Session T12: Mini-Symposium: The Neutron Lifetime Discrepancy

UCNτ+: Higher Precision on the Neutron Lifetime through Improved Loading of the UCNτ Magneto-Gravitational Trap

UCNτ+ is an upgrade underway to increase the sensitivity of the UCNτ experiment at the Los Alamos Neutron Science Center.  Neutron decay measurements determine the weak coupling constants for the nucleon which are free from nuclear structure corrections, providing powerful tests for beyond standard model effects and strongly motivating experimental progress.  The most precise measurement of the free neutron lifetime was recently reported by the UCNτ collaboration, with the planned UCNτ+  upgrade promising a factor of two or more improvement over current UCNτ precision. The UCNτ project utilizes the existing UCNτ magneto-gravitational trap, introducing an adiabatic transport system to improve the number of loaded ultracold neutrons (UCN) and implementing a higher rate (in situ) UCN detector.  An introduction to the UCNτ+ project is presented, highlighting Monte-Carlo simulations of a conceptual design for the adiabatic transport system.  Aspects of the transport strategy and the impact of transport parameters such as guide specularity, guide loss per bounce, and gaps between transport elements, are studied to understand their effects on the final detected UCN population. We also characterize the issue of depolarization while loading the trap.   

Neutron-Axion Scattering and the Neutron Lifetime Puzzle

Neutron lifetime anomaly is a discrepancy of results between two main types of experiment: the beam and the bottle methods. The difference between the most precisely measured results of each of the types is 10 ± 2 s, exceeding the 3σ bound. We consider a possible explanation for the anomaly, in which the neutrons in the experiments gain small recoil energy and are ejected from an apparatus due to the interactions with dark matter particles, as the solar system moves through the dark matter halo of the galaxy, with a mean velocity of 250 km/s. This could lead to an uncontrolled loss of neutrons, resulting in a shorter neutron lifetime in bottle experiments. Due to the unique capabilities of the UCNτ experiment to measure tiny recoil energies (∽10 neV), we base our calculations on the light dark matter models, using axions as convenient candidates. We present a calculation of the neutron-axion scattering cross-sections and use the lifetime differences to constrain various axion models, putting bounds on the dark matter's mass and its average density around the Earth's surface.   

The BL3 Beam Neutron Lifetime Experiment

BL3 is a next generation beam neutron lifetime experiment using the Sussex-ILL-NIST method. BL3 is based on the principles of the existing NIST apparatus but it will employ a larger and more uniform superconducting magnet, a larger and higher flux neutron beam, and a large diameter segmented silicon detector, giving a >100 times increase in proton trapping rate. A number of new features to test and explore systematic effects will be included. The goal of BL3 is twofold: 1) to search for an explanation for the neutron lifetime discrepancy and thoroughly validate systematics in the beam method at the 1 s level; and 2)reduce the ultimate precision in the beam method to <0.3 s. An overview of BL3 and its design will be presented.   

Proton Detection System for the BL3 Experiment

The BL3 experiment aims to measure the lifetime of the free neutron using the Sussex-ILL-NIST method to an uncertainty below 0.3 s.  Protons from in-flight decays of cold neutrons are trapped in a segmented electrode trap, which is “emptied” periodically and the protons guided to a detector via a magnetic field.  This approach requires absolute proton counting.  To reach the improved precision necessary for Standard Model tests with this approach requires a large area, segmented proton detector.  The design of the system will be discussed as well as the program of measurements to understand and characterize potential proton loss mechanisms.   

A New Neutron Fluence Calibration Monitor for the BL3 Experiment

The BL3 experiment determines the neutron lifetime by measuring the rate of decay protons emerging from a beam of cold neutrons at the NIST research reactor. To reach the precision goal of less than 0.03%, the fluence of the neutron beam passing through the decay volume needs to be determined to a precision better than accuracy attained in the previous calibration[A. Yue, et al., Metrologia 55, 460 (2018)]. To improve the performance beyond the current statistical limit, we will implement a new "Alpha-Gamma device" (AG) to accept a larger neutron beam (~35mm vs 10mm for BL2) with higher detection efficiency. The new device will employ the previous 4-step measurement procedure based on geometrical efficiency determination, but will also employ an alternative, independent, coincidence method that provides a "first principles" calibration [Gilliam, Greene, Lamaze G P 1989 Nucl. Instrum. Meth., A 284, 220, (1989)]. In this talk, we will describe the working principles of the new AG device that includes optimized detector positioning to reduce systematic effects from the extended beam profile and angular correlations between the reaction particles as well as precision in-situ target positioning for rapid target alternation.   

Experimental Search for $n \rightarrow n'$ Oscillations to Explain the Neutron Lifetime Discrepancy

The theory of "mirror matter" restores parity to the standard model by hypothesizing a copy of the standard model with right-handed weak interactions. One potential way to explain the $>4 \sigma$ discrepancy between beam and bottle neutron lifetime experiments invokes neutrons oscillating into their mirror neutron partners. Mirror neutron oscillations as a source of loss in bottle lifetime experiments are constrained by the agreement between $V_{ud}$ extracted from neutron $\beta$-decay and the value of $V_{ud}$ required for CKM unitarity. However, in the event of a small mass difference $\Delta m$ between the normal and mirror neutron states, the 4.6 T magnetic field present in the NIST Beam Lifetime experiment could cause an apparent increase in the measured $\tau_n$. An experiment at Oak Ridge National Lab probed this theory by searching for the hypothesized reappearance of neutrons passing through a B$_4$C absorber inside a magnetic field varying between a peak strength of -6.6 T and +6.6 T. This talk presents new limits on neutron oscillations into non-degenerate mirror matter, excluding this effect as an explanation of the neutron lifetime discrepancy. We will also discuss further efforts to improve these limits and search for other models of $n \rightarrow n'$ oscillation.