Session GL: Fundamental Symmetries I

Commissioning the Nab Experiment

The Nab experiment at the Spallation Neutron Source will precisely measure the neutron beta decay correlation coefficients $a$, relating the momenta of the produced electron and antineutrino, and $b$, the Fierz interference term. Precise extraction of these two parameters can probe physics beyond the Standard Model by investigating CKM unitarity and the existence of scalar and tensor currents in the weak interaction. We will describe Nab's principles of operation and introduce some of the systematic considerations in these measurements. The Nab experiment uses the world's largest cryogen-free superconducting magnet to guide electrons and protons from neutron decays into one of two silicon detectors. The electron energy and the difference in time of flight between electrons and protons in coincident decays can be used to determine $a$ and $b$. This talk will describe Nab's ongoing commissioning, including preliminary data taken during this process.   

The BL3 experiment at NIST

Neutron beta decay provides input to the primordial abundance of 4He in Big Bang Nucleosynthesis, and is a theoretically clean test of CKM unitarity unencumbered by nuclear structure effects. However, the experimental determination of the neutron lifetime is plagued by a 8.7 second (4 sigma) discrepancy between in-flight decay and trapped ultra-cold neutron measurements. The goal of the BL3 experiment is to examine this discrepancy by improving the in-flight decay determination to 0.3 seconds, examining various systematic effects which might explain the difference. Enhancements in this experiment involve a larger, improved proton trap to measure the absolute neutron decay rate, and a neutron detector with improved detection uniformity and absolute calibration to measure the neutron fluence of the beam. Both new detectors were designed to take advantage of the high-flux NG-C beamline at the NIST Center for Neutron Research. I will describe the in-flight decay measurement technique and some innovations distinguishing the BL3 experiment from other measurements using this method.   

UCNτ: Recent Improvements, Current Status, and Outlook

The UCNτ collaboration recently measured the free neutron lifetime τ_n to a relative precision of 0.04% by confining ultracold neutrons (UCN) without material interactions via a magnetic plus gravitational trapping potential and utilizing a novel in situ detector. This measurement reached, for the first time, a level of precision sufficient to probe the current theoretical understanding of neutron decay. However, further improvements in precision are necessary to decouple Standard Model tests from theoretical uncertainties associated with nuclear structure corrections. Throughout the last several run cycles at Los Alamos National Laboratory, continued evaluation of systematics and refinements to our experimental techniques, in particular ongoing detector development and improvements to our normalization strategy, have demonstrated that the experiment is still operating in a statistics-limited regime. The current status of and outlook for the UCNτ experiment will be discussed in light of these advances.   

Tau+: an upgrade to the UCNτ neutron lifetime experiment

Tau+ is a planned upgrade to the existing UCNτ experiment that measures the free neutron lifetime.  Tau+ will share many of the features of UCNτ, including storing ultracold neutrons (UCN) produced by the LANSCE UCN source in an asymmetric trap closed by a Halbach array of permanent magnets on the bottom and sides and by gravity on the top; using an in situ scintillating detector to count the surviving neutrons after storage for different periods of time; and using large area moveable plastic sheets to remove any loaded UCNs above the gravitationally trappable energy range.  Tau+ will differ by improving the method used to load UCNs into the trap: instead of loading them through a moveable trapdoor in the bottom of the Halbach array, it will load them by lowering them into the trap from above in a moving “elevator” volume.  Simulations show that for a given production rate of UCNs from the LANSCE source, a factor of 9 greater number of UCNs will be loaded into the trap using this technique versus the trap door of UCNτ.  Since the UCNtau results published thus far have been statistically limited in overall precision, we expect an improvement in sensitivity in Tau+ of a factor of 3.  The status of the simulations and engineering design will be presented.

    

Reconstructing the Neutrino Energy Spectrum and Setting Tensor Current Limits through Precision Measurements of 8B Beta Decay

The beta decay of ⁸B is the dominant source of high-energy neutrinos produced in the Sun and is exceptionally sensitive to physics beyond the Standard Model. Preliminary results from a high-statistics experiment on ⁸B beta decay performed at Argonne National Laboratory using the Beta-decay Paul Trap (BPT) surrounded by four double-sided silicon strip detectors will be presented. By precisely measuring the alpha and beta particle energies from the beta decay of ⁸B to ⁸Be, which breaks up into two alpha particles, limits on the exotic tensor current contribution to the weak interaction can be determined. Additionally, the decay product kinematics can be leveraged to reconstruct the undistorted ⁸B neutrino energy spectrum, a vital ingredient for solar neutrino astrophysics experiments aiming to precisely measure neutrino oscillations.   

UCNA+: A precision measurement of the beta-asymmetry with ultracold neutrons

The UCNA experiment at the Ultracold neutron facility at Los Alamos National Lab measured the beta-asymmetry parameter A₀ to a precision of 0.6%. This parameter is used in determinations of the ratio of the axial and vector weak coupling constants and combined with measurements of the neutron lifetime is used to test the unitarity of the CKM quark mixing matrix and beyond the standard model physics searches. Upgrades of several key subsystems of UCNA are being investigated with the goal of reaching a precision of 0.2% in A₀. Upgrades of several key subsystems of UCNA will enable systematic uncertainties related to scattering and polarization to be reduced are in development.  The statistical sensitivity will be enhanced due to the recent upgrade of the LANL ultracold neutron source. A prototype of the electron detector is in development which would eliminate the MWPC gas counter and traditional PMT's in favor of an array of edge-coupled SiPM's. The SiPM array will provide energy, position, and timing data with resolution that is equal to or better than the original detector. This change will reduce systematic uncertainties that arose from scattering on the inactive material in the MWPC. We will present the status of the upgrade to UCNA and the development of a prototype detector array.   

Polarization Measurement of an Unpolarized Neutron Beam for Nab

The purpose of the Nab experiment is to accurately characterize unpolarized correlation coefficients in the neutron beta-decay. Any unwanted polarization of the neutrons would have a systematic impact on "a", which determines the angular correlation between the electron and antineutrino, for the neutron beta decay. The Nab experiment is trying to measure "a" to a relative accuracy on the order 10^-3 so which also poses an upper limit on the neutron beam polarization of 2*10^-5. By performing Adiabatic Fast Passage (AFP) to reverse the polarization of He-3 cell placed on the beamline and comparing the transmission rates of neutrons in different polarization states of the helium cell. We will present an overview of the method and results of polarimetry measurements.   

Characterization of Segmented Semiconductor Detectors for Neutron Beta Decay

The neutron beta decay process can be used to extract V_ud which is an extremely sensitive probe of CKM unitarity and Beyond-the-Standard-Model physics. The Nab collaboration will study this by collecting the decay protons (and electrons) in an asymmetric time-of-flight spectrometer. In this talk I will give an overview of the University of Manitoba’s low energy steerable proton source and discuss the characterization of the silicon detectors used in the Nab experiment with protons and radioactive sources. Specifically, an investigation of detector performance as a function of various system parameters will be presented. These parameters include, but are not limited to detector bias voltage, proton energy, temperature, and electronic noise.

    

Recent Progress from the BeEST Neutrino Experiment

The Beryllium Electron-capture in Superconducting Tunnel junctions (BeEST) experiment precisely measures the decay products of ⁷Be following nuclear electron capture (EC) to search for heavy neutrino mass states using superconducting quantum sensors.  Following up the first limits from the Phase-II data reported last year [PRL 126, 021803 (2021)] using a single detector pixel counting for a month, we have started work on Phase-III to scale the experiment to multi-pixel arrays of sensors.  In this talk I will present our progress towards increasing the sensitivity of the experiment by understanding the systematics associated with secondary particles from the decay (Auger and shaking electrons), materials effects, and sensor development for scaling to 32- and 128-pixel arrays.