UCNA Experiment
UCNA is an experiment that will probe neutron decay with unprecedented precision. Previous experiments measuring the correlation between a neutron's spin and its decay electron's momentum (the "A-correlation") have arrived at inconsistent results. The UCNA experiment will measure this correlation with a factor of four improvement in precision and will bring our understanding of the weak interaction responsible for neutron decay to a new level. The results may offer a glimpse into new phenomena beyond the Standard Model of particle physics.
The experiment is part of a broader program to develop a high intensity source of Ultra-Cold Neutrons (UCN) in the United States; a first generation source is running now at Los Alamos National Laboratory. We have been taking asymmetry data since 2008 and have recently published a precision result on the neutron's axial vector weak coupling (read the article).
This web page is a resource for the involvement of Caltech in this project.
We are also involved in another UCN experiment searching for an electric dipole moment of the neutron.
This material is based upon work supported by the National Science Foundation under grant number 1205977. Any opinions findings and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.
Physics of UCN
The UCNA experiment will measure the difference in the number of electrons emitted along vs. against the spin direction of the decaying neutron. This asymmetry can be related to up and down quark weak mixing angle, sometimes called V_ud. A measurement of V_ud can be used to test the unitarity of the quark weak mixing matrix (the so-called CKM matrix) that is expected in the standard model. Any deviations from unitarity would be a signature of new physics beyond the standard model. The sensitivity of the UCNA experiment ("proposed" in the figure) is shown in the figure below.An overview of the experiment can be found in some of our recent TALKS and in our very long PROPOSAL.
UCNA/Caltech Experiment Overview
Experimental Apparatus Ultra-cold neutrons (UCN) from the UCN source are first polarized by a 7T superconducting solenoid. Their spin state can then be selected with an Adiabatic Fast Passage (AFP) RF spin flipper. A five meter long superconducting solenoid acts as a spectrometer for the decay electrons which are detected with Multi-Wire Proportional Chambers (MWPC) and plastic scintillator detectors.
Source
Ultra-cold neutrons (UCN) are produced by phonon downscattering from a solid deuterium "superthermal" moderator. The neutrons are produced via proton spallation from a Tungstan target at 800 MeV.
You can see an animated version of this picture here by clicking on Nuclear Science and Ultra-Cold Neutrons
Collaborators
California Institute of Technology
R. Carr, B. Filippone, T. M. Ito, J.W. Martin, R. McKeown, B. Tipton, J. Yuan
Institut Laue-Langevin
P. Geltenbort
Japan Atomic Energy Research Institute
K. Soyama
Los Alamos National Laboratory
T. Bowles (co-PI), M. Fowler, R. Hill, A. Hime, G. Hogan, S. Lamoreaux, C. Morris, A. Pichlmaeir, A. Saunders, S. Seestrom, P. Walstrom, J. Wilhelmy
North Carolina State University
A.R. Young (co-PI)
Petersburg Nuclear Physics Institute
A. Alduschenkov, A. Kharitonov, M. Lassakov, Yu. Rudnev, A. Serebrov, A. Vasilev
Princeton University
S. Hoedl, C-Y. Liu
Tohoku University
T. Kitagaki
Tokyo Institute of Technology
K. Asahi
University of Kyoto
M. Hino, T. Kawai, M. Utsuro
University of Tokyo
T. Miyachi
Virginia Polytechnic Institute and State University
M. Makela, M. Pitt, R.B. Vogelaar