For some time my research has been about neutrinos, and particularly about their mass. When I was at Los Alamos, our group developed a new type of tritium beta decay experiment using a source of gaseous molecular tritium, and we were able to push down the laboratory limits on the mass of the electron neutrino to below 10 eV, ruling out a claim that it was around 30 eV. In 1988 I joined the Sudbury Neutrino Observatory, and in 2001 SNO was able to show that electron neutrinos are actually mixtures of at least two mass eigenstates. Super-Kamiokande had shown that muon neutrinos also mix, and thus at least two of the three neutrino states have non-zero mass. SNO also resolved the "solar neutrino problem" and confirmed the astrophysical calculations of the flux of solar neutrinos.
Neutrino oscillation experiments like SNO and Super-K tell us neutrinos have mass, but not how much. For that we still need tritium beta decay, and the largest such experiment ever undertaken is the KATRIN project in Karlsruhe, Germany. UW is the lead US institution in the project, and our group delivered the detector system for it. KATRIN will be sensitive to a mass as small as 0.2 eV (current upper limit is 1.8 eV), and will begin running in 2016.
Oscillations do tell us the average mass of the 3 mass eigenstates cannot be less than 0.02 eV. If the mass is below KATRIN's sensitivity, is there another way to reach that final 0.2 to 0.02 eV range? Project 8 is a new idea that might get us there. Instead of measuring the energy of electrons by magnetic or electrostatic means, we detect the cyclotron radiation emitted as they spiral in a magnetic field, and measure the frequency. We built a proof-of-principle experiment to see if this idea works, and it was very successful! For that experiment we used Kr-83m. Now we are pushing ahead to develop a small-scale experiment with tritium.
Another thing we would like to know about neutrinos is whether they are their own antiparticles or not. If they are, it could be a key to the puzzle of why there is much more matter in the universe than antimatter. The only known way to get this answer is via double beta decay. If that can go without emission of neutrinos, it would show that neutrinos and antineutrinos are indeed the same. The Majorana Demonstrator experiment uses single-crystal isotopically enriched detectors made of germanium-76, and is being built in the Sanford Underground Research Facility in Lead, SD.
HALO is a detector ready to catch neutrinos from a supernova explosion. It was built from the neutron counters once used in SNO, and 79 tonnes of lead. It is sited in SNOLAB, and has been running since May 2012. Because it uses lead, it is uniquely sensitive to electron neutrinos (rather than antineutrinos). No supernova neutrinos have been seen in HALO yet.
Final states in the beta decay of tritium; why LANL and LLNL did not get zero; arXiv:1502.03497
A summary for NOW2014 of direct mass measurements; arXiv 1502.00144
The electron capture spectrum of Ho-163 is more complicated arXiv 1411.2906
Proof of concept for Project 8; arXiv 1408.5362
The "white paper" on Project 8 submitted for Snowmass 2013; arXiv 1309.7093
A study, perhaps surprising, of the "dead" layer on Si PIN-diode detectors; arXiv 1310.1178 .
In the theory of neutrinoless double beta decay, it seems the matrix element and phase space are inversely correlated; arXiv 1301.1323.
A description of the detector system for KATRIN; arXiv 1404.2925.
A review of solar neutrinos can be found in Ann. Rev. Astron. Astrophys. 51, 21 (2013)
The last, big paper on SNO; arXiv 1109.0763.
A colloquium talk on neutrino mass given at the Ohio State University February 2, 2015.
A plenary talk on the history and status of beta decay, given at the APS April meeting, Savannah GA on April 8, 2014.
A talk on neutrino mass given at the Symposium in memory of Stuart Freedman, Berkeley CA, January 11, 2014.
A summary talk on neutrino mass given at the Snowmass Meeting at the University of Minnesota, August 2, 2013.
Physics 585 is the Nuclear Seminar, which I manage. Seminars are usually held in CENPA. As a rule, it is informal and students do not register for it. Check the CENPA website for seminar announcements. I presently have 2 grad students I am advising.
I am a Professor in the Physics Department , hold a Boeing Distinguished Professorship, and am Director of the Center for Experimental Nuclear Physics and Astrophysics (CENPA). A brief bio and full CV and publication list are available.