Transport of Polarized Ultra Cold Neutrons for UCN-A Fundamental Symmetry Study

Virginia Polytechnic Institute And State University, Blacksburg VA

Investigators

Abstract

Nuclear beta decay revealed that Nature indeed violates what had been thought to be a very fundamental symmetry: parity (looking at the world through a mirror). Greatly improved precision of this measurement lets us again probe beyond the 'Standard Model'. By measuring the directional correlation between a polarized neutron and the electron emitted when it decays into a proton - coupled with the neutron and muon lifetimes - we can test whether our understanding of quark 'mixing' is complete. (A neutron has two 'down' and one 'up' quarks, while the proton has two 'up' and one 'down' quarks - in neutron beta decay a 'down' quark becomes an 'up' quark.) If the current picture is correct, the quark mixing matrix should be unitary (much like the length of a vector doesn't change when you rotate its coordinate system). Any deviation from unitarity would show new physics at work and be of fundamental importance. To measure this correlation, we create 'Ultra-Cold Neutrons' (UCN) at the Los Alamos National Laboratory which undergo total reflection - much like light in fiber optics - meaning these neutrons can be transported long distances in special guides and are also easily polarized. This dramatically reduces systematic errors otherwise present due to beam-induced backgrounds and incomplete polarization. We have shown that the best guides are diamond coated, which we fabricate at Virginia Tech (with this grant) using 'pulsed laser deposition' of 'diamond-like carbon'. Upstream of where the neutrons get polarized we can also use Nickel-58 coatings, which we are also able to to. The experiment in in its final commissioning stages, and several test runs have already been made. We anticipate getting our first one percent measurement of the neutron's beta asymmetry this year, and bringing to bear all the advantages of a UCN experiment to reach the goal of 0.2% before we are finished. This grant also supports our efforts for the data acquisition system to run the experiment.