RUI: The aCORN Experiment to Measure the Beta-Neutrino Asymmetry in Neutron Decay

Hamilton College, Clinton NY

Investigators

Abstract

The aCORN collaboration intends to measure the beta-neutrino asymmetry, a, in neutron decay with a relative uncertainty of 1%. The decay of the free neutron provides a nearly ideal system in which to probe the limits of the Standard Model of the electroweak interaction. Some current tests of the self-consistency of the Standard Model are limited by the 4% experimental uncertainty in a. An improved measurement of a also provides a new method to measure a related quantity known as Vud that does not require neutron polarimetry. The aCORN experiment relies on separating neutron decays from a cold neutron beam at NIST into two classes; one in which the beta and neutron are emitted nearly parallel, and one in which they are anti-parallel. A set of electric and magnetic fields guide the particles to detectors at opposite ends of a 3m long vacuum vessel reducing the determination of a to counting the numbers of protons in the two classes. Considerable effort has gone into studying potential sources of systematic error, and the apparatus has been designed to reduce systematic errors below 0.5%. The apparatus is currently being assembled at Indiana University in preparation for a move to NIST. A principle source of systematic error is mis-alignment between the magnetic field and the collimators that define the experimental axis. We have devised a mapping and alignment procedure that will be tested at Indiana, and we now propose to repeat the procedure in the more difficult magnetic environment posed by NIST. In addition we will directly measure the alignment of the system by measuring the distribution of electrons from a hot filament at the proton detector position. After prototype testing at Hamilton, we will implement the final alignment check in the aCORN apparatus at NIST. The work will continue to involve significant numbers of undergraduates, both at Hamilton and at NIST. This will allow undergraduates from a small college to take ownership of a portion of the experiment Hamilton, and then to see their piece integrated into the final system in the new environment of a national laboratory.