Session LL: Mini-Symposium on Fundamental Symmetries: Theory and Experiment IV

Status and Prospects in Neutron Beta Decay

Neutron beta decay is an exceptional laboratory for sensitive experimental tests which can reveal new particles and interactions beyond our Standard Model of Particle Physics. Recent advances in both theory and experiment have opened the door for the neutron to play an increasingly important role as a reference system for unitarity tests of the Cabibbo-Kobayashi-Maskawa quark-mixing matrix and constrain extensions to the Standard Model which predict non-V--A currents. These low energy precision measurements have sensitivity to new physics at energy scales comparable to or above the reach of the Large Hadron Collider. This presentation will review the state of the art in experimental determinations of neutron beta decay observables and outlook for precision tests and probes for new physics in the next generation.   

The BL2 Experiment: An In-Beam Measurement of the Neutron Lifetime

Neutron beta decay is the simplest example of semi-leptonic decay. A precise measurement of the neutron lifetime and $\lambda$, the ratio of axial vector and vector coupling constants of the weak interaction, allow for a determination of the CKM matrix element V$_{ud}$ that is free from nuclear structure effects. The neutron lifetime provides an important test of unitarity and consistency of the Standard Model. The neutron lifetime is also the largest uncertainty in Big Bang Nucleosynthesis calculations of light element abundance. A new measurement of the neutron lifetime using the in-beam method is ongoing at the NIST Center for Neutron Research. This method requires the absolute counting of the decay protons in a neutron beam of precisely known flux. Improvements in the neutron and proton detection systems as well as the use of a new analysis technique should allow for a thorough investigation of major systemic effects. The experimental status, systematic tests, analysis techniques and early data will be presented.   

Current Status of the UCNtau Experiment

The UCNtau experiment uses a combination of permanent magnets and gravity to contain polarized ultracold neutrons (UCN's) produced at Los Alamos National Lab for measuring the free neutron lifetime. This value is a major input parameter for Big Bang nucleosynthesis predictions and, with neutron decay correlation coefficients, can be used to probe beyond Standard Model physics. Additionally, over the past two years a significant discrepancy between lifetime measurements that count surviving neutrons, like UCNtau, and those that detect decay products has prompted the creation of many exotic and exciting explanations. UCNtau is now collecting data to achieve a total uncertainty around 0.25 seconds. Improvements for this cycle include a non-magnetic buffer volume upstream of the trap, a new dagger in-situ neutron detector, an upgrade to the giant spectrum cleaner that allows for active detection of removed neutrons and a monitor expressly for setting empirical limits on depolarization. These enhancements will be described and an update on the analysis of the blinded data from the 2018 data cycle will be presented.   

Measurement of the Free-Neutron Lifetime Using Space-based Neutron Data from NASA's MESSENGER Mission

Precise knowledge of the free neutron lifetime, $\tau_n$, is required to test the consistency of the standard model and uncertainties in $\tau_n$ dominate those in predicted primordial $^{\rm 4}$He abundance from Big Bang nucleosynthesis. Presently, there exist two classes of experiments that have successfully made measurements of $\tau_n$. The `Beam' class involves measuring the activation of cold neutron beams and the `Bottle' class uses storage (material, magnetic and/or gravitational) to trap neutrons and measure the rate of decay during storage. However, there currently exists a 4\ $\sigma$ disagreement between the `beam' and `bottle' measurements. We have developed a new technique for using space-based neutron spectroscopy measurements to determine $\tau_n$. Under this technique the change in planet-originating neutron flux with planet-to-spacecraft distance yields a measure of $\tau_n$. Here, we will present an analysis of data from the neutron spectrometer on NASA's MESSENGER mission as a proof-of-principle demonstration of a space-based $\tau_n$ measurement. In this talk, we will discuss the basis of the technique, statistical and systematic errors of the measurement, and preliminary results will be presented.   

Tau2: A next generation neutron lifetime experiment based on UCN$\tau $

The UCN$\tau $ experiment measures the free neutron lifetime by in situ counting of surviving ultracold neutrons (UCNs) after different storage times in an asymmetric magneto-gravitational storage volume.~ This experiment has acquired sufficient data for a measurement of the neutron lifetime with a statistical uncertainty of about 0.3 s and has demonstrated a systematic uncertainty of 0.28 s; it is expected to ultimately reach a total uncertainty of about 0.2 s, limited primarily by the efficiency with which UCN$\tau $ utilizes the neutrons supplied by the Los Alamos UCN facility.~ In this talk, we will describe the conceptual Tau2 experiment, which is intended to use the techniques learned during the UCN$\tau $ experiment to minimize systematic uncertainties while maximizing the statistical reach possible with UCNs supplied by the Los Alamos source.~ Replacing the permanent magnet-based trap of UCN$\tau $ with a larger volume superconducting trap is expected to enable improving the total uncertainty on the neutron lifetime to 0.1 s and beyond, permitting investigation of the weak nuclear force, when taken with precision beta decay correlation experiments, with physics reach comparable to LHC-based high energy experiments.   

Simulations of the UCN velocity distribution at Los Alamos National Lab

Ultracold neutron (UCN) velocity distributions are particularly important to precision UCN experiments due to the velocity dependence in material bottle interactions, and other energy-sensitive effects such as magnetic interactions and bounce height. While the UCNs for the Spallation Neutron Source's neutron Electric Dipole Moment experiment at Oak Ridge National Lab will be created in situ (cold neutrons downscattered to UCN in the measurement cell), components such as the measurement cell are characterized at the UCN beamline at Los Alamos National Lab. A velocity spectrometer is being designed and created to measure the velocity distribution in this UCN beamline using the bounce height distribution of the neutrons based on ongoing development of this detector technology by members of the UCNTau collaboration. Simulations of this method will show its effectiveness, considering specularity, contaminants, and UCN spectrum evolution as they bounce down the beamline to the detector.   

BL3: A next generation "beam" experiment to measure the neutron lifetime

Neutron beta decay is an archetype for all semi-leptonic charged-current weak processes. A precise value for the neutron lifetime is required for consistency tests of the Standard Model and is needed to predict the primordial 4He abundance from the theory of Big Bang Nucleosynthesis. An effort is under way for an in-beam measurement of the neutron lifetime that is able to evaluate the systematic uncertainties at the 0.3 s level. This effort is part of a phased campaign of neutron lifetime measurements based at the NIST Center for Neutron Research, using the Sussex-ILL-NIST technique. Recent advances in neutron fluence measurement techniques as well as new large area silicon detector technology address the two largest sources of uncertainty of in-beam measurements, paving the way for a new measurement. The experimental design, schedule, and projected uncertainties for the main subsystems will be discussed.