Precision measurements of the Co-60 beta-asymmetry parameter in search for tensor currents in weak interactions

The β-asymmetry parameter Afor the Gamow-Teller decay of 60Cowasmeasured by polarizing the radioactive nuclei with the brute-force low-temperature nuclear-orientation method. The 60Co activity was cooled down to milliKelvin temperatures in a 3He-4He dilution refrigerator in an external 13-T magnetic field. The β particles were observed by a 500-μm-thick Si PIN diode operating at a temperature of about 10 K in a magnetic field of 0.6 T. Extensive GEANT4 Monte Carlo simulations were performed to gain control over the systematic effects. Our result, A = −1.014(12)stat(16)syst, is in agreement with the standard-model value of −0.987(9), which includes recoil-order corrections that were addressed for the first time for this isotope. Further, it enables limits to be placed on possible tensor-type charged weak currents, as well as other physics beyond the standard model.status: publishe