The Nab experiment: A precision measurement of unpolarized neutron beta decay

¹ Department of Physics, University of Virginia, Charlottesville, VA 22904–4714, USA
² Department of Physics, Arizona State University, Tempe, AZ 85287–1504, USA
³ Physics Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA
⁴ Universidad Nacional Autónoma de México, México, D.F. 04510, México
⁵ Department of Physics, North Carolina State University, Raleigh, NC 27695–8202, USA
⁶ Department of Physics and Astronomy, University of Tennessee, Knoxville, TN 37996, USA
⁷ Department of Physics and Astronomy, University of Kentucky, Lexington, KY 40506, USA
⁸ Department of Physics and Astronomy, University of Sussex, Brighton BN19RH, UK
⁹ University of Michigan, Ann Arbor, MI 48109, USA
¹⁰ Department of Physics, University of Manitoba, Winnipeg, Manitoba, R3T 2N2, Canada
¹¹ KIT, Universität Karlsruhe (TH), Kaiserstraße 12, 76131 Karlsruhe, Germany
¹² Department of Physics and Astronomy, University of South Carolina, Columbia, SC 29208, USA
¹³ Department of Chemistry and Physics, University of Tennessee at Chattanooga, Chattanooga, TN 37403, USA
¹⁴ Department of Physics, North Carolina State University, Raleigh, NC 27695–8202, USA
¹⁵ Los Alamos National Laboratory, Los Alamos, NM 87545, USA
¹⁶ Department of Physics, University of Winnipeg, Winnipeg, Manitoba R3B2E9, Canada
¹⁷ TRIUMF, Vancouver, Canada, V6T 2A3, USA

^a e-mail: jaf2qc@virginia.edu

Published online: 12 December 2019

Abstract

Neutron beta decay is one of the most fundamental processes in nuclear physics and provides sensitive means to uncover the details of the weak interaction. Neutron beta decay can evaluate the ratio of axial-vector to vector coupling constants in the standard model, λ = g_A/g_V, through multiple decay correlations. The Nab experiment will carry out measurements of the electron-neutrino correlation parameter a with a precision of δa/a = 10⁻³ and the Fierz interference term b to δb = 3 × 10⁻³ in unpolarized free neutron beta decay. These results, along with a more precise measurement of the neutron lifetime, aim to deliver an independent determination of the ratio λ with a precision of δλ/λ = 0.03% that will allow an evaluation of V_ud and sensitively test CKM unitarity, independent of nuclear models. Nab utilizes a novel, long asymmetric spectrometer that guides the decay electron and proton to two large area silicon detectors in order to precisely determine the electron energy and an estimation of the proton momentum from the proton time of flight. The Nab spectrometer is being commissioned at the Fundamental Neutron Physics Beamline at the Spallation Neutron Source at Oak Ridge National Lab. We present an overview of the Nab experiment and recent updates on the spectrometer, analysis, and systematic effects.