First Name
Last Name
TRIUMF Res.Sc./Adjunct, PHAS
Office Room
Tel (Office)
(604) 222-1047 x6371

Students Wanted
actively recruiting

Bachelor's Degree
Caltech, 1983, B.Sc. in Physics

Doctoral Degree
University of Washington, 1991, Isospin mixing in giant dipole resonances, advisor Kurt Snover

Employment History

Postdoc at SUNY Stony Brook with Gene Sprouse
Postdoc at Simon Fraser with Otto Hausser, mostly on laser traps for radioactive atoms.

Research Area
Atomic, Molecular & Optical Physics

Research Field
Subatomic Physics

Research Title
TRIUMF Neutral Atom Trap


We use laser trapping and cooling techniques with a table-top sized apparatus to precisely test the Standard Model of the electroweak interaction.

Radioactively decaying atoms are trapped in a 1 mm-sized cloud. These undergo nuclear beta decay, producing 3 products: a positron, a neutrino and a recoiling final nucleus. Although the nucleus has a very low energy, it escapes freely from the trap, and by detecting it in coincidence with the positron, we can reconstruct the neutrino momentum.

The Standard Model predicts the angular distribution of the neutrinos with respect to the positrons, and by accurately measuring this we can look for new forces not in the Standard Model.

We have pioneered these techniques, and now are polarizing the atoms, to test whether parity is fully violated in the weak interaction. I.e. we ask the question: "Is Nature completely left-handed?" We're made the best beta asymmetry measurement, in agreement with the Standard Model at 0.35% accuracy, and are trying to extend the measurement to other observables sensitive to different new physics. 

We are also looking for time-reversal violation in beta-neutrino-gamma coincidences, a very challenging extra coincidence for us that needs some creativity. When parity violation was discovered in the late 1950's in beta and muon decay, people immediately proposed experiments testing time-reversal symmetry, hoping it might be large. This particular observable has never been measured in the first generation of particles, and could still have a large breaking of time reversal.

We are also trapping francium atoms, the heaviest alkali atom, with an eventual goal of measuring weak neutral current effects in atoms.

Selected Publications

Best beta decay asymmetry: B. Fenker et al. Phys Rev Lett  120 162502 (2018) "Precision Measurement of the β Asymmetry in Spin-Polarized 37K Decay"

Talk on the time-reversal proposal and status:

Atom trap decay experiments in general:

Public writeup on wrong-handed neutrino sensitivity:

``Standard model tests with trapped radioactive atoms', J.A. Behr and G. Gwinner, J.Phys. G: Nucl. Part. Phys. 36 (2009) 033101 or arXiv:0810.3942v3

``Beta decay angular correlations with  neutral atom traps'', J.A. Behr and A. Gorelov,  J. Phys. G: Nucl. Part. Phys. 41 114005 doi:10.1088/0954-3899/41/11/11400