Reanalysis of the ß-ν[over ¯]_{e} Angular Correlation Measurement from the aSPECT Experiment with New Constraints on Fierz Interference.

On the basis of revisions of some of the systematic errors, we reanalyzed the electron-antineutrino angular correlation (a coefficient) in free neutron decay inferred from the recoil energy spectrum of the protons which are detected in 4π by the aSPECT spectrometer. With a=-0.104 02(82) the new value differs only marginally from the one published in 2020. The experiment also has sensitivity to b, the Fierz interference term. From a correlated (b,a) fit to the proton recoil spectrum, we derive a limit of b=-0.0098(193) which translates into a somewhat improved 90% confidence interval region of -0.041≤b≤0.022 on this hypothetical term. Tighter constraints on b can be set from a combined [shown as superscript (c)] analysis of the PERKEO III (ß asymmetry) and aSPECT measurement which suggests a finite value of b with b^{(c)}=-0.0181±0.0065 deviating by 2.82σ from the standard model.

New Constraint on the Local Relic Neutrino Background Overdensity with the First KATRIN Data Runs.

We report on the direct search for cosmic relic neutrinos using data acquired during the first two science campaigns of the KATRIN experiment in 2019. Beta-decay electrons from a high-purity molecular tritium gas source are analyzed by a high-resolution MAC-E filter around the end point at 18.57 keV. The analysis is sensitive to a local relic neutrino overdensity ratio of η<9.7×10^{10}/α (1.1×10^{11}/α) at a 90% (95%) confidence level with α=1 (0.5) for Majorana (Dirac) neutrinos. A fit of the integrated electron spectrum over a narrow interval around the end point accounting for relic neutrino captures in the tritium source reveals no significant overdensity. This work improves the results obtained by the previous neutrino mass experiments at Los Alamos and Troitsk. We furthermore update the projected final sensitivity of the KATRIN experiment to η<1×10^{10}/α at 90% confidence level, by relying on updated operational conditions.

Bound on 3+1 Active-Sterile Neutrino Mixing from the First Four-Week Science Run of KATRIN.

We report on the light sterile neutrino search from the first four-week science run of the KATRIN experiment in 2019. Beta-decay electrons from a high-purity gaseous molecular tritium source are analyzed by a high-resolution MAC-E filter down to 40 eV below the endpoint at 18.57 keV. We consider the framework with three active neutrinos and one sterile neutrino. The analysis is sensitive to the mass, m_{4}, of the fourth mass state for m_{4}^{2}≲1000 eV^{2} and to active-to-sterile neutrino mixing down to |U_{e4}|^{2}≳2×10^{-2}. No significant spectral distortion is observed and exclusion bounds on the sterile mass and mixing are reported. These new limits supersede the Mainz results for m_{4}^{2}≲1000 eV^{2} and improve the Troitsk bound for m_{4}^{2}<30 eV^{2}. The reactor and gallium anomalies are constrained for 100<Δm_{41}^{2}<1000 eV^{2}.

Ion source for tests of ion behavior in the Karlsruhe tritium neutrino experiment beam line.

An electron-impact ion source based on photoelectron emission was developed for ionization of gases at pressures below 10(-4) mbar in an axial magnetic field in the order of 5 T. The ion source applies only dc fields, which makes it suitable for use in the presence of equipment sensitive to radio-frequency (RF) fields. The ion source was successfully tested under varying conditions regarding pressure, magnetic field, and magnetic-field gradient, and the results were studied with the help of simulations. The processes in the ion source are well understood, and possibilities for further optimization of generated ion currents are clarified.

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.

Scalar interaction limits from the beta-nu correlation of trapped radioactive atoms.

We have set limits on contributions of scalar interactions to nuclear beta decay. A magneto-optical trap provides a localized source of atoms suspended in space, so the low-energy recoiling nuclei can freely escape and be detected in coincidence with the beta. This allows reconstruction of the neutrino momentum, and the measurement of the beta-nu correlation, in a more direct fashion than previously possible. The beta-nu correlation parameter of the 0(+)-->0(+) pure Fermi decay of (38)K(m) is a =0.9981+/-0.0030+0.0032 / -0.0037, consistent with the standard model prediction a =1.

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.

Is the unitarity of the quark-mixing CKM matrix violated in neutron beta-decay?

We report on a new measurement of neutron beta-decay asymmetry. From the result A(0) = -0.1189(7), we derive the ratio of the axial vector to the vector coupling constant lambda = g(A)/g(V) = -1.2739(19). When included in the world average for the neutron lifetime tau = 885.7(7) s, this gives the first element of the Cabibbo-Kobayashi-Maskawa (CKM) matrix V(ud). With this value and the Particle Data Group values for V(us) and V(ub), we find a deviation from the unitarity condition for the first row of the CKM matrix of Delta = 0.0083(28), which is 3.0 times the stated error.