Precision measurement of the neutron β -asymmetry A with spin-polarized ultracold neutrons

1. Precision measurement of the neutron β-asymmetry A with spin-polarized ultracold neutrons B.W. Filippone, K.P. Hickerson, T.M. Ito, J. Liu, J.W. Martin, M. Mendenhall, A. PérezGalván, B. Plaster, R. Schmid, B. Tipton, and J. Yuan W.K. Kellogg Radiation Laboratory, California Institute of Technology, Pasadena, CA 91125 USA and the UCNA Collaboration Institutions Duke University, Idaho State University, Indiana University, Institut Laue-Langevin, Los Alamos National Laboratory, North Carolina State University, Shanghai Jiao Tong University, Texas A&M University, Triangle Universities Nuclear Laboratory, University of Kentucky, University of Washington, University of Winnipeg, Virginia Tech Spin-polarized neutron β-decay Solid deuterium ultracold neutron (UCN) source at LANL UCNA experimental setup • The differential probability that a polarized neutron will decay into an electron, a proton, and an electron anti-neutrino with momenta in specified directions relative to the neutron spin is given by [1] • Theneutron-spin, electron-momentum angular correlation parameter, A,(the “beta asymmetry”) can be expressed in terms of the ratio of the axial-vector and vector coupling constants,λ = gA / gV , as [2] • Measurements of A provide the definitive value for gA, the weak axial-vector coupling constant of the nucleon. Further, measurements of the neutron lifetime, τn , and A, determine the CKM matrix element Vud as [3] • What are ultra-cold neutrons (UCN) ? • Neutrons with speeds < 8 m/s (< 350 nano-eV) that can undergo total external reflection at all angles from various material surfaces and can be reflected (μ·B interaction) from ~few Tesla magnetic fields How do we produce UCN ? • Short (few hundred μs long) pulse of 800 MeV protons incident on a tungsten spallation target produces a flux of MeV neutrons that are moderated to < 100 K • Downscattered into UCN regime (< 4 mK) via phonon interactions in a ~2-liter solid deuterium source maintained at < 6 K How do we transport and polarize UCN ? • Transported along cylindrical guides coated with 58Ni or a diamond-like film evaporated onto the surface of the guides • Spin-polarized via transport through a 7-Tesla magnetic field 7-Tesla polarizing magnets UCN guides UCN “flapper valve” AFP spin flipper walls of source coated with 58Ni electron detectors UCN from source solid deuterium (< 6 K) PDG 2010 Values −0.103(4) −0.1173(13) 0.9807(30) LHe cryostat UCN beamline 1-Tesla solenoidal spectrometer 800 MeV proton beam polyethylene moderator (< 100 K) muon vetoes tungsten spallation target electron detectors beryllium reflector UCNA β-decay spectrum and asymmetries UCN production in solid deuterium was pioneered at LANL in a prototype solid deuterium source [9]. UCNA: motivation and first results • Past measurements of A with beams of polarized cold neutrons suffered from significant discrepancies. • The UCNA experiment has provided the first-ever measurement of A with stored ultracold neutrons (UCN). The use of UCN has significant advantages in terms of the neutron polarization and neutron-generated backgrounds. The first UCNA results [4,5] are in good agreement with the most recent, and most precise, cold neutron-based experiment, PERKEO II [6]. • Current (2010) status of A, lifetime, and neutron-based Vud results : S/B ~ 40:1 from 275–625 keV UCNA experimental concept • Spin-polarized UCN transported into 3-m long guide situated within a 1-Tesla superconducting solenoidal spectrometer [10] • Fraction of the UCNs undergo β-decay • Emitted decay electrons spiral around the field lines and are detected in (identical) MWPC/scintillator detector packages [11] located at both ends of the spectrometer • Measured asymmetry in the rates in the two detector packages yields a value for A [4]: UCNA Proof-of-Principle [6]: PERKEO II Binned values for A after corrections for backscattering and  β cosθ  acceptance [5]: UCNA First Precision Result Size of systematic corrections for backscattering and  β cosθ  acceptance for different experimental configurations • Electron detectors [10,11]: • Plastic scintillator for measurement of energy and timing information (trigger) • MWPC for position information and background (gamma) rejection (coincidence between scintillator and MWPC) entrance window vacuum housing for scintillator, light guides, and PMTs References MWPC: Gamma Background Rejection [7] MWPC: Position Reconstruction [8] MWPC [1] J.D. Jackson, S.B. Treiman, and H.W. Wyld, Jr., Phys. Rev. 106, 517 (1957). [2] S. Gardner and C. Zhang, Phys. Rev. Lett. 86, 5666 (2001). [3] A. Czarnecki, W.J. Marciano, and A. Sirlin, Phys. Rev. D 70, 093006 (2004). [4] R.W. Pattie et al., Phys. Rev. Lett. 102, 012301 (2009). [5] J. Liu et al., Phys. Rev. Lett. 105, 181803 (2010). [6] H. Abele et al., Phys. Rev. Lett. 88, 211801 (2002). [7] A. Serebrov et al., Phys. Lett. B 605, 72 (2005). [8] I.S. Towner and J.C. Hardy, Rep. Prog. Phys. 73, 046301 (2010). [9] C.L. Morris et al., Phys. Rev. Lett. 89, 272501 2002); A. Saunders et al., Phys. Lett. B 593, 55 (2004). [10] B. Plaster et al., Nucl. Instrum. Methods Phys. Res. A 595, 587 (2008). [11] T.M. Ito et al., Nucl. Instrum. Methods Phys. Res. A 571, 676 (2007). gas handling system 113Sn: 368 keV Detector cart for the MWPC/scintillator on the floor prior to insertion into the spectrometer.