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

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

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

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

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

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) p n e- Beta-Asymmetry Neutrino-Electron-Correlation Neutron lifetime

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

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

The Beta Asymmetry: PERKEO II Electron Detector (Plastic Scintillator) p+ n e- Decay Electrons Polarized Neutrons Split Pair Magnet PERKEO II Magnetic Field Beam time Result Publication 1995 A = -0.1189(12) Phys. Lett. B 407, 212 (1997) 1997 A = -0.1189(7) Phys. Rev. Lett. 88, 211801 (2002) 2004 A = -0.1198(5) (preliminary)

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

Unitarity: Situation 2004 Neutron Measurements needed: . 9 8 τn [PDG2006] A [PERKEO II] Unitarity of the CKM Matrix . 9 7 5 Neutron Measurements needed: Neutron lifetime τn Beta Asymmetry A(λ) Neutrino-Electron-Correlation a(λ) d V u 0+→ 0+ . 9 7 ; λ = gA/gV . 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 Asymmetry A(λ) Neutrino-Electron-Correlation a(λ) ; λ = gA/gV . 9 8 Unitarity of the CKM Matrix . 9 7 5 τn [Serebrov 2005] Vud Nuclear 0+→ 0+ decays . 9 7 τn [PDG2006] . 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.

Uncertainty Budget PERKEO II, last run Error Analysis Correction Uncertainty PERKEO II Statistical uncertainty 0.26% Background 0.1% Neutron beam polarization 0.3 % Spin flip efficiency 0% Magnetic mirror effect 0.11% 0.01% Edge Effect -0.22% 0.05% Detector response … H. Abele, 2006, preliminary All newer spectrometers use the same principle as PERKEO II

New attempts: UCNA (ultracold neutrons) Superconducting solenoidal magnet (1.0 T) Be coated mylar foil MWPC Detector housing Plastic scintillator PMT Decay volume Polarizer / Spin flipper 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.

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 velocity selector chopper cold beam dump neutron beam 2 m B. Maerkisch, D. Dubbers (Heidelberg), H. Abele (Vienna), T. Soldner (ILL) et al.

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

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

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

aSPECT (Mainz, Munich, ILL, Virginia) Proton Detector Magnetic field Decay rate w(E) response function @ U=375V Spectrum for a = +0.3 … for a = -0.103 (PDG 2008) 200 400 600 Analyzing Plane Electrode 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%

aCORN e- p n Tulane (F. Wietfeldt), Indiana, NIST, et al. a = -0.103: Magnetic field Transverse proton- and electron momentum restricted by collimator Neutrino-direction inferred from proton-TOF Asymmetry I/II ,measured 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

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

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

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

More observables: Neutrino Asymmetry p n e- 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)

More observables: R/N correlation p n e- 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 27

R/N correlation σn pp e: N gives up-down asymmetry 50 cm 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: Bodek scintillator MWPC scintillator K. Bodek (Cracow), Villigen, CAEN, Leuven, Kattowice, Accepted in PRL, 2009

Thank you for your interest !! 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 !! Größer