Determination of the electron$ndash$antineutrino angular correlation coefficient a0 and the parameter |$\lambda$| $equal$ |GA/GV | in free neutron $\beta$-decay from measurements of the integrated energy spectrum of recoil protons stored in an ion trap

Abstract

Recoil protons from the β-decay of unpolarized neutrons are stored in a cryogenic ion trap formed by the superposition of an axisymmetric electrostatic field on a coaxial magnetic field, which varies uniformly in space between regions of high (∼4.30 T) and low (∼0.61 T) homogeneous field. Protons generated in the high field region are adiabatically focused onto a ‘mirror’ electrode in the low field region. The integrated energy spectrum is determined by counting the number of stored protons as a function of mirror potential. Violation of the exact adiabatic conditions is corrected for by carrying out alternate measurements at trapping times which differ by a factor of 2. The results: a0 = −0.1054 ± 0.0055, |λ| = 1.271 ± 0.018 are in agreement with previous measurements of these quantities.

Precision experiments with cold and ultra-cold neutrons

“…Examples are the electron-antineutrino correlation coefficient a [6][7][8], the beta asymmetry parameter A [9][10][11][12][13], the neutrino asymmetry parameter B [14,15] (reconstructed from proton and electron momenta), the proton asymmetry parameter C [16], the triple correlation coefficient D [17,18], the Fierz interference term b, and various correlation coefficients involving the electron spin [19,20]. Each coefficient in turn relates to an underlying broken symmetry.…”

The Beta-Neutrino Correlation in Sodium-21 and Other Nuclei

“…The vertical grey band is allowed by the limit on the Fierz term from superallowed (0 + → 0 + ) Fermi decays [12]. The systems are 6 He [13] (blue), neutron [14] (pink) and [15] (orange), 21 Na (this work, red), 38m K [4] (green), and 32 Ar [16] (violet). A measurement for 23 Ne [17] is not shown, but is used to calculate the allowed region.…”

Cold and Ultra-cold Neutrons as Probes of New Physics

“…The most precise published measurements have reached accuracies of a few times 10 −3 for beta A [86,87,88] and neutrino asymmetry parameters B [89,90] and on the percent level for proton asymmetry parameter C [91] and electron-neutrino correlation coefficient a [92,93]. The Fierz interference term b has not yet been resolved in neutron decay, but is promising for probing scalar and tensor interactions [94,95].…”