Precision measurement of the neutronβ-decay asymmetry

Abstract

A new measurement of the neutron β-decay asymmetry A 0 has been carried out by the UCNA Collaboration using polarized ultracold neutrons (UCNs) from the solid deuterium UCN source at the Los Alamos Neutron Science Center. Improvements in the experiment have led to reductions in both statistical and systematic uncertainties leading to A 0 = −0.11954(55) stat (98) Precision measurements of neutron β decay are an essential ingredient in understanding the electro-weak interaction in the light quark sector. In particular the axial-vector weak coupling constant, g A , is an important input to understanding the spin and flavor structure of the nucleon [1,2] and is being actively studied in detailed lattice QCD calculations [3,4]. It also plays an important role in a variety of astrophysical processes, including solar fusion cross sections important for energy and neutrino production in the Sun [5].The angular distribution of emitted electrons from decays of a polarized neutron ensemble can be expressed as [6]where A(E) specifies the decay asymmetry for electron energy E, v ≡ βc is the electron velocity, P is the mean neutron polarization, and θ is the angle between the neutron spin and the electron momentum. can be expressed aswhere λ ≡ g A /g V is the ratio of the vector to axial-vector weak coupling constants. Combining g A with independent measurements of the Fermi coupling constant G F , the Cabibbo-Kobayashi-Maskawa matrix element V ud , and the neutron lifetime τ n allows a precision test of the consistency of measured neutron β-decay observables [7]. The ultracold neutron asymmetry (UCNA) experiment is the first experiment to use ultracold neutrons (UCNs) in a precision measurement of neutron decay correlations. Following the publication of our earlier results [7][8][9], the UCNA Collaboration implemented a number of experimental improvements that led to reductions in both statistical and systematic uncertainties. These improvements, described below, include enhanced UCN storage, improved electron energy reconstruction, and continuous monitoring of the magnetic field in the spectrometer. This refined treatment of the systematic corrections and uncertainties begins to address issues of consistency in the world data set for A 0 .The UCNA experiment ran in 2010 using the "thin window geometry D" as described in [7,9], and collected a total of 20.6 × 10 6 β-decay events after all cuts were applied. We used the UCN source [10] Copyright by the American Physical Society. Mendenhall, M. P. ; Pattie, R. W., Jr. ; Bagdasarova, Y. ; et al., Mar 25, 2013. "Precision measurement of the neutron beta-decay asymmetry," PHYSICAL REVIEW C 87(3): 032501.

Global effective-field-theory analysis of new-physics effects in (semi)leptonic kaon decays

“…This is indeed the case for g A , which has been measured precisely [82,83] and for which there are ongoing LQCD efforts [84], with results in the physical point currently at the few-percent level [85][86][87]. As recently pointed out in ref.…”

Precision half-life measurement of theβ+decay ofK37

“…The most thoroughly studied and theoretically cleanest example of such a transition is the decay of the neutron [4][5][6], but a number of other cases have attracted attention recently: the decays of 19 Ne, 21 Na, 29 P, 35 Ar and the subject of the present work, 37 K. NaviliatCuncic and Severijns [7] used the f t values for these five transitions to determine V ud , though less precisely than has been achieved from the 0 + → 0 + transitions. With more precise measurements, however, the T = 1/2 result for V ud could be improved enough to provide a valuable cross-check on the result from the 0 + → 0 + transitions.…”

Limits on tensor coupling from neutronβdecay

“…For this analysis we consider the β-asymmetry from UCNA [17] and Perkeo II [12], and the proposed limits are from the PERC experiment [26].…”

“…Careful analysis of a simultaneous experiment would be required to determine the impact a non-zero Fierz term would have on the extracted λ. Using the results from Perkeo II [12] and UCNA [17], A m = −0.11931(46), we then obtain −0.0026 < C T /C A < 0.0024 (95% C.L.). Note that the 30% reduction in the limit is due in large part to the increased error bar on A 0 used by the PDG to account for the variations in the current measurements; including this factor would increase the limit to 2σ = 0.0039.…”