Precision measurements of theCo60β-asymmetry parameter in search for tensor currents in weak interactions

Abstract

The β-asymmetry parameter A for the Gamow-Teller decay of 60 Co was measured by polarizing the radioactive nuclei with the brute force low-temperature nuclear-orientation method. The 60 Co activity was cooled down to milliKelvin temperatures in a 3 He-4 He 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.

Prospects for precision measurements in nuclear decay in the LHC era

“…However, the allowed Gamow-Teller matrix element is strongly reduced in 60 Co decay. The SM value of the asymmetry parameter has therefore a theoretical uncertainty which is of the same order of magnitude than the experimental error [43] due to the contribution of recoil corrections. The total relative uncertainties achieved were respectively 1.4% and 2.0% at 1σ for 114 In and 60 Co and do not significantly improve the limits obtained from P F /P GT .…”

“…As pointed out in Sec. 4.2, recent measurements of the β asymmetry parameter in nuclei [42,43] have a rather moderate impact in constraining exotic couplings compared to measurements of the longitudinal polarization ratio, P F /P GT . New measurements of the β asymmetry parameter in mirror nuclei are primairly motivated by the determination of the GamowTeller to Fermi mixing ratios, ρ [66].…”

“…Absolute measurements of the β asymmetry parameter, A, have recently been performed in Gamow-Teller decays with the explicit purpose to probe tensor couplings via the Fierz term [42,43]. The experiments used the low-temperature nuclear orientation technique in the decays of 114 In [42] and 60 Co [43].…”

“…The experiments used the low-temperature nuclear orientation technique in the decays of 114 In [42] and 60 Co [43]. These transitions have a relatively low end point energies and hence large sensitivity to the Fierz term.…”

A new spin-oriented nuclei facility: POLAREX

“…In this context, LTNO is a great tool to probe symmetries in the nuclear interaction. The measured anisotropy shape of the emitted beta particles allows weak interaction tests such as CPT violation test, and an accurate determination of the β-asymmetry parameter [7].…”