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Exploring Neutron Beta Decay Spin Correlations and Lifetime

This presentation covers detailed design and performance of the Area B UCN source, physics motivation for the UCNA experiment, analysis techniques and results from the 2007 run, as well as the status report on the 2008 run. It delves into topics such as neutron β-decay spin correlations, electron-neutrino correlations, and various correlation coefficients related to neutron lifetime. The talk also addresses the status of A and λ measurements, challenges in resolving discrepancies, and the advantages of conducting these measurements via ultra-cold neutrons. The UCNA experiment's approach to reducing systematic errors, background corrections, and advancements in electron detection methods are discussed, highlighting the significance of studying neutron beta decay for precision physics measurements.

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Exploring Neutron Beta Decay Spin Correlations and Lifetime

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  1. LANL 2008 B. Plaster First Results from the UCNA Experiment Leah Broussard (Duke/TUNL) Kevin Hickerson (Caltech) Adam Holley (NCSU) Russ Mammei (Virginia Tech) Michael Mendenhall (Caltech) Robert Pattie (NCSU) Raymond Rios (Idaho/LANL) Anne Sallaska (U Washington) Riccardo Schmid (Caltech) Sky Sjue (U Washington) Junhua Yuan (Caltech) Yanping Xu (NCSU) Several New Collaborators as of 2008 Run Brad Plaster, University of Kentucky LANL P-25 — November 5, 2008

  2. Detailed design/performance of the Area B UCN source Mark Makela, Chris Morris, Andy Saunders Will talk about Physics motivation for UCNA Review of UCNA experiment Analysis techniques/results from 2007 run Status report on 2008 run LANL 2008 B. Plaster This talk Not talk about

  3. = 1 (CVC) σ gV = f1 (q2  0 ) GF |Vud| weak magnetism θp θe p n e e gA = g1 (q2  0 ) GF |Vud| gA  = eiφ gV O (5%) lattice QCD LANL 2008 B. Plaster Neutron β-decay

  4. spin σ θp θe e− correlation −0.103(4) electron, β-asymmetry −0.1173(13) neutrino-asymmetry 0.983(4) p n e e 1 – 2 2 +  2 -  a0 = A0 = −2 B0 = 2 1 + 32 1 + 32 1 + 32 Highest Sensitivity to  LANL 2008 B. Plaster Correlation coefficients

  5. 1 GF2|Vud|2(1 + 32) (1 + RC)  n LANL 2008 B. Plaster Neutron lifetime “Master Formula” Marciano and Sirlin (2006) 1.0389 ± 0.0004 (0.0008) δ(Vud) = ± 0.0002

  6. ΔA/A [ % ] PERKEO II update: PERKEO II 0.6 H. Abele, Prog. Part. Nucl. Phys. 60, 1 (2008) PDG 2008  ILL-TPC 1.3 IAE-PNPI 1.2 Average of 4 published A results + 1 simultaneous A/B result [ Mostovoi et al. (2001) ] PERKEO I 1.7  = −1.2695 ± 0.0029 [0.23%] PDG Mean 1.1 LANL 2008 B. Plaster Status of A and  Published 0.28%

  7. Vud PDG 2008 neutron Vud = 0.9746 (4)τ(18)(2)RC LANL 2008 B. Plaster Status of n and neutron Vud Serebrov et al. (2005) Still no new (published) results since

  8. Improvement in neutron sector Vud ? Resolve discrepancies in A   If achieve agreement, , factor of 2 improvement Plus improvement in bottom-line precision on A: 0.6%  ?? But must resolve n discrepancy !! LANL 2008 B. Plaster Status of Vud pion 0+ 0+ neutron PDG nuclear mirror transitions 19Ne, 21Na, 35Ar Naviliat-Cuncic and Severijns, arXiv:0809.0994

  9. LANL 2008 B. Plaster Why measurement of A via UCN ? Systematic Corrections [ % ] Polarization / Spin-Flip Backgrounds Other PERKEO I (1986) 2.6 ~ 3 ~13 magnetic mirroring ILL (1997) 2.9 ~ 3 ~15  cos θ  PNPI (1997) 23 small ~3  cos θ  PERKEO II (2002) 1.1 0.5 small

  10. LANL 2008 B. Plaster Why measurement of A via UCN ? Advantages of UCNA experiment (A via UCN) T < 335 nano-eV, v < 8 m/s, stored in material bottles 1) Polarization Expect to achieve ~100% polarizations via transport of UCN through 7-Tesla magnetic fields Spin-state selected via μ • B interaction, ± 60 neV/Tesla

  11. prompt backgrounds environmental LANL 2008 B. Plaster Why measurement of A via UCN ? Advantages of UCNA experiment (A via UCN) T < 335 nano-eV, v < 8 m/s, stored in material bottles 2) Beam-related backgrounds Greatly reduced by operating “pulsed mode” spallation source coupled to superthermal UCN source

  12. scint scint MWPC MWPC β-decay electron LANL 2008 B. Plaster Why measurement of A via UCN ? Advantages of UCNA experiment (A via UCN) T < 335 nano-eV, v < 8 m/s, stored in material bottles 3) New approach to electron detection MWPC + plastic scintillator Low sensitivity to gammas (primary bkg’d in cold neutron expt’s) Low-threshold MWPC for detection of low-energy-deposition Coulomb backscattering events

  13. LANL 2008 B. Plaster Why measurement of A via UCN ? Advantages of UCNA experiment (A via UCN) T < 335 nano-eV, v < 8 m/s, stored in material bottles 4) Reduced neutron-generated backgrounds Long storage times for UCN in apparatus For given rate, need relatively smaller number of stored UCN as compared to cold neutron beam

  14. vβ W(θ)  1 + PnA cos θ c LANL 2008 B. Plaster UCNA experiment 0.6-T field expansion Angular Distribution

  15. LANL 2008 B. Plaster UCNA experiment 7-Tesla Polarizers spin flipper muon vetoes UCN UCN Beamline 1-Tesla Electron Spectrometer photograph 12/2007

  16. LANL 2008 B. Plaster UCN spin polarization 7-Tesla Polarizer/Spin-Flipper Field Profile 420 neV μ•B barrier UCN flux 1-Tesla AFP spin-flip region Spin-flipping via Adiabatic Fast Passage [AFP] Resonator with nominal frequency of ~29 MHz Spin-flipping cancels systematic errors

  17. LANL 2008 B. Plaster Spin-flip efficiency + Cu foil UCN flux AFP resonator g UCN detector

  18. LANL 2008 B. Plaster Spin-flip efficiency Transmission measured to be 0.40 ± 0.05 % Spin-Flip Efficiency > 99.6%

  19. off on LANL 2008 B. Plaster In-situ depolarized fraction 1-T electron spectrometer UCN detector B 7-T Polarizer AFP β-decay running look for “wrong spin” drain “right spin” UCN from SD2 source

  20. LANL 2008 B. Plaster In-situ depolarized fraction Upper Limit of 0.65% on Depolarized Fraction Present During Any Run Quote 1.3% Systematic Spin Flipper State Change Dt for switcher leakage signal

  21. LANL 2008 B. Plaster Electron detection Baseline requirements Low-sensitivity to backgrounds Minimal electron backscattering Reasonable energy resolution 25-μm entrance and exit windows 2007 Geometry 3.5-mm plastic scintillator: energy, trigger Cu decay trap e low-pressure MWPC with low-Z fill gas 2.5-μm mylar foil

  22. 3-m long Cu decay trap LANL 2008 B. Plaster Electron spectrometer 4.5-m long superconducting solenoid 1.0-Tesla field 0.6-Tesla field-expansion region [ BP et al., NIMA 595, 587 (2008) ]

  23. PMT PMT LANL 2008 B. Plaster Electron detection MWPC 100-Torr neopentane 2.54-mm wire spacing on anode and 2 cathode planes (163 х 163) mm2 active area (x,y) reconstruction Fiducial volume definition [ T.M. Ito et al., NIMA 571, 676 (2007) ] Scintillator PMTs in 100-Torr N2 Axial fields ~300 Gauss

  24. LANL 2008 B. Plaster Spectrometer performance suppression of gamma background [ PERKEO II dominant background ] MWPC: gamma suppression 113Sn 368 keV 2007 calibrations: 113Sn (368 keV), 207Bi (503 keV, 995 keV) ~310 p.e. / MeV, 5.6% at 1 MeV [ ~ 100 p.e./MeV in PERKEO II ]

  25. LANL 2008 B. Plaster Spectrometer performance 113Sn 368 keV reconstruction of the center of Larmor spiral fiducial volume radius cut [decay trap walls]

  26. Correct Type I MISID Type II Type III MISID Type IV + scattering off of decay trap foils + “lost events” LANL 2008 B. Plaster Event reconstruction scintillator scintillator decay trap foil decay trap foil MWPC MWPC

  27. Evis (Etrue)  Erecon (Evis) residuals small, ~5 keV LANL 2008 B. Plaster Monte Carlo: GEANT4 / PENELOPE 1) Reconstruct β-decay energy on event-by-event basis from measured “visible energy” deposition in scintillators “Invisible energy” loss in decay trap foils, MWPC windows/interior, and scintillator dead layer 2) Form experimental asymmetry, Aexp(Erecon) = P β cos θA

  28. energy loss tail in steep part of β-decay spectrum Aexp (Erecon) β cos θ Global Backscattering LANL 2008 B. Plaster Monte Carlo: GEANT4 / PENELOPE 3) Correct experimental asymmetry for subtle effects β cos θ “acceptance” Unobservable backscattering β cos θ for all triggering events

  29. LANL 2008 B. Plaster Systematic corrections analysis window 2007 Correction  β cos θ −1.6% Backscattering +1.1%

  30. LANL 2008 B. Plaster Uncertainties in corrections Uncertainty in correction for β cos θacceptance ? How well is dE/dx energy loss modeled ? Assign (conservative) 25% uncertainty for geometry considerations

  31. LANL 2008 B. Plaster Uncertainties in corrections Uncertainty in backscattering correction ? GEANT4 Simulations tend to under-predict backscattering PENELOPE Assign 30% uncertainty to correction

  32. 778.4 (6.6) 783.2 (6.2) LANL 2008 B. Plaster β-decay energy spectra UCNA 2007 PERKEO II S/N 7:1 S/N 21:1

  33. LANL 2008 B. Plaster Asymmetry extraction Recoil-order corrections were applied to asymmetries (weak magnetism, gAgV interference, nucleon recoil) Standard calculational procedure of Wilkinson (1982) Included Fermi Function

  34. LANL 2008 B. Plaster UCNA 2007 result UCNA 2007 A = −0.1138 ± 0.0046 ± 0.0021

  35. 2008 Run : Statistics + Systematics < 1% Run I: 25-μm MWPC windows, 0.7-μm decay trap foils “Calibrate” MCs Completed July–September, ~10.5M collected Run II: 25-μm MWPC windows, 13-μm decay trap foils Completed Monday 6AM, ~10.0M collected Run III: 6-μm MWPC windows, 0.7-μm decay trap foils Optimal Geometry Starts this weekend, thru end-of-run LANL 2008 B. Plaster UCNA 2008 running 2007 Run 25-μm MWPC windows, 2.5-μm decay trap foils 0.781M β-decay events 0.396M passed cuts (tight fiducial cut), 4.1% statistical error Need ~8.5-10M events for 1% result (depending on fiducial cut)

  36. 25 μm MWPC Windows 6 μm MWPC Windows Factor ~2 LANL 2008 B. Plaster Reduction in 2008 systematics Bias to Asymmetry Decay Trap Foils β cos θ Backscattering 0.7 μm 1.5% −0.5% 13 μm 3.4% −2.7%

  37. LANL 2008 B. Plaster UCNA 2008 running Run I Run II

  38. LANL 2008 B. Plaster Projected impact of 2008/09 results

  39. LANL 2008 B. Plaster Summary UCNA experiment has reported first-ever measurement of any neutron β-decay correlation parameter with UCN 4.5% “proof-of-principle” result 2008 data collection proceeding well Poised to produce very interesting and competitive result at 1% level Sub-1% level in 2009 running Higher rate: improved Fermi potential, beam current, SD2

  40. Backgrounds vs. Spin State 0 – 800 keV background rates (all event types) East West no evident correlation (AFP-induced noise) 0.184 ± 0.004 AFP off 0.274 ± 0.005 AFP on 0.188 ± 0.004 0.268 ± 0.005

  41. Decay Trap (x,y) Spectra 40 mm cut

  42. Background Muons

  43. NaI optical fibers β-scintillator 60Co GMS Gain Corrections GMS reference PMT LED box light guides to β-PMTs gain correction factor for ithβ-PMT position of LED peak in GMS reference PMT position of 60Co peak(s) in GMS reference PMT position of LED peak in ithβ-PMT

  44. 1.173 MeV 1.333 MeV GMS Gain Corrections East 60Co West 60Co

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