Search

In the standard model of particle physics, the weak interaction is described by vector and axial-vector couplings only. Non-zero 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., Comment: Phys. Rev. Lett., https://journals.aps.org/prl/

In addition to the common 3-body decay of the neutron $n\rightarrow p e^-\overline{\nu_e}$ there should exist an effective 2-body subset with the electron and proton forming a Hydrogen bound state with well defined total momentum, total spin and magnetic quantum numbers. The atomic spectroscopic analysis of this bound system can reveal details about the underlying weak interaction as it mirrors the helicity distributions of all outgoing particles. Thus, it is unique in the information it carries, and an experiment unravelling this information is an analogue to the Goldhaber experiment performed more than 60 years ago. The proposed experiment will search for monoenergetic metastable BoB H atoms with 326 eV kinetic energy, which are generated at the center of a throughgoing beamtube of a high-flux reactor (e.g., at the PIK reactor, Gatchina). Although full spectroscopic information is needed to possibly reveal new physics our first aim is to prove the occurrence of this decay and learn about backgrounds. Key to the detection is the identification of a monoerergtic line of hydrogen atoms occurring at a rate of about 1 $\rm{s}^{-1}$ in the environment of many hydrogen atoms, however having a thermal distribution of about room temperature. Two scenarios for velocity (energy) filtering are discussed in this paper. The first builds on an purely electric chopper system, in which metastable hydrogen atoms are quenched to their ground state and thus remain mostly undetectable. This chopper system employs fast switchable Bradbury Nielsen gates. The second method exploits a strongly energy dependent charge exchange process of metastable hydrogen picking up an electron while traversing an argon filled gas cell, turning it into manipulable charged hydrogen. The final detection of hydrogen occurs through multichannel plate (MCP) detector., Comment: 7 pages, 18 figures, conference report

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 $\lambda = g_A/g_V = -1.27641(45)_\mathrm{stat}(33)_\mathrm{sys}$ which confirms recent measurements with improved precision. This corresponds to a value of the parity violating beta asymmetry parameter of $A_0 = -0.11985(17)_\mathrm{stat}(12)_\mathrm{sys}$. We discuss implications on the CKM matrix element $V_{ud}$ and derive a limit on left-handed tensor interaction.

The authors demonstrate an enhanced electron time-of-flight (ToF) method that overcomes limitations of conventional ToF experiments. They use an inverse magnetic mirror configuration in which the ToF is almost independent of emission angle. In particular at lower $\ensuremath{\beta}$-decay energies, where flight times are long, one can obtain the electron kinetic energy with significantly improved accuracy. Applications range from the precise calibration of energy-sensitive scintillators to future neutron-decay experiments.

The beta asymmetry in neutron beta decay is used to determine the ratio of axial-vector coupling to vector coupling most precisely. In electron spectroscopy, backscattering of electrons from detectors can be a major source of systematic error. We present the determination of the correction for undetected backscattering for electron detection with the instrument PERKEO III. For the electron asymmetry, undetected backscattering leads to a fractional correction of $5\times 10^{-4}$, i.e. a change by 40% of the total systematic uncertainty., Proceedings of the International Workshop on Particle Physics at Neutron Sources PPNS 2018, Grenoble, France, May 24-26, 2018

The PERC collaboration will perform high-precision measurements of angular correlations in neutron beta decay at the beam facility MEPHISTO of the Forschungs-Neutronenquelle Heinz Maier-Leibnitz in Munich, Germany. The new beam station PERC, a clean, bright, and versatile source of neutron decay products, is designed to improve the sensitivity of neutron decay studies by one order of magnitude. The charged decay products are collected by a strong longitudinal magnetic field directly from inside a neutron guide. This combination provides the highest phase space density of decay products. A magnetic mirror serves to perform precise cuts in phase space, reducing related systematic errors. The new instrument PERC is under development by an international collaboration. The physics motivation, sensitivity, and applications of PERC as well as the status of the design and preliminary results on uncertainties in proton spectroscopy are presented in this paper.