New result for the neutron β -asymmetry parameter A0 from UCNA

Abstract

Background:The neutron β-decay asymmetry parameter A 0 defines the angular correlation between the spin of the neutron and the momentum of the emitted electron. Values for A 0 permit an extraction of the ratio of the weak axial-vector to vector coupling constants, λ ≡ g A /g V , which under assumption of the conserved vector current hypothesis (g V = 1) determines g A . Precise values for g A are important as a benchmark for lattice QCD calculations and as a test of the standard model. Purpose: The UCNA experiment, carried out at the Ultracold Neutron (UCN) source at the Los Alamos Neutron Science Center, was the first measurement of any neutron β-decay angular correlation performed with UCN. This article reports the most precise result for A 0 obtained to date from the UCNA experiment, as a result of higher statistics and reduced key systematic uncertainties, including from the neutron polarization and the characterization of the electron detector response. Methods: UCN produced via the downscattering of moderated spallation neutrons in a solid deuterium crystal were polarized via transport through a 7 T polarizing magnet and a spin flipper, which permitted selection of either spin state. The UCN were then contained within a 3-m long cylindrical decay volume, situated along the central axis of a superconducting 1 T solenoidal spectrometer. With the neutron spins then oriented parallel or anti-parallel to the solenoidal field, an asymmetry in the numbers of emitted decay electrons detected in two electron detector packages located on both ends of the spectrometer permitted an extraction of A 0 .

Search for dark matter decay of the free neutron from the UCNA experiment: n→χ+e+e−

“…In this work we use the latest data from the Ultracold Neutron Asymmetry (UCNA) experiment [13][14][15][16][17] to put direct constraints on one of the proposed dark matter decay channels: n → χ + e + e − . In this decay mode, the e + e − sum energy is approximately equal to the entire mass difference between the neutron and the χ .…”

“…Older datasets show small systematic timing drifts, which had minimal impact on the β-decay asymmetry analysis [15], but would introduce significant additional background in this work. Thus, a total of approximately 14.55 ×10 6 neutron decays are considered for the present analysis, after applying all cuts and correcting for conventional β-decay detection efficiency [25].…”

“…The large signal with a peak at ∼25 ns is caused by backscattering electrons from neutron β-decay events. This is explicitly shown by the overlaid, normalized, resolutioncorrected, GEANT4 simulation of neutron β-decay backscatters which includes our spectrometer and detector effects [15,25]. Thus, the timing region for dark matter neutron decay is where the time difference is <16 ns.…”

“…Of course, the energy resolution of the detectors broadens this signal and is accounted for in this analysis. Over the analysis energy range (discussed below) the energy resolution is measured [14][15][16] to be E/E = 0.05 × √ 1 MeV/E. Furthermore, while the detector energy response for incident electrons has been carefully calibrated [15], the response for incident positrons is determined from a GEANT4 simulation using our full detector geometry.…”

“…Over the analysis energy range (discussed below) the energy resolution is measured [14][15][16] to be E/E = 0.05 × √ 1 MeV/E. Furthermore, while the detector energy response for incident electrons has been carefully calibrated [15], the response for incident positrons is determined from a GEANT4 simulation using our full detector geometry. 1 This simulation indicates that the fraction of positrons that deposit only their full kinetic energy in the scintillator is approximately 15% smaller than that of electrons over the energy range of interest, primarily due to annihilation.…”