UCNA+ Neutron Beta-Asymmetry Experiment

The UCNA+ experiment measures the angular correlation (A_o) between the neutron's spin and the emitted electron's momentum after beta decay. The axial coupling constant extracted using A_o alongside neutron lifetime determines the strength of the quark-mixing element (V_ud) of the Cabibbo-Kobayashi-Maskawa (CKM) matrix. The measurement allows for a check on the unitarity of the top row of the CKM matrix and, hence, provides an avenue to find traces of physics beyond the Standard Model and sanity check for nuclear beta-decay estimates.

UCNA+ is an upgrade to the UCNA experiment and takes advantage of the increased ultracold neutron density after the source upgrades for better statistical precision. The experiment will have sophisticated EJ200 scintillator-based detectors to reconstruct electron position and energy. It will also have a precision source scanner for in-situ calibration inside the superconducting magnet. The combined effect of the new detector and the scanner minimizes the systemic uncertainties in the energy reconstruction of the electron events. The simulations for the experiment also suggest more than ~ 50% reduction in back-scattering event corrections from magnetic-mirror effects. The experiment aims to achieve a statistical precision of ~ 0.2%, similar to the best single measurement with a cold neutron beam. In this contribution, I will highlight the main components and status of the experiment with the expected error budget for the measurement.