Physics with Ultracold and Thermal Neutron Beams

Technical Report · Tue Aug 10 00:00:00 EDT 2004

DOI:https://doi.org/10.2172/1012914· OSTI ID:1012914

Steyerl, Albert

This project has been focused on a measurement of the mean lifetime {tau}{sub n} of the free neutron with a precision better than 0.1%. The neutron {beta}-decay n {yields} p + e{sup -} + {bar {nu}}{sub e} + 783 keV into a proton, electron and electron antineutrino is the prototype semi-leptonic weak decay, involving both leptons and hadrons in the first generation of elementary particles. Within the standard V-A theory of weak interaction, it is governed by only two constants: the vector coupling constant g{sub V}, and axial vector constant g{sub A}. The neutron lifetime has been measured many times over decades, and the present (2004) world-average, {tau}{sub n} = 885.7 {+-} 0.8 s, has a weighted error of {approx}0.1% while individual uncertainties are typically 2-10 seconds for high precision data. The highest precision claimed by an individual measurement is {approx}0.15%. An improvement is required to resolve issues of the Standard Model of the electro-weak interaction as well as of astrophysics and of Big Bang theories. The focus in astrophysics is the solar neutrino deficit problem, which requires a precise value of g{sub A}. Big Bang theories require a precise {tau}{sub n}-value to understand the primordial He/H ratio. The strong interest of particle physicists in {tau}{sub n} is mainly based on a possible difficulty with the Cabibbo Kobayashi Maskawa (CKM) matrix, which describes the mixing of quark mass states by the weak interaction. Nuclear, neutron, and pion decay data, probing the mixing amplitude V{sub ud} within the first quark generation, in combination with K and B meson decay data, which probe the second and third generation (V{sub us} and V{sub ub}), indicate a departure from the unitarity demanded by all gauge-invariant theories. The deviation of the first-row sum |V{sub ud}|{sup 2} + |V{sub us}|{sup 2} + |V{sub ub}|{sup 2} from unity is on the 2.3 sigma level. Including a new value for V{sub us} would remove the discrepancy; but the authors of note an inconsistency requiring clarification. The largest contribution to this sum is |V{sub ud}|{sup 2} which is determined most sensitively by the neutron lifetime and the neutron decay asymmetry parameter A. Confirmation of nonunitarity would imply that the Standard Model of particle physics may have to be extended. To prepare for an improved {tau}{sub n} measurement based on ultracold neutron (UCN) storage our project had two main goals: (a) To investigate the suitability of a new type of per-fluorinated oil for low-loss wall coating. Like Fomblin oil, which has been used in several previous high-precision {tau}{sub n} measurements, the new oil consists only of carbon, oxygen and fluorine. These elements have very low neutron absorption cross sections. However, due to weak intermolecular binding the new polymer solidifies at a lower temperature ({approx}150 K vs. {approx}230 K for Fomblin) and can, therefore, be used in liquid form at a lower temperature. This is important since a liquid perfectly seals small gaps and the low temperature ensures that the loss due to thermal-inelastic and quasi-elastic scattering is also small. The new types of oil have become known as 'Low Temperature Fomblin' (LTF). (b) If indeed the anticipated low losses were obtained we planned to perform first direct UCN storage experiments in a gravitational storage system coated with this oil. This system in principle allows measurement of the storage lifetime as a function of UCN energy and trap size, and an extrapolation to zero loss yields the neutron lifetime.

• Research Organization: University of Rhode Island, 70 Lower College Rd., Kingston, Rhode Island 02881
• Sponsoring Organization: USDOE
• DOE Contract Number: FG02-02ER45970
• OSTI ID: 1012914
• Report Number(s): DOE/ER/45970 Final Report; TRN: US1102635
• Country of Publication: United States
• Language: English

Related Subjects

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS
B MESONS
COUPLING CONSTANTS
CROSS SECTIONS
DECAY
ELECTRON ANTINEUTRINOS
ELEMENTARY PARTICLES
LIFETIME
NEUTRONS
PHYSICS
QUASI-ELASTIC SCATTERING
SOLAR NEUTRINOS
STANDARD MODEL
STORAGE
TESTING
THERMAL NEUTRONS
ULTRACOLD NEUTRONS
V-A THEORY
WEAK INTERACTIONS