Ultra-Cold Neutrons (UCN) provide unique system to study fundamental neutron properties

Transcription

1 Neutron β-decay with Ultra-Cold Neutrons (UCN) Ultra-Cold Neutrons (UCN) provide unique system to study fundamental neutron properties UCN experiments have different systematics compared to cold neutron beams Polarization process and background sources differ significantly UCNA First experiment to measure neutron decay correlation - A - with UCN First UCNA data: 12/07 (Actually 12/15-12/17/07) Plans for future data (6-12/08, 6-12/09) UW 10/9/2008

2 The Caltech UCN group Justin Chen Nick Hutzler Gary Cheng Jenny Hsiao Riccardo Schmid Kevin Hickerson Junhua Yuan Brad Plaster Bob Carr Michael Mendenhall Jianglai Liu BF

3 UCNA Collaboration California Institute of Technology R. Carr, B. Filippone, K. Hickerson, J. Liu, J. Martin, M. Mendenhall, B. Plaster, R. Schmid, B. Tipton, J. Yuan Institute Lau-Langevin P. Geltenbort Los Alamos National Laboratory J. Anaya, T. J. Bowles, T. Brun, M. Fowler, R. Hill, G. Hogan, T. Ito, K. Kirch, S. Lamoreaux, C.-Y. Liu, C. L. Morris, M. Makela, A. Pichlmaier, A. Saunders (co-spokesperson), S. Seestrom, P. Walstrom, J. Wilhelmy North Carolina State University/TUNL H. O. Back, L. Broussard, A. T. Holley, R. K. Jain, R. W. Pattie, K. Sabourov, A. R. Young (cospokesperson), Y.-P. Xu Petersburg Nuclear Physics Institute A. Aldushenkov, A. Kharitonov, I. Krasnoshekova, M. Lasakov, A. P. Serebrov, A. Vasiliev Tohoku University S. Kitagaki University of Kyoto M. Hino, T. Kawai, M. Utsuro University of Washington A. Garcia, S. Hoedl, D. Melconian, A. Sallaska, S. Sjue Virginia Polytechnic Institute and State University R. Mammei, M. Pitt, R. B. Vogelaar

4 Neutron Beta Decay or in quark picture u d d n W - u d u p e - ν e

5 Precision neutron decay measurements < 0.3% measurements can be sensitive to new physics (from loops in electroweak field theory) Radiative Corrections W W q q χ 0

6 CKM Matrix and Unitarity Weak eigenstates d s b w w w = V V V ud cd td V V V us cs ts V V V ub cb tb Mass eigenstates d s b d s b w w w = d 0.04 s 0.99 b Unitarity, eg. V ud 2 + V us 2 + V ub 2 = 1, (or lack thereof) of CKM matrix tests existence of further quark generations and possible new physics (eg. Supersymmetry)

7 Sensitivity to New Physics? Kurylov&Ramsey-Musolf Phys. Rev. Lett. 88, (2002) V ud in Standard Model (from µ vs. β-decay) µ - ν µ G G F V F d ud u W - e - W - e - Supersymmetric particles produce loop corrections ν e µ - ν χ~ 0 µ d µ~ ν~ W - e - ν e ~ d ν e χ~ 0 u u ~ W - e- ν e

8 What is Big A? r r r r r d Γ = Γ + p + σ r e.pν n.pe + σ n.pν n 1 + p r exp r ν a A B D.σ r E E E E E E e ν Differential neutron decay rate With no e - polarization e ν e ν n a, A and B are correlations that depend on the axial and vector weak coupling constants G A and G V

9 r r r r r d Γ = Γ + p + σ r e.pν n.pe + σ n.pν n 1 + p r exp r ν a A B D.σ r E E E E E E e ν e ν e ν n Γ n = 1/τ n is total decay rate Thus A and τ n gives V ud

10 Uncertainties in V ud Electroweak corrections (Z 0 and hadron loops) Neutron has potential to Recently cut in half by

11 CKM Summary: Particle Data Group recommended New τ n!! UCNA

12 Why UCNA? For accurate measurement of A & G A /G V (and V ud via neutron lifetime) need to characterize and minimize systematic uncertainties Different experimental approaches are critical to reducing systematic uncertainties PERKEO II/III Supermirror polarizer Cold neutron beam from CW reactor Scintillator β detector UCNA Superconducting magnet polarizer Trapped UCN from pulsed proton beam Scintillator & MWPC β detector

13 UCN Polarization via high B-field Low field seekers High field seekers

14 Reduced Background with pulsed Source of UCN PERKEO II (99) A-correlation experiment (at Reactor) UCNA UCNA Proposal data 4/00 12/07 (pulsed source)

15 What are UCN? Very slow neutrons (v < 8 m/s λ > 500 Å ) that cannot penetrate into certain materials Neutrons can be trapped in bottles or by magnetic field

16 UCN Properties g UCN 3m

17 Fermi Pseudo-potential E UCN For E UCN < V F, UCN are trapped Attractive potential can also lead to neutron absorption but often L mfp >> λ n (~10-5 probability per bounce)

18 Typical Fermi Potentials Material V F (nev) Al Ni 350 Ti - 48 neutron velocity v n ~ 8 m/s Cu 171 Stainless Steel 188 Diamond-Like Carbon (DLC) 282

19 How to make UCN? Conventional Approach: Start with neutrons from nuclear reactor core Use collisions with nuclei to slow down neutron Some of neutron s energy lost to nuclear recoil in each collision Gives a Maxwell- Boltzmann Distribution Record density at Institut Laue-Langevin (ILL) reactor in Grenoble (40 UCN/cm 3 )

20 Higher Density UCN Sources Use non-equilibrium system (aka Superthermal) Superfluid 4 He (T<1K) 11K (9Å ) incident neutron produces a phonon & becomes a UCN Used in on-going NIST τ n Experiment & Future new Neutron EDM exps. (neutron) Very few 11K phonons if T<1K minimal upscattering

21 Solid deuterium (SD 2 ) Gollub & Boning(83) Small absorption probability Faster UCN production Small Upscattering if T < 6K Cold Neutron UCN Phonon Can be optimally used at a pulsed source (e.g. accelerator-based spallation neutron source)

22 Schematic of prototype SD 2 source 58 Ni coated stainless guide Flapper valve Liquid N 2 Be reflector LHe Solid D 2 77 K polyethylene UCN Detector Tungsten Target Caltech, LANL, NCState, Princeton, VaTech, Russia, France, Japan Collaboration

23 Demonstration of a solid deuterium source of ultra-cold neutrons [Phys. Lett. B 593, 55 (2004)] New World Record UCN Density Previous record for bottled UCN = 41 UCN/cm 3 (at ILL) Measurements of Ultra Cold Neutron Lifetimes in Solid Deuterium [PRL 89, (2002)]

24 UCNA Experiment Design UCN Decay Trap (7.0 T & Adiabatic Fast Passage Spin Flipper in 1T) From

25 Asymmetry Measurement with UCN UCNA e - θ n

26 Overview of UCNA experiment SD 2 Superthermal UCN source UCN guides Stainless Steel, Cu, Diamond-coated Quartz Polarizer and spin-flipper system Spectrometer & β-decay detectors First data...

27 UCNA Experiment Layout Neutron Polarizing Magnets UCN Source Superconducting Spectrometer Electron Detectors

28 UCNA experiment Liquid N 2 Be reflector LHe Solid D 2 77 K poly Tungsten Target

29 UCNA SD 2 source Flapper Valve Solid Deuterium Tungsten Target Proton bunch

30 UCN Guides From source through first polarizing magnet = Stainless Steel Dairy Guide May be depolarizing (mildly ferromagnetic) Through AFP spin-flipper = Diamond coated Quartz Into 1T spectrometer = Electro-polished Copper maintains neutron polarizaion

31 UCNA neutron polarization Pre-polarizing 6T magnet allows good UCN transport through vacuum window (isolates source and detector system for safety 2 nd 7T magnet further filters UCN and allows for spin flip Adiabatic Fast Passage (AFP) resonator UCNA Polarimetry = measuring depolarized fraction When polarization is very high only modest measurement of depolarization fraction needed

32 Polarizer/AFP Spin Flipper e - 7T

33 Depolarization Measurements UCN detector 7T AFP 7T Polarizer/AFP UCN in Crossed polarizer: Uses AFP to flip UCN to low field seekers UCN in Sample during bottle emptying: change state of AFP at end of run cycle and monitor depolarized UCN leaking back to detector

34 UCN transmission through crossed polarizers vs AFP frequency

35 Spin-Flipper Tuning during β-decay Side View Polar izer AFP Spectro meter Magnetized foil UCN detector

36 Depolarization Trapping (in situ) Switcher Pol AFP Spec α (UCN from B+C+D) + β (UCN from A+Switcher Leakage) Draining of A+B+C +D Flow-Through Leakage LOAD 1hr CLEAN 150s UNLOAD 100s BACKGROUND AFP OFF AFP OFF AFP ON AFP ON AFP ON AFP ON AFP OFF AFP OFF

37 Superconducting Spectrometer 1 Tesla Central Field with 0.6 T field expansion to suppress backscattering Neutron Decay Tube Decay Electron Detectors

38 Solenoid Bore 35 cm diameter SS warm bore 12.5 cm diameter electropolished Cu decay tube Be-coated Mylar endcap windows (2.5 µm in 07, 0.7 µm in 08) 110 cm diameter Lucite β collimator β-detector Decay Tube β-detector

39 β-detector System Requirements: Low Background, Reasonable Energy Resolution, Minimal e - Backscattering Design: 25 or 6 µm Exit Window 25 or 6 µm Entrance Window 3.5 mm Scintillator e - Low Pressure MultiWire Proportional Chamber MWPC (100 torr) Scintillator + MWPC reduces room background > factor of 25

40 Full Detector Schematic PMT e - PMT Fe Magnetic Shields (also vacuum seal) MWPC Preamp Cards 100 torr N torr Neopentane

41 Assembled Detector PMT PMT

42 Detector Studies At Caltech with 130 kev electron gun At LANSCE with 113 Sn (E β ~ 370 kev) and 207 Bi (E β ~ 505 & 995 kev) sources At LANSCE with neutron β-decay

43 Detailed Backscattering studies completed at Caltech (comparison with GEANT4 and PENELOPE Monte Carlo) "New measurements and quantitative analysis of electron backscattering in the energy range of neutron beta-decay", J.W. Martin et al., Phys. Rev. C. 73, (2006). "Measurement of electron backscattering in the energy range of neutron beta decay", J.W. Martin et al., Phys. Rev. C 68, (2003).

44 Spectrometer studies at LANSCE with 113 Sn source in 1T field σ ~ 1.5mm Fiducial Volume Cut

45 Cosmic ray induced events (x,y) 80 mm diameter fiducial cut

46 UCN Decay Events (x,y) 110 mm diameter β collimator 80 mm diameter fiducial cut

47 LANSCE (Los Alamos Neutron Science CEnter) Proton Linac (½ mile long): 0.8 GeV, 1-2 ma UCN Source (Area B)

48 Recent Pictures of LANSCE Area B (10/5/08)

49 Recent Pictures of LANSCE Area B

50 Liquid He farm (~ 2000 liters/day)

51 UCN Switcher

52 UCN Accelerator...

53 UCNA-07 results First measurements of depolarized fraction Crossed polarizers & trapped depol. Neutrons Detailed measurements of UCN β-decay spectra First measurements of UCN asymmetry Note: All 07 data acquired with 2X25 mm b detector windows (ΔE ~ 20 kev) 2.5 mm endcap windows < 50 hours total acquisition time For robustness & safety

54 Statitistical sensitivity of UCNA Experiment can only run on nights & weekends (Due to nuclear weapon stockpile stewardship studies proton radiography) Modest downtime for polarization flips & depolarization measurements A sensitivity: σ A /A ~ 3%/month/sqrt(Hz) < 0.2 Hz in 05 ~ 1.7 Hz in 06 ~ 7 Hz in 07 ~ 18 Hz in 08 PERKO II 02: 0.7% UCNA goal: <0.5%

55 Reduced Background with pulsed Source of UCN Hz PERKEO II (99) A-correlation experiment (at Reactor) UCNA UCNA Proposal data 4/00 12/07 (pulsed source)

56 Scintillator rate increases during beam pulses Room Background

57 First results from UCNA

58 0 World Asymmetry data A ,970 1,980 1,990 2,000 2,010 TIME (year) UCNA

59 UCNA(07) UCNA(08) PERKEOII(02) Systematic Corr. Error Corr. Error Corr. Error Polarization <0.5% 1.0% <0.2% 0.2% 1.1% 0.3% Spin-flip <0.2% 0.1% 0.3% 0.3% Background <0.1% 0.2% <0.1% 0.1% 0.5% 0.3% Detector Linearity 1.5% <0.3% 0.2% Width/Ped 0.1% <0.1% 0.1% Drifts 0.2% <0.1% 0.006% Edge Effects -0.24% 0.1% Angle Effects -1.6% 0.5% <-0.4% 0.2% e - Trajectories Mirror Effect -0.06% 0.06% -0.06% 0.06% 0.09% 0.02% Backscatterin g Asymmetry Systematics Preliminary Projected 1.1% 0.5% 0.5% 0.2% 0.2% 0.1% Statistics 4.0% <0.8% 0.45% Total 0.5 +/- 4.4% 0.5% +/- 0.9% 2.0 +/- 0.7 %

60 Improvements for 2008 Thinner endcap/detector windows 2.5 µm/25 µm 0.7 µm/6 µm Reduces systematic corrections/uncertainties Improved energy calibrations Three sources ( 113 Sn, 85 Sr, 207 Bi, 114 In?) and variable pulsed LED system Reduces systematic uncertainties Higher beam current & more hermetic UCN guides Typically running at 15-18Hz compared to 7 Hz in 2007 Plans for DLC Coated decay tube Raises spectrometer Fermi potential Should ~ double β-decay rate

61 2007 Run Total counts vs Time (08)

62 Beyond 2008 with UCN Big A measurement with <0.5% precision Neutron lifetime with Halbach magnetic array Correlation experiments with Si detectors a & B measurements possible with proton detection

63 Summary High density UCN source developed at LANL First UCN correlation experiment underway Future high precision measurements possible with UCN