Investigating Parity Violation in Neutron Decay

Transcription

1 Investigating Parity Violation in Neutron Decay Neutral Currents W,Z-Bosons Stefan Baeßler 3 Quark Generations Higgs-Bosons Low energy structure of Weak Interactions

2 Outline 1. Theory of Neutron Beta Decay 2. The Lifetime of the Free Neutron 3. The Beta Asymmetry and PERKEO-II 4. Results of the first test beam time with the Neutron Decay Spectrometer aspect 5. Improvements for aspect 6. Further measurements

3 The Beta Decay Hamiltonian Task: Construction of Parity-Violating Hamiltonian, which couples leptons and hadrons µ µ 5 ' H weak,elementary = GF u γ γ γ d e γµ γµ γ5 νe V A 60 Co I e - 60 Co e- I Properties: Helicity of (relativistic) fermions is -1, of antifermions is 1 Complication: Nucleons aren t elementary particles: H = G V p γ λγ γ n e γ γ γ ν µ µ 5 weak F ud µ µ 5 e Coupling constants are unknown. Fermi-Transitions: g V = G F V ud Gamow-Teller-Transitions: g A = G F V ud λ

4 Determination of the Coupling Constants Fermi-Decay: g V = G F V ud p 1 2 e - ν e e - ν e A = 0 S = 0, m S = 0 n Gamow-Teller-Decay: p 1 2 e - ν + e e - ν e A = 0 S = 1, m S = 0 g A = G F V ud λ p Two unknown parameters, g A and g V, need to be determined in 2 experiments 1. Neutron-Lifetime: 1 ( 2 2 ) n gv 3gA 2. Beta-Asymmetry: τ + τn 885 s 2 λ + λ e - A = λ ν e λ = g A gv A = -1 S = 1, m S = 1 v dw 1+ A cos pe, c ( σ ) n

5 Outline 1. Theory of Neutron Beta Decay 2. The Lifetime of the Free Neutron 3. The Beta Asymmetry and PERKEO-II 4. Results of the first test beam time with the Neutron Decay Spectrometer aspect 5. Improvements for aspect 6. Further measurements

6 Neutron Lifetime Measurements Decrease of Neutron Counts N with storage time t: N(t) = N(0)exp{-t/τ eff } 1/ τ eff = 1/τ β +1/ τ wall losses Neutron lifetime [s] Spivak Nesvizhevsky 92 Byrne Mampe Arzumanov 00 Mampe Dewey 03 PDG2006 MAMBO Serebrov Year of Publication Many new attempts planned, mostly with magnetic bottles

7 Primordial Nucleosynthesis Before Phase Transition: p e - p ν e p e - Equilibrium W ± W ± W ± n After about three minutes, the cosmic hydrogen to helium ratio was fixed....nothing of interest has happened since ν e n + ν e p + e - After Phase Transition (about 1 min after Big Bang): n e + n + e + p + ν e n + p d + γ d + d 3 He + n t + p d + 3 He 4 He + p Then the Primordial Nucleosynthesis stops, since there are no stable nuclei with A = 5, 8 and since the free neutrons die out. Lower τ n more n p reactions Phase Transition later less 4 He n ν e n p + e - + ν e.. Steven Weinberg Lecture, Harvard University, 1975

8 Outline 1. Theory of Neutron Beta Decay 2. The Lifetime of the Free Neutron 3. The Beta Asymmetry and PERKEO-II 4. Results of the first test beam time with the Neutron Decay Spectrometer aspect 5. Improvements for aspect 6. Further measurements

9 The Beta Asymmetry Electron Detector (Plastic Scintillator) p + n ν e e - Decay Electrons v dw 1+ A cos pe, c A N N up up N + N ( σ ) down down n Polarized Neutrons Split Pair Magnet Magnetic Field PERKEO II Beam time Result Publication A = (12) H. Abele, S. B. et al., Phys. Lett. B 407, 212 (1997) A = (7) H. Abele, S. B. et al., PRL 88, (2002) A = (4) (preliminary)

10 Possible Tests of the Standard Model 1. Search for Right-handed Currents W R? 2. Search for Scalar and Tensor interactions Leptoquarks? Charged Higgs Bosons? 3. Search for Supersymmetric Particles (Loop corrections to Beta Decay change Coupling Constants) 4. Test of the Unitarity of the Cabbibo-Kobayashi-Maskawa-Matrix d ' Vud Vus Vub d s ' = Vcd Vcs Vcb s b ' Vtd Vtd V tb b V ud + Vus + Vub = 1

11 The Unitarity of the CKM Matrix Cabbibo-Kobayashi-Maskawa-Matrix d ' Vud Vus Vub d s ' = Vcd Vcs Vcb s b ' Vtd Vtd V tb b V Unitarity Condition ud + Vus + Vub = 1 B/D mesons Superallowed Fermi Decays Neutron Decay n p + e - + ν e Pion Decay Kaon (Ke3) Decays K e + π + ν e Kaon (Kµ2) Decays K µ+ ν µ Hyperons

12 Determination of λ = g A /g V -1,255-1,26 λ -1,265 PERKEO, ,27-1,275 Yerozolimskii, 1997 Liaud, 1997 PERKEO II, ,28 PERKEO II, 1997 PERKEO II is the systematically cleanest experiment Still the disagreement with older measurements is not explained

13 Determination of the Coupling Constants Fermi-Decay: g V = G F V ud p 1 2 e - ν e e - ν e a = 1 n Gamow-Teller-Decay: p 1 2 e - ν + e e - ν e a = 1 g A = G F V ud λ p Two unknown parameters, g A and g V, need to be determined in 2 experiments 1. Neutron-Lifetime: 1 ( 2 2 ) n gv 3gA 2b. Neutrino-Electron-Correlation a: τ + τn 885 s e - ν e 2 1 λ a = ~ λ λ = a = -1 v dw 1+ a cos p, p c g A gv ( ) e υ e

14 Determination of λ = g A /g V -1,255-1,26 Stratowa, 1978 λ -1,265 PERKEO, ,27-1,275-1,28 Yerozolimskii, 1997 Liaud, 1997 PERKEO II, 1997 PERKEO II, 2002 Byrne, 2002 A measurement of a is independent of possible unknown errors in A, systematics are entirely different. Present experiments have a/a ~ 5%, our aim is a/a ~ 0.3%

15 Outline 1. Theory of Neutron Beta Decay 2. The Lifetime of the Free Neutron 3. The Beta Asymmetry and PERKEO-II 4. Results of the first test beam time with the Neutron Decay Spectrometer aspect 5. Improvements for aspect 6. Further measurements

16 The Neutrino Electron Correlation and the Proton Spectrum in Neutron Decay The correlation coefficient a p n ν e e - v dw 1 + a cos( pe, pυ ) e c Sensitivity of the Proton Spectrum to a: ν e e - e - n n p Decay rate w(e) p Proton kinetic energy is big ν e Proton kinetic energy is small Proton spectrum for a = +0.3 and for a = (PDG 2006) Proton kinetic energy E [ev]

17 MEPHISTO Detector aspect - SPECTROMETER Analyzing Plane 0 to 800 V Beam Stop Protons Neutrons Beam Line Decay volume

18 Principle of a Retardation Spectrometer Analyzing Plane p p Adiabatic conversion B A B A = 0,35 T Potential Barrier eu Magnetic Field Proton Trajectory Decay rate w(e) Proton spectrum for a = +0.3 and for a = (PDG 2006) Transmission U=375V 1,0 0,8 0,6 0,4 0,2 Transmission function T U (E) p p B 0 = 1,8 T Decay Volume Proton kinetic energy E [ev] 0,0 Transmission function T U (E) in the adiabatic limit: 0 ; E < eu TU ( E) = 1 1 B0 BA ( 1 eu E) ; otherwise 1 ; E > eu 1 ( - B B ) A 0

19 The Proton Detector Segmented Si - PIN - Diode Active Area mm² Segmented in 25 Stripes Thin Entrance Window Dead Layer: 67 nm Energy Loss for 30 kev protons: ~ 8 kev

20 Typical Proton Event Proton Pulse Shape Pulse Height Energy Event Length: about 2 µs

21 Proton Spectra Pulse height spectrum: after background subtraction: Electronic noise Proton peak Proton spectrum looks ok, Count rate ~ 500 Hz Signal to background ratio > 10:1 Proton Signal not well separated from electronic noise

22 Proton Spectra Count rate [s -1 /channel] AP Voltage: 50 V 800 V (usually) 800 V (sometimes) Proton-like Peak Proton-like peak: not stable in time leads to non-statistical fluctuations in the proton count rate additional systematic uncertainty, limits our present accuracy Pulse height [ADC channels] 100

23 Extraction of a Total proton count rate [s -1 ] Simulation: a = (PDG2006) Simulation: a = +0.3 Experimental Data PRELIMINARY ( ) ( ) ( ) N U T E w E de U Analyzing Plane Voltage U [V] Function of aspect is demonstrated Data Analysis will be finished in a couple of weeks

24 Systematic Uncertainties Magnetic field measurements: Accurate Field Mapping in Decay Volume and Analyzing Plane, but: Hysteresis Electric Potential measurements: Accurate Voltage Measurement with Multimeter, but Surface Charges and Inhomogeneities of the Work Function Signal to Background Ratio 10:1, Background is measured, but problems with instabilities exist. Test of the adiabatic approximation: Magnetic field 100% 50% 40% 30% 20% a/a % 4% 20% First principles calculation of effects of the rest gas

25 Outline 1. Theory of Neutron Beta Decay 2. The Lifetime of the Free Neutron 3. The Beta Asymmetry and PERKEO-II 4. Results of the first test beam time with the Neutron Decay Spectrometer aspect 5. Improvements for aspect: New Proton Detector, Calibration Source, Kelvin Probe, Anti-Magnetic Screen 6. Further measurements

26 Kelvin Probe: Tool to measure Work Functions V Material 1 Material 2 Φ 2 EF,2 Material 1 Material 2 Material 1 Material 2 V C Φ 2 Φ 1 Φ 1 Φ 2 Φ 1 E F,2 E F,1 E F,1 E _ F,2 + _ + _ + E F,1 + 2 Materials with different work functions, isolated 1 st material: to be tested 2 nd material: tip with known work function Electrical Connection: Charging, until Fermi levels are equal External electric field If Material 2 is moved: Capacitance changes, Measurable Current V Bias = -V C Bias Voltage Charge disappears, no external electric field No current if Material 2 is moved

27 Design of an Antimagnetic Screen Magnetic Flux Density B [Gauss] Influence on the External Magnetic Field: ,1 0,01 Calculation w/o Anti-Magnetic Screen Calculation with Anti-Magnetic Screen Experimental Data 1E-3 0, Radial Distance from Decay Volume r [m] Influence on the Internal Magnetic Field: influence quite small the value of B 0 /B A changes Planned: Online-monitoring of B 0 /B A with 2 static NMR probes

28 The aspect collaboration Institut für Physik, Universität Mainz, Germany: F. Ayala Guardia, M. Borg, L. Cabrera Brito, F. Glück, W. Heil, G. Konrad, N. Luquero Llopis, R. Muñoz Horta, M. Orlowski, Ch. Palmer, Y. Sobolev, S. B. Institut für Kernchemie, Universität Mainz, Germany: K. Eberhardt Physik-Department E18, TU München, Germany: I. Konorov, G. Petzoldt, M. Simson, H.-F. Wirth, O. Zimmer Forschungsneutronenquelle Heinz-Maier-Leibnitz, Garching, Germany: D. Rich

29 Outline 1. Theory of Neutron Beta Decay 2. The Lifetime of the Free Neutron 3. The Beta Asymmetry and PERKEO-II 4. Results of the first test beam time with the Neutron Decay Spectrometer aspect 5. Improvements for aspect 6. Further measurements: Proton Asymmetry, Fierz Interference Term

30 Modification to measure the Proton Asymmetry Detector aspect - SPECTROMETER Analyzing Plane 0 to 800 V Beam Stop Protons Neutrons Beam Line Decay volume

31 Expected Proton Asymmetry Angular dependence of the proton emission: 0,0 Neutron Spin N N θ Probability of Proton Emission Proton Asymmetryα P,exp (E) -0,1-0,2-0,3-0,4 Standard Model α P = <α P (E)> Proton Kinetic Energy E [ev] Present best (only) measurement: (integrated over all energies) N N α P = = 0.238(11) N + N

32 The future FRM-2, SNS?): PERC Neutron guide Polarizer + Spin flipper neutron decay channel (neutron guide, 8 m long) neutron beam stop B 0 B 1 B 2 charged decay products Velocity selector Chopper PERC example: B 0 = 2 T, B 1 = 8 T, B 2 = 0.5 T: count rates: beam time: s 1, continuous unpolarized n-beam ½ h s 1, continuous beam polarized to 98% 2 h 6000 s 1, pulsed unpolarized beam 10 h 370 s 1, pulsed beam polarized to 99.5% 4 d Analyzing plane for ~10 4 statistical error Proton Detector

33 Summary Investigation of Parity Violation is 50 years old, but still necessary More precision and reliability are needed with the neutron measurements New experiments will hopefully give new insights into Parity Violation Thank you for your interest!!