Sharpening the Tools of the Astrophysical Trade

Workers are only as good as their tools, the old saying goes. And while skill, of course, matters, it certainly does help to have quality tools on hand—or better yet, state-of-the-art tools—to do the job well.

Sharpening astrophysicists' tools and developing brand new ones is the raison d'être of the Detector/Instrument Development research program at the Kavli Institute for the Physics and Mathematics of the Universe (Kavli IPMU) at the University of Tokyo. About 30 Kavli IPMU members currently comprise the membership of the program. The range of scientific instruments these members work on represents many major fields in astronomy, astrophysics, particle, and plain ol' physics (which underlies everything else, natch).

The program looks to equip principal investigators, or PIs, who work directly on experiments with the best gear to enable scientific breakthroughs.

"We are trying to share our knowledge in detectors among the Kavli IPMU PIs who are involved in experiments in order to maximally increase the productivity of detector R&D and the following physics outputs," says Takeo Higuchi, an Associate Professor at Kavli IPMU and a member of the Detector/Instrument Development research program.

Given the many pursuits by Kavli IPMU researchers, the quarry of the detectors the Detector/Instrument Development members contribute to accordingly runs the gamut as well. One such target are ghostly neutrinos. These particles' infinitesimal, yet non-zero mass is in marked defiance of predictions by the Standard Model, the otherwise extremely successful framework encapsulating nature's particles and forces (excepting gravity, but that's another story). Kavli IPMU researchers are deeply involved in neutrino detections by the Super-Kamiokande instrument, a giant underground vessel full of ultra-pure water that acts as a detection medium for those rare times when neutrinos interact with ordinary matter. Ongoing work involves doping the water with the element gadolinium, which can boost the ability to detect neutrinos blasted out by stellar explosions called supernovae.

For Higuchi and colleagues, a particular—pun intended—particle focus is the Belle II experiment, installed at the SuperKEKB accelerator complex in Japan. Belle II is cleverly named given its scientific goal of studying "beauty" or B mesons. These are heavy particles that contain a so-called bottom antiquark. On account of their unusual and rare decays, B mesons offer unique insights into the strong interaction, the force of nature that glues composite particles together. Some of the B meson decay characteristics that continue to accumulate evidence through experiments like Belle II do not readily conform to Standard Model predictions, offering—like neutrinos—a path forward into the unknown.

"The main objective of Belle II is the discovery of new particles and new particle interactions and the establishment of a new particle theory beyond the Standard Model of particle physics," says Higuchi.

For Belle II, Kavli IPMU researchers have produced an apparatus, called the silicon vertex detector (SVD), which very precisely locates particle positions. A new physics effect would likely appear as a slight deviation of a particle's location from the Standard Model-predicted location, thus making the precise determination of the particle location crucial for the experiment's success. "In this context," Higuchi says, "we say we produced the heart of the Belle II experiment." Belle II started gathering data in 2018 and will gather data in the coming years that researchers hope can move physics boldly ahead.

Multiple other detectors and instruments with Kavli IPMU involvement are similarly pushing the envelope. Such efforts involve ultra-purifying the gases argon and xenon for ever-more sensitive searches for neutrinos, along with dark matter particles, the hypothesized material thought to outnumber "normal" matter six times over. Other materials, such as scintillating and ultra-pure crystals, are also being actively pursued. The researchers additionally have a hand in developing a spectrograph for the Subaru Telescope for examining loads of galaxies simultaneously, and an advanced sensor for detecting the universe's oldest light, the cosmic microwave background.

These varied pursuits all, amazingly, fit under one roof at Kavli IPMU. This interdisciplinarity is one of the main features that drew Higuchi to the Institute, where he's been an Associate Professor since 2016. "I knew Kavli IPMU offers excellent opportunities to make interdisciplinary discussions. Indeed, this was true," Higuchi says. "Kavli IPMU is a unique place, where mathematicians, string theorists, particle-physics theorists, astrophysicists, and experimentalists live together."