A Caltech Library Service

New result for the neutron β-asymmetry parameter A_0 from UCNA

Brown, M. A.-P. and Blatnik, M. and Carr, R. and Filippone, B. W. and Hickerson, K. P. and Martin, J. W. and Mendenhall, M. P. and Pérez Galván, A. and Picker, R. and Slutsky, S. and Sun, X. and Swank, C. (2018) New result for the neutron β-asymmetry parameter A_0 from UCNA. Physical Review C, 97 (3). Art. No. 035505. ISSN 2469-9985.

[img] PDF - Published Version
See Usage Policy.

[img] PDF - Submitted Version
See Usage Policy.


Use this Persistent URL to link to this item:


Background: The neutron β-decay asymmetry parameter A_0 defines the angular correlation between the spin of the neutron and the momentum of the emitted electron. Values for A_0 permit an extraction of the ratio of the weak axial-vector to vector coupling constants, λ≡gA/gV, which under assumption of the conserved vector current hypothesis (gV=1) determines gA. Precise values for gA are important as a benchmark for lattice QCD calculations and as a test of the standard model. Purpose: The UCNA experiment, carried out at the Ultracold Neutron (UCN) source at the Los Alamos Neutron Science Center, was the first measurement of any neutron β-decay angular correlation performed with UCN. This article reports the most precise result for A_0 obtained to date from the UCNA experiment, as a result of higher statistics and reduced key systematic uncertainties, including from the neutron polarization and the characterization of the electron detector response. Methods: UCN produced via the downscattering of moderated spallation neutrons in a solid deuterium crystal were polarized via transport through a 7 T polarizing magnet and a spin flipper, which permitted selection of either spin state. The UCN were then contained within a 3-m long cylindrical decay volume, situated along the central axis of a superconducting 1 T solenoidal spectrometer. With the neutron spins then oriented parallel or anti-parallel to the solenoidal field, an asymmetry in the numbers of emitted decay electrons detected in two electron detector packages located on both ends of the spectrometer permitted an extraction of A_0. Results: The UCNA experiment reports a new 0.67% precision result for A_0 of A_0=−0.12054(44)_(stat)(68)_(syst), which yields λ=gA/gV=−1.2783(22). Combination with the previous UCNA result and accounting for correlated systematic uncertainties produces A0=−0.12015(34)stat(63)syst and λ=gA/gV=−1.2772(20). Conclusions: This new result for A0 and gA/gV from the UCNA experiment has provided confirmation of the shift in values for gA/gV that has emerged in the published results from more recent experiments, which are in striking disagreement with the results from older experiments. Individual systematic corrections to the asymmetries in older experiments (published prior to 2002) were >10%, whereas those in the more recent ones (published after 2002) have been of the scale of <2%. The impact of these older results on the global average will be minimized should future measurements of A0 reach the 0.1% level of precision with central values near the most recent results.

Item Type:Article
Related URLs:
URLURL TypeDescription Paper
Filippone, B. W.0000-0002-2618-2688
Hickerson, K. P.0000-0001-7647-119X
Sun, X.0000-0001-8817-4643
Additional Information:© 2018 American Physical Society. Received 3 December 2017; published 26 March 2018. This work is supported in part by the U.S. Department of Energy, Office of Nuclear Physics (DE-FG02-08ER41557, DE-SC0014622, DE-FG02-97ER41042) and the National Science Foundation (NSF-0700491, NSF-1002814, NSF-1005233, NSF-1102511, NSF-1205977, NSF-1306997, NSF-1307426, NSF-1506459, and NSF-1615153). We gratefully acknowledge the support of the LDRD program (20110043DR), and the LANSCE and AOT divisions of the Los Alamos National Laboratory. We thank M. González-Alonso for comments on this work.
Funding AgencyGrant Number
Department of Energy (DOE)DE-FG02-08ER41557
Department of Energy (DOE)DE-SC0014622
Department of Energy (DOE)DE-FG02-97ER41042
Los Alamos National Laboratory20110043DR
Issue or Number:3
Record Number:CaltechAUTHORS:20180327-130512791
Persistent URL:
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:85460
Deposited By: Tony Diaz
Deposited On:27 Mar 2018 20:35
Last Modified:09 Mar 2020 13:18

Repository Staff Only: item control page