Neutron Decay Status and the future of the beta asymmetry - - PowerPoint PPT Presentation

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 lifetime Angular correlation in polarized neutron decay (Jackson et al ‘57)

( ) ( )

2 2 2 2 2 2

3 1 Re 2 , 3 1 Re 2 , 3 1 1

• +
• =

+ +

• =

+

• =

B A a

V A

G G

• +
• +
• +
• =
• E

p B E p A E E p p a d d

e e n e e

1

Neutron β decay and Vud

(τ = 885.7±0.8 s PDG2006)

) 3 (

2 2 1 A V R

G G Kf + =

• Vud

2 =

GV

2

GF

2(1+ R V )

δλ/δa ~ 3.3; a = –0.103 ± 0.004 (PDG2006) δλ/δA ~ 2.6; A = –0.1173 ± 0.0013 (PDG2006) δλ/δB ~ 13.4; B = –0.981 ± 0.004 (PDG2006)

Vud Experimental Status

|GA/GV|

* δA/A=0.6%

*

|Vud|

Contribution to the Vud uncertainty

Vud

2 =

4908.7 s n(1+ 32)(1+ R)

2.7 total 1.9 ΔR 1.5 δC 1.1 δR δVud (10-4) Contribution

0+→ 0+

13 total 2 ΔR 12 λ (δA/A = 0.6%) 4 τn (δτn = 0.8 s) δVud (10-4) Contribution

neutron

Vud

2 =

2984.48 s ft(1+ RC)(1+ R)

Marciano and Sirlin, PRL 96, 032002 (2006)

Principle of the A-coefficient Measurement

B field

Detector 1 Detector 2 Polarized neutron Decay electron

• cos

) ( ) ( ) ( ) ( ) (

2 1 2 1 exp

A P E N E N E N E N E A

• +
• =

(End point energy = 782 keV)

n

e

θ

dW=[1+βPAcosθ]dΓ(E)

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

0.5%

(15% environment)

1.1%

• 0.1189 ± 0.0007

2002 PERKEO II 1.6%

(15% environment)

1.5%

• 0.1189 ± 0.0012

1997 PERKEO II 3% 1.9%

• 0.1160 ± 0.0015

1995 ILL-TPC small 27%

• 0.1116 ± 0.0014

1991 PNPI 3% 2.6%

• 0.1146 ± 0.0019

1986 PERKEO

Systematic Corrections P Background

A

(Beta-Asymmetry)

Experiment

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

Measures a Construction NIST aCorn Measures C (=A+B) Proposal ORNL-SNS PANDA Measures a Data taking/ Analysis FRM-II aSPECT Data taking completed in 2004 Analysis ILL Perkeo II Uses unpolarized neutrons to measure a, b Simultaneous measurement

• f a, b, B, A to δA/A = 0.1%

(stat), δa/a = 0.1% (stat) Uses UCN to measure A to δA/A = 0.2% Measures A Remarks Proposal ORNL-SNS Nab Proposal/ Detector development ORNL-SNS abBA Construction/ Commissioning LANL UCNA Construction ILL Perkeo III Status Location Experiment

Perkeo III Experiment

• Higher decay rate
• Lower background
• Smaller edge effect

UCNA Experiment — General Approach

• 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

Novel features: UCN from pulsed spallation source MWPC + plastic scintillator as β detector Goal: 0.2% measurement of A (δA/A = 0.2%)

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

kinetic energy < 3x10-7 eV wavelength > 500Å

• r temperature < 4 mK
• cf: Gravity: 10-7 eV/ meter.

Magnetic field (µB): 10-7 eV/ 1.7 T. 3m

g

UCNA Experiment — Apparatus

Neutron Absorber

Field Expansion Region

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 3He neutron polarizer
• Use of superconducting magnet providing 4π

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 – 3He 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 δA/A~10-3 with

different approaches on the way