Limit on the Fierz Interference Term b from a Measurement of the Beta Asymmetry in Neutron Decay.

In the standard model of particle physics, the weak interaction is described by vector and axial-vector couplings only. Nonzero scalar or tensor interactions would imply an additional contribution to the differential decay rate of the neutron, the Fierz interference term. We derive a limit on this hypothetical term from a measurement using spin-polarized neutrons. This method is statistically less sensitive than the determination from the spectral shape but features much cleaner systematics. We obtain a limit of b=0.017(21) at 68.27% C.L., improving the previous best limit from neutron decay by a factor of four.

Measurement of the Weak Axial-Vector Coupling Constant in the Decay of Free Neutrons Using a Pulsed Cold Neutron Beam.

We present a precision measurement of the axial-vector coupling constant g_{A} in the decay of polarized free neutrons. For the first time, a pulsed cold neutron beam was used for this purpose. By this method, leading sources of systematic uncertainty are suppressed. From the electron spectra we obtain λ=g_{A}/g_{V}=-1.27641(45)_{stat}(33)_{sys}, which confirms recent measurements with improved precision. This corresponds to a value of the parity violating beta asymmetry parameter of A_{0}=-0.11985(17)_{stat}(12)_{sys}. We discuss implications on the Cabibbo-Kobayashi-Maskawa matrix element V_{ud} and derive a limit on left-handed tensor interaction.

Constraints on the Dark Matter Interpretation n→χ+e

Discrepancies from in-beam- and in-bottle-type experiments measuring the neutron lifetime are on the 4σ standard deviation level. In a recent publication Fornal and Grinstein proposed that the puzzle could be solved if the neutron would decay on the one percent level via a dark decay mode, one possible branch being n→χ+e^{+}e^{-}. With data from the Perkeo II experiment we set limits on the branching fraction and exclude a one percent contribution for 95% of the allowed mass range for the dark matter particle.

Determination of the weak axial vector coupling λ = gA/gV from a measurement of the ß-asymmetry parameter A in neutron beta decay.

We report on a new measurement of the neutron ß-asymmetry parameter A with the instrument Perkeo II. The main enhancements are the high neutron polarization of P = 99.7(1)% from a novel arrangement of supermirror polarizers and reduced background from improvements in beam line and shielding. The leading corrections were thus reduced by a factor of 4, pushing them below the level of statistical error and resulting in a significant reduction of systematic uncertainty compared to our previous experiments. We derive the ß-asymmetry parameter A0 = -0.11972(45)(stat)((-44)(+32))(sys) = -0.11972((-65)(+53)) and the ratio of the axial vector to the vector coupling constant λ = gA/gV = -1.2761(12)(stat)((-12)(+9))(sys) = -1.2761((-17)(+14)).

Measurement of the proton asymmetry parameter in neutron beta decay.

The proton asymmetry parameter C in neutron decay describes the angular correlation between neutron spin and proton momentum. In this Letter, the first measurement of this quantity is presented. The result C= -0.2377(26) agrees with the standard model expectation. The coefficient C provides an additional parameter for new and improved standard model tests.

Measurement of the neutrino asymmetry parameter B in neutron decay.

A new measurement of the neutrino asymmetry parameter B in neutron decay, the angular correlation between neutron spin and antineutrino momentum, is presented. The result, B=0.9802(50), confirms earlier measurements but features considerably smaller corrections. It agrees with the standard model expectation and permits updated tests on "new physics" in neutron decay.

The Beta-, Neutrino- and Proton-Asymmetry in Neutron ß-Decay.

This article describes measurements of angular-correlation coefficients in the decay of free neutrons with the superconducting spectrometer PERKEO II. A method for measuring the ß-asymmetry coefficient A is presented, as well as a new method for determining the neutrino-asymmetry coefficient B, which allows a value for the proton-asymmetry coefficient C to be obtained for the first time. An ongoing experiment is trying to improve the accuracy of these quantities.