Neutron β Decay Status and the future of the beta asymmetry measurement Takeyasu Ito Los Alamos National Laboratory The 4th International Workshop on the CKM Unitarity Triangle December 12-16, 2006 Nagoya University Nagoya, Japan
Outline • Introduction • Principle of the A measurement • Current status • Ongoing and future experiments – Perkeo III – UCNA – abBA • Summary
Neutron β decay and V ud 2 G V 2 = V ud V ) 2 (1 + � R G F Neutron lifetime 2 2 1 R Kf ( G 3 G ) � ( τ = 885.7±0.8 s PDG2006) � = + V A Angular correlation in polarized neutron decay (Jackson et al ‘57) � � � � p p p p � d d 1 a e � A e B � � � � = � � + + � � + � � 0 n � � E E E E � � � � e e � � � � G 2 2 2 ( ) ( ) 1 Re Re � � � + � � � � A � � a , A 2 , B 2 = = � = G 2 2 2 1 3 1 3 1 3 + � + � + � V δλ / δ a ~ 3.3; a = –0.103 ± 0.004 (PDG2006) δλ / δ A ~ 2.6; A = –0.1173 ± 0.0013 (PDG2006) δλ / δ B ~ 13.4; B = –0.981 ± 0.004 (PDG2006)
V ud Experimental Status |V ud | * |G A /G V | * δ A/A =0.6%
Contribution to the V ud uncertainty 0 + → 0 + neutron 2984.48 s 4908.7 s 2 = 2 = V ud V ud � n (1 + 3 � 2 )(1 + � R ) ft (1 + RC )(1 + � R ) δ V ud (10 -4 ) δ V ud (10 -4 ) Contribution Contribution τ n ( δτ n = 0.8 s) 1.1 4 δ R λ ( δ A/A = 0.6%) 1.5 12 δ C 1.9 2 Δ R Δ R total 2.7 total 13 Marciano and Sirlin, PRL 96 , 032002 (2006)
Principle of the A- coefficient Measurement Detector 1 Detector 2 B field e θ n Polarized neutron Decay electron dW =[1+ β PA cos θ ] d Γ ( E ) N ( E ) N ( E ) � A ( E ) 1 2 P A � cos = � � exp N ( E ) N ( E ) + 1 2 (End point energy = 782 keV)
Potential Sources of Systematics • Neutron polarization determination • Background • Detector-related effects – Electron backscattering – Edge effect (fiducial volume selection) – Detector response, calibration
Summary of previous systematics Experiment A Systematic Corrections (Beta-Asymmetry) P Background -0.1146 ± 0.0019 PERKEO 1986 2.6% 3% -0.1116 ± 0.0014 PNPI 1991 27% small ILL-TPC 1995 -0.1160 ± 0.0015 1.9% 3% -0.1189 ± 0.0012 PERKEO II 1997 1.5% 1.6% (15% environment) -0.1189 ± 0.0007 PERKEO II 2002 1.1% 0.5% (15% environment) Next generation experiment should address all these potential systematic errors.
Perkeo II Experiment Electron energy spectrum •Dead layer: 20 keV •Threshold: 60 keV •S/N in ROI = 7
Ongoing/Future Experiments Experiment Location Status Remarks Perkeo II ILL Analysis Data taking completed in 2004 Perkeo III ILL Construction Measures A aSPECT FRM-II Data taking/ Measures a Analysis aCorn NIST Construction Measures a UCNA LANL Construction/ Uses UCN to measure A to δ A/A = 0.2% Commissioning abBA ORNL-SNS Proposal/ Simultaneous measurement of a, b, B, A to δ A/A = 0.1% Detector (stat), δ a/a = 0.1% (stat) development Nab ORNL-SNS Proposal Uses unpolarized neutrons to measure a, b PANDA ORNL-SNS Proposal Measures C (= A + B )
Perkeo III Experiment •Higher decay rate •Lower background •Smaller edge effect
UCNA Experiment — General Approach Novel features: UCN from pulsed spallation source MWPC + plastic scintillator as β detector Goal: 0.2% measurement of A ( δ A/A = 0.2%) Neutron Polarization • – UCN (can produce >99% polarization with 7T magnetic field) – Diamond-like carbon coated neutron guide (low depolarization) Background • – Pulsed UCN source – MWPC+Plastic scintillator Electron backscattering • – MWPC+Plastic scintillator Fiducial volume selection • – MWPC Detector Characterization • – Off-line calibration system – Larger light collection
Interlude — What are UCN? • Very slow neutrons (v < 8m/s) • Totally reflected by some materials • Hence, they can be totally confined within a bottle for periods in excess of 100 seconds. • Typically: velocity < 8m/s g kinetic energy < 3x10 -7 eV 3m wavelength > 500Å or temperature < 4 mK • cf: Gravity: 10 -7 eV/ meter. Magnetic field ( µ B): 10 -7 eV/ 1.7 T.
UCNA Experiment — Apparatus Field Expansion Region Neutron Absorber
UCNA Apparatus in LANSCE Area B
UCNA Experiment — Status and timeline • Status – UCN source: commissioning – Beta detector: completed – Polarizer/spin flipper: commissioning – Neutron guides: construction • Plan – 2007: Commissioning, 1% measurement – 2008–: Physics run for 0.2% measurement • Future plans – Measurements of other correlation coefficients – Use of silicon detector
Beta decay spectrum from UCN S/N in 200-400 keV = 13
abBA — General approach • Simultaneous measurement of a, b, B, A • Coincidence detection of decay electrons and protons • Use of segmented Si detector as β detector • Use of pulsed cold neutron beam from spallation source • Use of 3 He neutron polarizer • Use of superconducting magnet providing 4 π coverage Goal: 0.1 % measurement of A
ORNL-SNS-FNPB SNS: •Spallation neutron source driven by 1.4 GeV, 1mA proton linac •Construction completed in 2006 FNPB: •Beam line for fundamental neutron physics •Consists of cold line and UCN line •Start operation in 2007
abBA — Experimental Layout at FNPB
abBA — β spectrometer
abBA — Status and Plans • Status – Proposal approved by FNPB PRAC – 3 He polarizer exists – Detector technology proven – Preliminary spectrometer design • Plans – 2007: Spectrometer design – 2008: Construction – 2009–: Commissioning & Physics run
Summary • No new published A measurement results since 2002 • Several experiments aiming at δ A/A~10 -3 with different approaches on the way
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