The Beta-Asymmetry in Neutron Decay Jeffery W. Martin

1. GV=GFVud u d e- W- ne shield package beta-spectrometer magnet Figure: Status of UCNA Experiment May 2004. UCN exit the shield package on the right, passing through the “prepolarizer” magnet, which accelerates correct spin neutrons through an Al foil. UCN then enter the 7 T polarizer magnet achieving high (99.9%) polarization. Polarized UCN then enter the beta-spectrometer, where neutron decay is measured. We have constructed a new UCN source at Los Alamos. The source has two unique properties compared to previous UCN sources: 1. It uses proton-induced spallation to produce the neutrons. In spallation, a proton beam strikes a heavy nuclear target, in our case tungsten, producing fission fragments that emit neutrons. 2. The neutrons scatter off solid deuterium (SD2) in order to become UCN. The neutrons down-scatter from the SD2 lattice producing phonons. This type of source is called a “super-thermal” source. UCN port UCN guide insertion proton beam direction polarizer magnet Using a prototype source, we have demonstrated a quantitative understanding of the properties of solid deuterium relating to UCN production (see Ref. 3). We have also used our prototype source to produce the world’s largest density of UCN (see Ref. 4 and Figs. to the right). remote extraction future UCN guide path UCN to expt æ ö æ ö æ ö d V V V d ç ÷ ç ÷ ç ÷ w ud us ub Weak eigenstates Mass eigenstates “prepolarizer” magnet = s V V V s ç ÷ ç ÷ ç ÷ w cd cs cb ç ÷ ç ÷ ç ÷ b V V V b è ø è ø è ø w td ts tb previous record ILL, Grenoble Figure: Detector Schematic MWPC amps Fe shield PMT e- PMT 100 Torr neopentane, 6 m mylar windows. The Beta-Asymmetry in Neutron Decay Jeffery W. Martin W. K. Kellogg Radiation Laboratory, California Institute of Technology, Pasadena, CA 91125 for the UCNA Collaboration T. J. Bowles1 (co-PI), R. Carr2, B. W. Filippone2, A. Garcia3, P. Geltenbort4, R. E. Hill1, S. A. Hoedl3, G. E. Hogan1, T. M. Ito2, S. K. Lamoreaux1, C.-Y. Liu1, M. Makela5, R. Mammei5, J. W. Martin2, R. D. McKeown2, F. Merrill1, C. L. Morris1, M. Pitt5, B. Plaster2, K. Sabourov6, A. Sallaska3, A. Saunders1, A. Serebrov7, S. Sjue3, E. Tatar8, R. B. Vogelaar5, Y.-P. Xu6, A. R. Young6 (co-PI), and J. Yuan2 1Los Alamos National Laboratory, Los Alamos, NM 87545; 2W. K. Kellogg Radiation Laboratory, California Institute of Technology, Pasadena, CA 91125; 3Center for Experimental Nuclear Physics and Astrophysics, University of Washington, Seattle, WA 98195; 4Institut Laue-Langevin, BP 156, F-38042 Grenoble Cedex 9, France.; 5Virginia Polytechnic Institute and State University, Blacksburg, VA 24061; 6North Carolina State University, Raleigh, NC 27695.; 7St.-Petersburg Nuclear Physics Institute, Russian Academy of Sciences, 188350 Gatchina, Leningrad District, Russia; 8Idaho State University, Pocatello, ID 83209. Neutron Beta-Decay A New Way to Make Ultracold Neutrons The UCNA Experiment • Decay Rate • The neutron decays via the weak interaction into an electron, a proton, and an electron anti-neutrino. • The decay rate depends on the polarization of the neutron, and the momenta of the outgoing electron and anti-neutrino. • Measuring the total rate (the lifetime) and any of the above correlation parameters determines the vector (GV) and axial-vector (GA) couplings. • The “beta-asymmetry” parameter A is the most sensitive of the correlation parameters to GA. This parameter represents correlation between the electron momentum and the neutron polarization. Figure: Experimental Schematic for UCNA. Polarized UCN exit the polarizer magnet and enter the beta-spectrometer. While inside the spectrometer, a fraction of the UCN decay. The emitted electrons spiral around the magnetic field lines and are sensed in MWPC/scintillator detectors. The asymmetry in rates between the detectors at each end gives the decay parameter A. previous record ILL Improvements for a determination of A All previous experiments on A used a reactor source of cold neutrons, and used supermirror polarizers to polarize the neutrons. Using UCN , we can achieve: reduced backgrounds: We use a spallation source to produce neutrons, which can be pulsed to reduce prompt backgrounds, and can be switched off to determine remaining backgrounds. higher neutron polarization: We expect neutron polarizations at the level 99.9% and hence smaller systematic corrections. The Standard Model • In the standard model: • By the conserved vector current theorem (CVC), the neutron coupling constant GV is related to the fundamental quark coupling. GV is therefore related to the element Vudof the Cabibbo-Kobayashi-Maskawa (CKM) matrix. • The matrix must be unitary, so, theoretically: • With precise measurements of Vud and Vus, we can test this! • Status (2002): • There is currently a lack of self-consistency between neutron experiments on A. • The most precise measurement of A tends to give a value for Vud in disagreement with results from superallowed nuclear decay. • UCNA aims to settle these discrepancies with a 0.2% measurement of A. • A new, precise determination of Vud will be useful for comparisons with 1-|Vus|2 from kaon decay. Figure: The new UCN source for UCNA as construction finishes at Los Alamos April 2004. The UCN source also underwent successful commissioning in April 2004. Future programme Upgrades for UCNA are planned. One upgrade involves the use of silicon detectors in place of the existing scintillators. The silicon detectors would provide significantly improved energy resolution, allowing for more accurate beta-spectroscopy. Silicon detectors make it possible to extract energy-dependent recoil-order terms in the asymmetry, sensitive to weak magnetism; and to extract the energy-dependence of the unpolarized decay spectrum, sensitive to Fierz interference. Detectors for Accurate Beta-Spectroscopy The UCNA experiment uses a combination of a multi-wire proportional counter (MWPC) with a scintillation counter to detect the emitted betas. The scintillation counter provides a measurement of the full energy deposition and fast timing. The MWPC provides position sensitivity, background rejection, and a low detector threshold. Figure: prototype silicon detector for UCNA. Another planned upgrade involves the use of proton detectors. Protons would be accelerated into a thin foil producing secondary electrons. The secondaries would be sensed in the silicon detectors. Proton detection allows the possibility of extracting the remaining correlation parameters B and a. With the level of accuracy to be attained, systematic uncertainties relating to the accurate detection of the decay electrons become important. We used accelerator and radioactive sources to precisely calibrate the beta detectors. (see Refs. 5 and 6 and Fig.) MWPC Anode Scintillator sum counts References channels channels Figure: UCNA beta-detector ready for insertion into spectrometer. Figure: Digitized detector response for 120 keV incident electron beam from electron gun. • The Caltech UCNA website, http://www.krl.caltech.edu/ucn. My email address: jmartin@krl.caltech.edu. • T. Bowles and A. R. Young (co-principal investigators), A proposal for an accurate measurement of the neutron spin, electron angular correlation in polarized neutron beta-decay with ultracold neutrons (2000). • C. L. Morris et al, “Measurements of Ultracold-Neutron Lifetimes in Solid Deuterium,” Phys. Rev. Lett. 89, 272501 (2002). • A. Saunders et al, “Demonstration of a Solid-Deuterium Source of Ultracold Neutrons,” nucl-ex/0312021, submitted to Phys. Lett. • J. W. Martin et al, “Measurement of Electron Backscattering in the Energy Range of Neutron Beta-Decay,” Phys. Rev. C 68, 055503 (2003). • J. Yuan et al, “A Double-Focusing Helmholtz-Coil Spectrometer,” Nucl. Instrum. Methods Phys. Res. A 465, 404 (2001). • This work is supported by the National Science Foundation and the Department of Energy. Systematic effects due to backscattering of electrons from the MWPC windows and the scintillator face will necessitate a correction on the determination of A at the level 0.1%. To gain confidence in this correction, we have carried out detailed measurements of backscattering in the low-energy regime (see Ref. 5).