday particles, familiar to us as electricity—with positrons, their far rarer antimatter counterparts. The collisions will produce exotic particles called B mesons. The the "B" stands for the bottom/beauty quarks composing them, which are paired with one of four other quark types, excepting the top quark or another bottom quark. The "meson" bit, meanwhile, denotes that the particle pair is made of one part matter and one part antimatter.

B mesons have intrigued researchers because their decays do not wholly follow the Standard Model, the deeply fleshed-out rule book for how particles and forces interact. For instance, the decays seem to show a slight overall preference for producing matter versus antimatter in the downstream of particles resulting from B meson breakdowns. If further solidified, this asymmetry could factor in significantly in accounting for the universe's matter-antimatter misalignment, pointing to other such discordances, as well as new physics beyond the Standard Model.

The Kavli IPMU-built detector offers the necessarily exquisite sensitivity to capture the details of the B meson decays. Called the vertex detector, the detector is a lantern-shaped structure comprised of four cylindrical layers. Each layer is composed of a cylindrical arrangement of ladder-like arrays of silicon sensors, Higuchi explains. He and other Kavli IPMU members built the various key components of the vertex detector right on the Institute's campus in a cleanroom. In such a facility, stray dust motes, human hair, and other assorted, everyday detritus that can throw off instrument precision are kept to a minimum. 

As the collisions inside Belle pile up into the trillions upon trillions over the experiment's lifetime, the particles and energies collected by the Kavli IPMU-made detector will allow researchers to extrapolate back to the properties of the short-lived particles produced by the collisions inside Belle. Those parent particles will either adhere to tried-and-true physics or, more excitingly, not adhere, opening the door to discovery.

It's early days yet, and the first paper related to Belle II—about experimental performance milestones—has recently been accepted at a journal. Higuchi says some preliminary physics results will soon start to roll out, with likely announcements come springtime.

Stay tuned, science fans. Belle II might just end up as, well, the belle of the physics ball.

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