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Stefan Baeßler

Studies of Neutron Beta Decay. Stefan Baeßler. d. e -. d. u. u. u. p. d. n. How to discover new particles?. High Energy Physics Experiments. Low Energy Precision Experiments. Example: Production of W-Boson Search for extra (e.g., righthanded) W bosons. Example:

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Stefan Baeßler

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  1. Studies of Neutron Beta Decay Stefan Baeßler d e- d u u u p d n

  2. How to discover new particles? High Energy Physics Experiments Low Energy Precision Experiments Example: • Production of W-Boson • Search for extra (e.g., righthanded) W bosons Example: • Study of Neutron Beta • Search for abnormal properties of decay products

  3. Precision measurements in Astronomy Discovery of Neptune: Urbain Le Verrier, 1811-1877 John Couch Adams,1819-1892 Neptune • Theoretical prediction (Le Verrier, Adams, 1845) • Idea: Explain distortions in orbit of Uranus • Discovery (Galle, 1846) • Later: Similar story for Pluto Uranus Sun Distortions of Uranus orbits known since decades

  4. Precision measurements Non-Discovery of Vulcan: Vulcan • Idea: Explain extra perihelion precession of Mercury by presence of Vulcan • Convincing observation failed • But failure is more interesting than a success would have been: Extra precession (43 arcsec/100 y) • explained in General Relativity Neptune Uranus Perihelion Precession of a planet: For Mercury, perihelion precession angle is 1.5 deg/100 y Sun

  5. Precision measurements Modern Example: Lunar Laser Ranging to search (among other things) for violation of the Equivalence principle: Sun Earth Vulcan Moon Neptune Lessons: Discoveries can be made with precision measurements The discovered item might be unexpected Even with high precision, a discovery is not guaranteed Sun

  6. e- p n Observables in Neutron Beta Decay Jackson et al., PR 106, 517 (1957): Observables in Neutron beta decay, as a function of generally possible coupling constants (assuming only Lorentz-Invariance) Beta-Asymmetry Neutrino-Electron-Correlation Neutron lifetime

  7. The Standard Model Parameters Vud and λ A = 0 S = 0, mS = 0 A = 0 S = 1, mS = 0 A = -1 νe νe νe p e- e- e- νe νe S = 1, mS = 1 n p p 2. Beta-Asymmetry: Fermi-Decay: gV = GF·Vud e- e- Gamow-Teller-Decay: gA = GF·Vud·λ Two unknown parameters, gA and gV, need to be determined in 2 experiments 1. Neutron-Lifetime:

  8. Neutron Lifetime Measurements Decrease of Neutron Counts N with storage time t: N(t) = N(0)exp{-t/τeff} 1/ τeff = 1/τβ+1/τwall losses MAMBO Many new attempts underway, mostly with magnetic bottles: Under (at least) construction: Ezhov et al. (ILL, PNPI Gratchina), Bowman et al. (LANL), Paul et al. (TUM, PSI) see K.W. Schelhammer, 10:30 h

  9. The Beta Asymmetry: PERKEO II e- p+ n Electron Detector (Plastic Scintillator) Decay Electrons Polarized Neutrons Split Pair Magnet PERKEO II Magnetic Field

  10. Possible Tests of the Standard Model Multiple determinations (nuclear physics, other correlation coefficients) overconstrain problem, enable: • Search for Right-handed Currents • WR? • Search for Scalar and Tensor interactions • Leptoquarks? Charged Higgs Bosons? • Search for Supersymmetric Particles • (Loop corrections to Beta Decay change Coupling Constants) • Test of the Unitarity of the Cabbibo-Kobayashi-Maskawa-Matrix

  11. Unitarity of the CKM Matrix τn [PDG2006] A [PERKEO II] Unitarity: Situation 2004 0 . 9 8 0 0 . 9 7 5 • Neutron Measurements needed: • Neutron lifetime τn • Beta AsymmetryA(λ) • Neutrino-Electron-Correlationa(λ) d u 0+→ 0+ V 0 . 9 7 0 ; λ = gA/gV 0 . 9 6 5 - 1 . 2 5 - 1 . 2 6 - 1 . 2 7 - 1 . 2 8 λ = gA/gV

  12. Unitarity 2008 • Neutron Measurements needed: • Neutron lifetime τn • Beta AsymmetryA(λ) • Neutrino-Electron-Correlationa(λ) ; λ = gA/gV 0 . 9 8 0 Unitarity of the CKM Matrix 0 . 9 7 5 τn [Serebrov 2005] Vud Nuclear 0+→ 0+ decays 0 . 9 7 0 τn [PDG2006] 0 . 9 6 5 A [ P E R K E O I I ] - 1 . 2 5 - 1 . 2 6 - 1 . 2 7 - 1 . 2 8 l = g / g A V Neutron lifetime discrepancies have to be sorted out. To make A not limiting for neutron-based determination: ΔA/A < 0.2% needed.

  13. Uncertainty Budget PERKEO II, last run H. Abele, 2006, preliminary All newer spectrometers use the same principle as PERKEO II

  14. New attempts: UCNA (ultracold neutrons) Superconducting solenoidal magnet (1.0 T) Be coated mylar foil MWPC Detector housing Plastic scintillator PMT Polarizer / Spin flipper Decay volume Light guide Field Expansion Region Neutron absorber Diamond-coated quartz tube UCN source Short test run: A0=-0.1138(46)(21) A. Young (NCSU), A. Saunders (LANL), et al.

  15. 2 m velocity chopper selector Next generation: PERKEO III • Advantages: • very high countrate w/o pulsing • reduced background through pulsing • no edge effect detector (plastic scintillator) detector (plastic scintillator) decay volume, 150 mT cold beam dump neutron beam B. Maerkisch, D. Dubbers (Heidelberg), H. Abele (Vienna), T. Soldner (ILL) et al.

  16. New attempts at SNS: abBA / Nab / PANDA Proton Beam 60 Hz Spectrometer Shutter Adiabatic Choppers Flux LH2 Spin Flipper Monitor Neutron Guide Collimator 3He Mercury Polarizer Biological Shield Spallation Target Fast, segmented silicon detector: 21.2 MeV Jπ = 1-,2- 20.5 MeV 19.8 MeV 20.1 MeV Jπ = 0+ 3He+n Γ = 0.27 MeV p+t S. Wilburn (LANL), D. Bowman (ORNL) et al. Jπ = 0+ 4He

  17. Determination of the Coupling Constants a = 1 a = 1 a = -1 νe νe νe p e- e- e- νe νe n p p 2b. Neutrino-Electron-Correlation a: Fermi-Decay: gV = GF·Vud e- e- Gamow-Teller-Decay: gA = GF·Vud·λ Two unknown parameters, gA and gV, need to be determined in 2 experiments 1. Neutron-Lifetime:

  18. Determination of λ = gA/gV Stratowa, 1978 Yerozolimskii, 1997 UCNA, 2009 PERKEO, 1986 Liaud, 1997 Byrne, 2002 PERKEO II, 1997 PERKEO II, 2002 PERKEO II, ? • A measurement of a is independent of possible unknown errors in A, systematics are entirely different. • Present experiments have Δa/a ~ 5%, an order of magnitude improvement is desirable

  19. aSPECT (Mainz, Munich, ILL, Virginia) e- p n Proton Detector Magnetic field Decay rate w(E) response function @ U=375V Spectrum for a = +0.3 … for a = -0.103 (PDG 2008) Analyzing Plane Electrode 0 200 400 600 Proton kinetic energy E [eV] Protons Protons @ 15 kV Neutron Decay Present best experiments: Δa/a = 5% Present status of aSPECT: (Δa/a)stat = 2% per day Final aim: 0.3%

  20. e- p n aCORN Magnetic field a = -0.103: “pυ up” more likely Aim: Δa/a ~ 2%, maybe 0.5% after NIST upgrade Tulane (F. Wietfeldt), Indiana, NIST, et al.

  21. cut The cosθeν spectrometer Nab @ SNS p e- • Kinematics: • Energy Conservation • Momentum Conservation n electron and proton phase space 1.4 2 c / 1.2 2 V e 1 M ( 0.8 2 p p 0.6 pp2distribution 0.4 0.2 0 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 Ee = 550 keV E (MeV) e 0.0 0.5 1.0 1.5 pp2 [MeV2/c2]

  22. The cosθeν spectrometer Nab @ SNS Segmented Si detector Neutron beam 30 kV TOF region decay transition volume region acceleration region 107 • Spectrometer and detector shared with abBA • Will likely be converted in asymmetric configuration • Aim: ~0.1% 106 Simulated count rate pp2distribution 105 Ee = 300 keV Ee = 500 keV 104 Ee = 700 keV D. Pocanic, S.B. (Virginia), D. Bowman (ORNL), et al. Ee = 550 keV 103 0.0 0.5 1.0 1.5 0 . 0 0 0 . 0 2 0 . 0 4 0 . 0 6 0 . 0 8 1/tp2 [1/μs2] pp2 [MeV2/c2]

  23. e- p n More observables: Fierz Interference Term Jackson et al., PR 106, 517 (1957): • Fierz-Interference Term: • Signal expected forMSSM: b ~ 10-3 (Ramsey-Musolf, 2007) • Not measured in neutron beta decay, Nab might be able to.

  24. e- p n More observables: Neutrino Asymmetry Jackson et al., PR 106, 517 (1957): Neutrino-Asymmetry • Signal expected for MSSM at ΔB ~ 10-3 (Ramsey-Musolf, 2007) • Last measurements: B = 0.9802(50) (PERKEO II, 2007) • B = 0.9801(46) (Serebrov, 1998)

  25. e- p n More observables: R/N correlation Jackson et al., PR 106, 517 (1957): Electron polarization • Standard-Model: NSM= 0.07; RSM= 0.0066 ~ 0 • Scalar or Tensor Interactions lead to deviations (Leptoquarks, charged Higgs, Sleptons in SUSY) • Of special interest: R, as it is Time-Reversal violating, measures imaginary part of coupling constants

  26. 50 cm R/N correlation σn pp e: N gives up-down asymmetry Pb-foil pe p e: R gives forward-backward asymmetry Polarized n beam Pb-foil Detection of electron polarization through Mott scattering in Pb foil: The probability of having a V track is electron spin dependent. Result: scintillator MWPC scintillator K. Bodek (Cracow), Villigen, CAEN, Leuven, Kattowice, Accepted in PRL, 2009

  27. Summary • Rich experimental program with the study of neutron decay correlations • New physics might be found with precision measurements. Maybe soon! • Main problem: Neutron lifetime disagreement Thank you for your interest !!

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