First Full Dalitz Plot Measurement in Neutron $\beta$-Decay using the Nab Spectrometer and Implications for New Physics

TL;DR

This paper reports the first full Dalitz plot measurement of neutron beta decay using the Nab spectrometer, providing new insights into weak interactions and potential new physics beyond the Standard Model.

Contribution

It introduces a novel experimental setup and presents the first comprehensive Dalitz plot for neutron beta decay, enhancing the precision of decay correlation measurements.

Findings

First full Dalitz plot of neutron beta decay

New constraints on excited neutron states

Implications for physics beyond the Standard Model

Abstract

Precision measurements of observables in neutron $\beta$-decay are used to test the Standard Model description of the weak interaction and search for evidence of new physics. The Nab experiment at the Fundamental Neutron Physics Beamline at the Spallation Neutron Source was constructed to measure correlations in neutron decay by utilizing an asymmetric spectrometer and novel detection system to accurately reconstruct the proton momentum and electron energy for each $\beta$-decay. This work describes the detection of neutron $\beta$-decay products in the Nab spectrometer and presents the first full Dalitz plot representation of the phase space of neutron $\beta$-decay for all electrons >100 keV. In addition, new constraints are placed on a possible excited neutron state, hypothesized to explain the disagreement between the appearance and disappearance neutron lifetime techniques.