First full Dalitz plot measurement in neutron <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mi>β</mml:mi> </mml:math> decay using the Nab spectrometer and implications for new physics

Precision measurements of observables in neutron <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"> <a:mi>β</a:mi> </a:math> 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 <b:math xmlns:b="http://www.w3.org/1998/Math/MathML"> <b:mi>β</b:mi> </b:math> decay. This work describes the detection of neutron <c:math xmlns:c="http://www.w3.org/1998/Math/MathML"> <c:mi>β</c:mi> </c:math> -decay products in the Nab spectrometer and presents the first full Dalitz plot representation of the phase space of neutron <d:math xmlns:d="http://www.w3.org/1998/Math/MathML"> <d:mi>β</d:mi> </d:math> decay for all electrons <e:math xmlns:e="http://www.w3.org/1998/Math/MathML"> <e:mrow> <e:mo>&gt;</e:mo> <e:mn>100</e:mn> </e:mrow> </e:math> 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.