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 = gA/gV, 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 Vud 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.
Highlights
3|λ|2), where GF is the Fermi constant, Vud is the first diagonal term in the CKM matrix, and λ is the ratio of the axial vector to vector coupling constants, λ =
We present an overview of the Nab experiment and recent updates on the spectrometer, analysis, and systematic effects
The initial characterization of the detectors at the Triangle Universities Nuclear Laboratory (TUNL) proton accelerator showed a dead layer of 100 nm and a resolution near 3 kV [23, 24], meeting the needs of the Nab experiment
Summary
Where pe, pν, Ee, and Eν are the momenta and energy of the decay electron and neutrino and E0 is the endpoint. Independent extractions of λ from different correlation coefficients offer a different set of systematic uncertainties and consistency checks and are necessary to entangle Vud from the neutron lifetime. The neutron sector must measure the neutron lifetime to ∼ 0.3 s and λ to ∼ 3 × 10−4 to competitively test the most precise determination of Vud from 0+ → 0+ decays [17]. The Nab experiment aims for a high precision measurement of a with an expected error of a/a ∼ 1 × 10−3 or λ/λ ∼ 3 × 10−4, about a factor of 40 more precise than the most precise extractions to date [19,20,21] and a factor of 9 more precise than the preliminary results of the aSPECT experiment announced at this conference [7]
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