My research specialization is in neutrino physics and particle astrophysics. The specific topics I have studied include neutrino oscillation, nucleon decay, and the search for dark matter.
Neutrino oscillation: I was fortunate to be deeply involved in the discovery and measurement of neutrino mass by observing the flavor-changing oscillations of atmospheric neutrinos using the Super-Kamiokande detector in Japan. This work that was recognized in 2015 with the Nobel Prize in Physics. I coauthored, with T. Kajita and Y. Totsuka, an account of this discovery in the August 1999 Scientific American. I also participated in the now-completed MACRO and K2K experiments, both of which provided independent confirmation of the neutrino oscillation effect. In addition to continuing studies of atmospheric neutrinos with Super-K, I am also a member of the T2K (Tokai-To-Kamioka) experiment. With it, we have detected, for the first time, the appearance of a neutrino flavor in a nearly pure beam that lacks that flavor to begin with. This gives a measurement of the parameter theta13, which is the gateway to future studies of CP violation and mass hierarchy. These future measurements will be accomplished with the Deep Underground Neutrino Experiment, DUNE that will use an intense beam of neutrinos from Fermilab to a new cutting-edge detector located in South Dakota. To gain some experience with the DUNE liquid argon TPC technology, I am working with a prototype experiment called LArIAT at Fermilab.
Nucleon decay: An ongoing goal of Super-Kamiokande is to search for baryon number violation, by searching for proton decay and related processes. These processes are predicted by Grand Unified Theories that unite the strong, weak, and electromagnetic interactions. This has been a major research thrust in my group at BU; we have graduated four Ph.D. students who have worked in this area. The search for proton decay will continue with DUNE, which is particularly sensitive to supersymmetric modes that contain kaons in the final state.
Dark matter: I am involved in the search for dark matter using a technique based on cryogenic noble liquids. The particular detector that I and my group helped build is called MiniCLEAN. The detector is located in SNOLab, in Sudbury, Ontario. We hope to perform our first studies in 2016. I am also interested in indirect dark matter detection, where we can observe neutrinos from dark matter annihilation in Super-Kamiokande. Super-K is the leading neutrino detector for this physics for dark matter mass below roughly 100 GeV.
- Massachusetts Institute of Technology, B.S. 1982
- Harvard University, A.M. 1984, Ph.D. 1990
In the news:
- Super-K and T2K awarded Breakthrough Prize in Fundamental Physics
- Neutrino Nobel has BU connections
- T2K experiment confirms electron neutrino appearance
- First Long-Baseline Neutrino in T2K
- Professors Kearns and Sandvik Named APS Fellows
MACRO Experiment: Monopole Astrophysics and Cosmic Ray Observatory
The deep underground MACRO detector operated at the Laboratori Nazionali del Gran Sasso in Abruzzo, Italy from 1990 to 2000. MACRO had a geometrical acceptance of 10,000 square meters at an average depth of 3.8 kilometers of water equivalent under the mountainous overburden of the Gran Sasso d'Italia. The MACRO detector was used to research several topics. The specialty of MACRO was the search for magnetic monopoles: particles with bare north or south magnetic charge. These particles are a natural consequence of Grand Unified Theories, which also predict that the monopole will be very massive, perhaps 10 to the power 16 GeV. Such particles can only be produced by the intense energies available during the big bang. MACRO operated like a giant Time-Of-Flight counter to detect the unique signature of a slow moving but penetrating massive particle. It was equipped with tanks of liquid scintillator, planes of streamer tubes and plates of track etch material in the hopes of recording a convincing signature from a single candidate event. MACRO's high-resolution tracking and timing were also used to perform high statistics measurements of cosmic ray muons; in particular, the scintillator timing was used to distinguish upward going muons produced by neutrino interactions in the rock. This was an opportunity to investigate the possible flavor oscillation of massive neutrinos as suggested by the atmospheric neutrino puzzle, and MACRO was the first experiment to independently confirm the atmospheric neutrino oscillation signal seen by Super-Kamiokande.
Neutrino Physics and Astrophysics
As a sequel to the combined effort of the two earlier deep underground ring-imaging detectors, IMB and Kamiokande, Super-Kamiokande started recording data deep inside a lead and zinc mine in the Japanese Alps in 1996. The detector is a 40-meter high by 40-meter diameter stainless steel tank, filled with 50,000 tons of ultra-pure water. The walls of the tank are lined with 11,200 photomultiplier tubes, each an enormous 50 centimeters in diameter. One half of the surface of the tank is covered by photosensitive material. These tubes record the Cherenkov light from charged particles as they pass through the water.
Physicists in the US and in Japan designed Super-K to search for, in part, the radioactive decay of the proton, a rare event never before observed. Detecting proton decay would confirm the Grand Unified Theory of particle physics.
Though Super-K has yet to detected a single candidate, it observed something equally intriguing: convincing evidence that neutrinos have mass. Because neutrinos carry no charge, they rarely interact with other particles. Billions go through each of us every second without effect. But since Super-K is so large, it detects a handful of these neutrinos each week. Using the data collected by Super-K, researchers including Boston University physicists Jim Stone (US co-spokesman), Larry Sulak and Ed Kearns confirmed results from the two precursor experiments that about half the neutrinos expected were unseen.
Neutrinos come in three species – the tau neutrino, the muon neutrino, and the electron neutrino. Super-K discovered that these neutrino species transform into each other. If oscillations occur, muon neutrinos could transform into tau neutrinos, and the missing neutrinos observed by Super-K would be explained. This was documented in the 1999 Ph.D. thesis of Boston University graduate student Mark Messier. Quantum mechanically, for these oscillations to occur, the neutrino must have mass. Several experiments since this revelation in 1998 have reinforced this interpretation.
The discovery of massive neutrinos has forced theorists to rethink the Standard Model of particle physics; neutrino mass is not anticipated in this most accurate and predictive of theories. Further, with mass, the neutrinos in the universe account for nearly as much mass as all the stars. Hence, they would influence the formation of galaxies in the early universe. This discovery is the first indication of new physics beyond the Standard Model.