Precision measurement of the nuclear polarization in laser-cooled, optically pumped³⁷K

Abstract

We report a measurement of the nuclear polarization of laser-cooled, optically pumped 37 K atoms which will allow us to precisely measure angular correlation parameters in the b + -decay of the same atoms. These results will be used to test the V−A framework of the weak interaction at high precision. At the TRIUMF neutral atom trap (TRINAT), a magneto-optical trap confines and cools neutral 37 K atoms and optical pumping spin-polarizes them. We monitor the nuclear polarization of the same atoms that are decaying in situ by photoionizing a small fraction of the partially polarized atoms and then use the standard optical Bloch equations to model their population distribution. We obtain an average nuclear polarization of¯=  P 0.9913 0.0009, which is significantly more precise than previous measurements with this technique. Since our current measurement of the β-asymmetry has 0.2% statistical uncertainty, the polarization measurement reported here will not limit its overall uncertainty. This result also demonstrates the capability to measure the polarization to <0.1%, allowing for a measurement of angular correlation parameters to this level of precision, which would be competitive in searches for new physics.

New physics searches in nuclear and neutron β decay

“…The nuclear polarization was measured in situ by photoionizing a small fraction of the polarized atoms and then using the optical Bloch equations to model the population distribution ( Fig. 7) [369]. Following previous developments [370,371] this was the first time the nuclear polarization in an atom trap could be determined with a precision smaller than 0.1%.…”

“…Recently, first results for the asymmetry parameter using laser optical pumping in a MOT, obtained with TRINAT, have been reported [78,183]. Atoms of the mirror nucleus 37 K were confined in an alternating-current MOT and were spinpolarized to 99.13(9)% via optical pumping [369]. The nuclear polarization was measured in situ by photoionizing a small fraction of the polarized atoms and then using the optical Bloch equations to model the population distribution ( Fig.…”

“…The authors of Ref. [369] estimate that by further increasing the light polarization and decreasing the transverse magnetic field, which will both increase the average nuclear polarization and decrease its uncertainty, a polarization uncertainty of ∼0.04% could be achieved in planned new measurements, allowing for an uncertainty of ∼0.1% on this and other polarized correlation parameters.…”

The MORA project

“…The best constraints so far on such T-violating interaction arise from the neutron decay, with a measurement yielding D n = (−0.9 ± 2.1 · 10 −4 ) [30,37]. Weaker constraints were obtained for nuclear decays using the decay of 19 Ne, with a combination of measurements yielding a limit D19 N e < 6 · 10 −4 [38]. In terms of the coupling constants from the effective Hamiltonian of beta decay, the present limits on D n translate into a limit on the relative phase between the vector and axial couplings: Im(C V /C A ) = (1.6 ± 6.3) · 10 −4 (90% C.L.)…”

Precision half-life measurement of Al25

“…The mixing ratio has been derived from measurements of one of the following three correlation parameters: the beta asymmetry parameter A β , the neutrino asymmetry parameter B ν , and the beta-neutrino asymmetry angular correlation parameter a βν [10]. Recent efforts to determine the mixing ratio ρ in mirror transition nuclei include a polarization measurement on 37 K in anticipation of a measurement of A β at the TRINAT experiment of TRIUMF [13]. Another effort is the Versatile Ion-polarized Techniques online (VITO) [14] at ISOLDE, which aims at measuring A β in 35 Ar [15], as well as in 21 Na and 23 Mg in the future [16].…”