“Table-top” experiments can still probe physics complementary to particle searches at high-energy accelerators. A beta-neutrino correlation experiment using TRIUMF’s Neutral Atom Trap (TRINAT) has now set the best limits on general scalar interactions contributing to nuclear beta decay.
TRINAT uses the radiation pressure of laser light to capture radioactive atoms in a 1 mm-sized cloud. Laser light of a frequency slightly below an atomic resonance is shone from all sides of the trap. Atoms moving away from the trap then “see” along their direction of motion light that is blueshifted closer to the resonance, while away from their direction of motion they see light redshifted further away from resonance. The net effect is of radiation pressure opposite to the direction of motion, as the atom absorbs more light that is closer to its resonance.
The trapped atomic nuclei undergo beta decay, which produces three decay products: a positron (β+), a neutrino (ν) and the recoiling daughter nucleus. The daughter nucleus has a kinetic energy of 0-430 eV; while it would stop in 1 nm of material, it can escape the trap. By measuring the momentum of the nucleus in coincidence with that of the β+, the TRINAT team can deduce the momentum of the neutrino more accurately than in previous experiments (which did not measure the recoil energy).
These techniques have been pioneered at TRIUMF using potassium isotopes with 1 s half-lives produced at the Isotope Separation and Acceleration (ISAC) facility with the main TRIUMF cyclotron – this “table-top” experiment admittedly is driven by the world’s largest cyclotron. Results are also becoming available from other experiments based on neutral-atom traps at Berkeley and Los Alamos.
In the Standard Model the weak interaction is mediated by spin-1 vector bosons, the W+, W– and Z. Measurements of the β-ν angular distribution in the decay of 38Km → 38Ar + β + ν where both parent and daughter have no nuclear spin allow the search for contributions from hypothetical spin-0 scalar bosons. The TRINAT result for the β-ν correlation parameter a is 0.9981 ± 0.0030 ± 0.0037, consistent with the Standard Model value a =1.
The previous best result, by a Seattle-Notre Dame collaboration using beta-delayed proton emission of 32Ar produced at the ISOLDE facility at CERN, is in the process of being re-evaluated after new measurements of the mass of parent and daughter. Such results constrain the existence of spin-0 bosons with mass:coupling ratios as great as four times the W+ mass, and are complementary to other measurements.
TRINAT can determine detector response functions in situ from the data itself. This is routinely done in high-energy experiments but never before for low-energy beta decay. The experiment has also used the equivalent of the missing-mass construction in high-energy physics to constrain the admixture of possible sterile neutrinos of million-electron-volt mass with the electron-neutrino.
TRINAT is also investigating other physics topics. These include measuring the neutrino asymmetry from polarized nuclei to search for evidence of non-Standard Model right-handed neutrinos (using a complementary measurement to the purely leptonic muon-decay studied at TRIUMF and PSI); measuring the spin asymmetry of the daughter nuclei in pure Gamow-Teller decays; and testing hints of a nonzero tensor interaction reported in π→νeγ by the PIBETA collaboration at PSI.
Further reading
A Gorelov et al. 2005 Phys. Rev. Lett. 94 142501.
