Current Research
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 and we recently published our first result, a limit of 1.1 eV, a factor of 2 below the previous limit. We have just completed an experiment called TRIMS (Tritium Recoil-Ion Mass Spectrometer) to find out why some experiments disagree with a prediction of the molecular theory used in KATRIN, and a paper is on the way.
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! Now we are taking data in a small-scale experiment with tritium.
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.
Selected Publications
Direct measurements of neutrino mass, a review; arXiv 2102.00594
Beta decay of molecular tritium; resolves a 60-year mystery; arXiv 2001.11671
First result on neutrino mass from KATRIN; arXiv 1909.06048
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.
Selected Talks
An invited talk on neutrino mass for Oak Ridge in October 2020.
An invited talk on neutrino mass given at the Inauguration of the T.D. Lee Institute in Shanghai, November 29, 2016.
A plenary talk on the history and status of beta decay, given at the APS April meeting, Savannah GA on April 8, 2014.
Teaching
I'm not teaching classes, having recently retired. I am still active in research and have been advising undergrads and grads.
Biography
I am a Professor Emeritus in the Physics Department , and the Center for Experimental Nuclear Physics and Astrophysics (CENPA). A brief bio and full CV and publication list are available.