The Nab Experiment: A Precision Measurement of Unpolarized Neutron Beta Decay

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, $\lambda =g_A/g_V$, through multiple decay correlations. The Nab experiment will carry out measurements of the electron-neutrino correlation parameter $a$ with a precision of $\delta a/a=10^{-3}$ and the Fierz interference term $b$ to $\delta b=3\times 10^{-3}$ 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 $\lambda$ with a precision of $\delta \lambda /\lambda =0.03\%$ 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.

arXiv commentPresented at PPNS2018

References in corpus (14)

• Measurement of the neutron lifetime using a gravitational trap and a low-temperature Fomblin coating
• Reduced hadronic uncertainty in the determination of $V_{ud}$
• Improved Determination of the Neutron Lifetime
• Superallowed 0+ --> 0+ nuclear beta decays: 2014 critical survey, with precise results for Vud and CKM unitarity
• New physics searches in nuclear and neutron $\beta$ decay
• Measurement of the Weak Axial-Vector Coupling Constant in the Decay of Free Neutrons Using a Pulsed Cold Neutron Beam
• Measurement of the neutron lifetime using an asymmetric magneto- gravitational trap and in situ detection
• Determination of the Weak Axial Vector Coupling from a Measurement of the Beta-Asymmetry Parameter A in Neutron Beta Decay
• Neutron lifetime measurements with the big gravitational trap for ultracold neutrons
• New result for the neutron $\beta$-asymmetry parameter $A_0$ from UCNA
• A clean, bright, and versatile source of neutron decay products
• Nab: Measurement Principles, Apparatus and Uncertainties
• Fundamental Neutron Physics Beamline at the Spallation Neutron Source at ORNL
• New precision measurements of free neutron beta decay with cold neutrons

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• Measurement of the neutron decay electron-antineutrino angular correlation by the aCORN experiment
• Precise Measurements of the Decay of Free Neutrons
• Limit on the Fierz Interference Term b from a Measurement of the Beta Asymmetry in Neutron Decay
• Radiative corrections to semileptonic beta decays: Progress and challenges
• All orders factorization and the Coulomb problem
• Field Theory of the Fermi Function
• Renormalization of beta decay at three loops and beyond
• Pseudo-neutrino versus recoil formalism for 4-body phase space and applications to nuclear decay
• NoMoS: An $R\times B$ Drift Momentum Spectrometer for Beta Decay Studies
• A Novel, Steerable, Low-Energy Proton Source for Detector Characterization
• Characterization of Low-energy Ionization Signals in Silicon Detectors for the Nab Experiment