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.
We live in a universe in which all observable phenomena appear to be explained in terms of four fundamental forces; gravity, electromagnetic force, the strong nuclear force, and the weak force. Gravity holds galaxies together, holds the earth in orbit round the sun, and holds us onto the earth. The electromagnetic force lights our lamps, drives our motors, and is responsible for most of the physics we see around us from friction to burning gasoline to make cars move. The strong force holds the nuclei in all our atoms together and the weak force causes the radioactive decay that allows a few kinds of nuclei to change their nature. The aCORN experiment seeks to measure one of the properties of this weak force in order to probe our understanding of its nature. Neutrons, the uncharged particles that make up about half the mass of all matter, are heavier than the positively charged protons that make up the other half. So long as it is not bound together in a nucleus by the strong force, a neutron will spontaneously decay into a proton, an electron, and an antineutrino. The Standard Model, our current model of the weak interaction, predicts that there will be a small difference in the probability that the electron and the antineutrino will emerge traveling in the same direction and the probability that they will emerge in opposite directions. The difference between those probabilities is the beta-neutrino asymmetry which is usually described by a parameter called ‘a’. aCORN is an experiment to measure the a parameter as a test of the Standard Model. The aCORN experiment is the result of a collaboration between Tulane University, Indiana University, De Pauw University, the National Institute of Standards and Technologies (NIST), and Hamilton College. The apparatus is installed on a reactor at the National Institute of Standards and Technology (NIST), and took preliminary data over the winter of 2010-2011. Hamilton College has been involved many aspects of the experiment including the design and computational modeling of the magnetic and electric fields, the design and testing a system to verify the alignment of the experiment, and analysis of the data. The aCORN experiment has been a valuable training ground for a large number of undergraduates at Hamilton College. With the individual attention associated with a small college, they have been able to take a part of a large-scale experiment and later see how their part fits into the greater whole. Over the three years, 10 undergraduates have done their senior theses on the aCORN experiment or worked on it during a summer. Of the six who have graduated two are now in physics graduate school and one is heading toward a joint MD/PhD program. We have been able to bring the physics of this current experiment into several of our classes, helping make the physics of the classroom more concrete for our students. In addition, our students have been involved with bringing the excitement of physics to our local elementary and middle schools.
