The Neutrino Electron Correlation Coefficient in Free Neutron Decay ...

The Neutrino Electron Correlation Coefficient 

in Free Neutron Decay: 

Latest Results with the aSPECT Spectrometer 

Gertrud Konrad 

University of Mainz / Germany 

7th International UCN Workshop 

Saint-Petersburg, Russia, 8 - 14 June 2009 

The aSPECT collaboration 

F. Ayala Guardia 1 , S. Baeßler 2 , M. Borg 1 , F. Glück 3 , W. Heil 1 , I. 

Konorov 4 , K.K.H. Leung 5 , R. Muñoz Horta 1 , M. Simson 4,5 , Y. 

Sobolev 1 , T. Soldner 5 , H.-F. Wirth 6 , O. Zimmer 4,5 , G. K. 1 

1 

University of Mainz, Germany 4 

TU Munich, Germany 

2 

University of Virginia, USA 5 

ILL Grenoble, France 

3 

University of Karlsruhe, Germany 6 

LMU Munich, Germany

Outline 

• The Antineutrino-electron angular correlation 

coefficient a in free neutron decay 

• The Neutron decay spectrometer aSPECT 

• Preliminary results from the beam time at ILL, 

Grenoble (Spring 2008) 

• Improvements and Outlook 

• Summary 

G. Konrad, University of Mainz The aSPECT Experiment, 7th Int’l UCN Workshop Saint -Petersburg 11 June 2009

Neutron Decay Parameters (SM) 

H G V n γ γ γ p ν γ γ γ e 

5 

weak F ud e μ μ 5 

 

 

 

p + e - 

n 

n 

Jackson et al., PR 106, 517 (1957) 

dW 

 

2 2 

2 p e e 

e e 

ud 

1 3 1 a p 

m p p p p 

GFV 

b 

n 

EE A e 

Ee 

E B 

e 

E D 

 

 

 

 

 

 

 

 

EE 

e 

 

ν e 

 

Neutron lifetime 

 

1 

G 2 

n 

V F ud 

 

2 1 3 

2 

 

Neutrino-Electron Correlation 

2 

1 

 

a 

2 

1 3 

Beta-Asymmetry 

2 

cos 

A 2 

2 

1 

3 

Coupling constant ratio 

 

g 

g 

A 

V 

e 

i 

, where phase = 180° (within the SM) 

λ can be extracted from measurements of either a, A or B 

over-determined process 


Possible Tests of the Standard Model 

• Search for Right-handed Currents 

‣ W R ? 

• Search for Scalar and Tensor Interactions 

‣ Leptoquarks? Charged Higgs Bosons? 

• Search for Supersymmetric Particles 

‣ (Loop corrections to Beta Decay change Coupling Constants) 

• Test of the Unitarity of the Cabibbo-Kobayashi-Maskawa-Matrix 

d' 

Vud 

Vus 

Vub 

d 

 

s' V V V s 

cd cs cb 

b ' Vtd Vtd V 

tb 

b 

 

V 

ud 

2 

 

 

4908.7 1.9 

 

n 

 

2 

1 

3 

2 2 2 

V V V 1 

ud 

us ub 

 

 

Kaon decays 

s 

B/ D mesons 


Search for Scalar and Tensor Interactions 

Left-handed Scalar and Tensor Model: 

(Time reversal invariance) 

g g g g g g 

1, 1, , and 

g g g g g g 

' 

' 

' 

' 

V A S S 

T T 

V A V V A A 

P.A. Vetter et al., Phys. Rev. C 77, 

035502 (2008) 

Right-handed Scalar and Tensor Model: 

(Time reversal invariance and b = 0) 

g g g g g g 

1, 1, , and 

g g g g g g 

' 

' 

' 

' 

V A S S 

T T 

V A V V A A 

g T 

/g A 

0.3 

0.2 

95% CL 

90% CL 

68.27% CL 

g T 

/g A 

0.3 

0.2 

95% CL 

90% CL 

68.27% CL 

g T 

/g A 

0.3 

0.2 

95% CL 

90% CL 

68.27% CL 

0.1 

0.1 

0.1 

0.0 

0.0 

0.0 

-0.1 

-0.1 

-0.1 

-0.2 

-0.3 

a PDG = -0.103(4) 

A PDG = -0.1173(13), B PDG = 0.9807(30) 

-0.2 

-0.3 

a PDG = -0.1030(3) 

A PDG = -0.1173(13), B PDG = 0.9807(30) 

-0.2 

-0.3 

a PDG = -0.1048(3) 

A PDG = -0.1173(13), B PDG = 0.9807(30) 

-0.3 -0.2 -0.1 0.0 0.1 0.2 0.3 

-0.3 -0.2 -0.1 0.0 0.1 0.2 0.3 

-0.3 -0.2 -0.1 0.0 0.1 0.2 0.3 

g S 

/g V 

g S 

/g V 

g S 

/g V

Determination of A V 

g 

g 

λ from neu utron beta decay 

-1.250 Measurements of A 

Measurements of a 

-1.255 

-1.260 

-1.265 

-1.270 

-1.275 

-1.280 

λ=-1.2695(29) from PDG2008 

• PERKEO II is the 

systematically cleanest 

experiment 

• Still the disagreement with 

older measurements is not 

explained 

• A measurement of a is 

independent of possible 

unknown errors in A; 

systematics are entirely 

different 

Present best experiments 

have Δa/ a ~ 5 % 

Aim of aSPECT 

is Δa/ a ~ 0.3% 


The Neutrino-Electron Correlation a 

and the Proton Spectrum in Neutron Decay 

The Correlation coefficient a 

dW 

v e 

 

 

1 a cos p , p 

 

 

n 

 

e 

c 

 

 

 

e - e - 

 

p + e - 

Sensitivity of the Proton Spectrum to a 

θ 

ν e 

n 

ν e 

n 

p + 

Decay rate w p(T) p 

p + 

Proton kinetic 

energy is big 

ν e 

Proton kinetic 

energy is small 

Proton spectrum for a = + 0.3 

and for a = - 0.103(4) (PDG 2008) 

0 200 400 600 

Proton kinetic energy T [eV] 


Setup of aSPECT at PF1b/ ILL 

and Spectrometer sketch 

Proton detector 

Proton 

Detector 

-15kV 

Magnetic 

Field 

Analyzing Plane 

Electrode 

U A 

E B 

(0V to 

+800V) 

Analyzing 

Plane 

Protons 

Neutron decay 

 

 

Decay 

Volume 

Anti-magnetic screen 

+1kV 

Electrostatic 

Mirror 

Neutron 

Beam 


Principle of a Retardation spectrometer 

p || 

p || 

p 

Adiabatic 

conversion 

p 

Analyzing plane 

Potential barrier 

eU B A = 0.44 T 

Magnetic field 

Proton trajectory 

B 0 = 2.2 T 

Decay volume 

Decay rate w p (T) 

Proton spectrum for a = + 0.3 

and for a = - 0.103(4) (PDG 2008) 

Transmission function @ U A = 375V 

0 200 400 600 

Proton kinetic energy T [eV] 

1.0 

0.8 

0.6 

0.4 

0.2 

0.0 

Transm mission function F tr (T;U A ) 

Transmission function F tr (T;U A ) in the adiabatic limit 

 

0 ; T eU 

A 

 

Ftr 

T; UA1 1B0 

BA 

1 eUA 

E 

; otherwise 

 

 

1 ; T eU 

A 1B 

B 

A 0 

 


Principle of extraction of a 

and Statistical sensitivity 

Pulse height spectrum 

U A 

Total proton count rate [s -1 ] 

500 

400 

300 

200 

Integrated Proton Spectrum 

Simulation for a = + 0.3 

and for a = - 0.103(4) (PDG 2008) 

Experimental data (offset subtracted) 

0 

0 100 200 300 400 500 600 700 

• About 470 events per second at U A = 50 V (on one detector pad) 

• Statistical sensitivity to a about 2 % per 24 hours measurement time 

100 

U 

; U 

N 

A 

Ft 

r 

T 

A 

wp 

T dT 

Analyzing plane voltage U A [V] 

M. Simson et al., Proceedings of the IWPPSN, Nucl. Instr. and Meth. A, accepted, arXiv:0811.3851 

G. Konrad et al., Proceedings of the PANIC08, submitted to Nucl. Phys. A

Systematic effects I: Trap Cleaning 

 

Decay protons can be shifted with an E Belectrode, that 

removes charged particles stored between AP and Mirror 

 

E B 

vd 

 

2 

B 

Background measured @ U A = 780V 

 

for two different settings of the E B electrode 

780V 

Analyzing 

Plane 


beam off 


beam on 


beam off 

U 1 

U 2 

+1kV 

Electrostatic 

Mirror 

G. Konrad, University of Mainz The aSPECT The aSPECT Experiment, Experiment, 7th Int’l DPG UCN Spring Workshop Meeting Saint Munich -Petersburg 09 

11 June 2009

Systematic effects II: Edge effect 

Detector 

Detector 

B 

B 

Neutron beam 

intensity 


Intensity 


Projected area 

• Protons produced outside the directly 

projected volume can hit the detector 

• and protons produced within the 

projected area might miss the detector 

This two effects cancel if the beam 

profile is flat, independent of the proton 

gyration radius 



Detector 

Detector 

B 

B 




Intensity 


Transition region 

… But if the profile is non-flat the 

probabilities for the two cases become 

different, and dependent on the proton 

gyration radius 

The gyration radius depends on the 

initial angle and energy of the proton 

The beam profile has to be 

sufficiently flat or well known 





normalized to maximum 

1.0 

0.8 

06 0.6 

0.4 

0.2 

0.0 

Neutron beam profile 

at the exit flange of the spectrometer 

for two different apertures 

Full neutron beam profile 

Profile with additional aperture 

-40 -20 0 20 40 

Projected 

area 

Width y [mm] 

Uncorrected neu utrino electron 

correlation coe efficient a exp 

The Correlation coefficient a 

-0.08 

-0.10 

-0.12 

a=-0.103(4) from PDG2008 

Full beam 

Narrow 

beam 

Each data point corresponds to about 

14 hours of measurement time 

… Monte Carlo simulations 

are currectly on the way 


Systematic effects III: The Magnetic field ratio 

External Helmholtz coils 

allow us to change magnetic 

field ratio r B B 

B 

For 0 A: r B = 0.2030; For 50 A: r B ‘= 0.2049 

 

0 ; T eU 

A 

 

Ftr 

T; UA1 1B0 

BA 

1 eUA 

E 

; otherwise 

 

 

1 ; T eU 

A 1B 

A 

0 

B 

A 0 

 

Uncorrected neutrino electron 

correlation coefficient a exp 

-0.08 

-0.10 

-0.12 

Helmholtz 

coils off 

a=-0.103(4) from PDG2008 

F tr (T) with 

r B = 0.2030 

Helmholtz 

coils on 

F tr (T) with 

r B = 0.2030 

Helmholtz 

coils on 

F tr (T) with 

r B ‘ = 0.2049 

Correlation coefficient a exp 

changes with the magnetic 

field ratio r B , as described 

by a slightly different 

transmission function 

F tr (T;U A ) 

Each data point corresponds to about 

14 hours of measurement time 


Improvements and Outlook 

… But a Δa/ a ~ 0.3% measurement requires 

U 

10mV 

4 

B 

B 

 

10 

• Electric potential measurements: accurate voltage measurement with 

multimeter, but: Surface charges 

 

 

 

A 

0 

Decay Volume 


‣ Investigations of the work function of the electrodes (Kelvin probe) 

• Online monitoring of the magnetic field ratio 

‣ NMR system with polarized 3 He 

• Calibration source 

U A 


Online monitoring of the magnetic field ratio 


Electrode 

Capacitors 

Amplitude V 

0.2 

0.1 

ν = γ He-3 B 0 

γ He-3 ≈ 32.436 [MHz/T] 

NMR coil 

0.0 

-0.1 

T 2 ≈ 6 ms 

-0.2 

0 2 4 6 8 10 

Copper tube 

for He-3 filling 

Amplitude 

0.004004 

0.003 

FFT transformation 

Time ms 

0.002 

0.001 

Electrode 

systeme 

0.000 

0 10000 20000 30000 40000 50000 

• Proof of principle 

• Accuracy better than 10 -4 

r B B 

Frequency Hz 

• Magnetic field ratio stable in time 

B 

A 

0 


Summary 

• aSPECT extracts a from the measured integrated proton spectrum 

in free neutron decay 

• Statistical accuracy about 2 % per 24 hours measurement time 

• Studied several systematic effects 

• Total accuracy expected to be well below the present literature 

value of 5 % 

• Data analysis is currently on the way 

• A statistical accuracy of Δa/ a ~ 0.3% can be obtained within one 

further beam time at ILL 


Backup 


Why investigate Neutron Beta Decay? 

A decaying neutron is an easy system. 

p = (udu) 

u 

e - 

32 

Cl (17p+15n) 

d e + 

e - 

μ 

d 

n = (ddu) 

W ± 

e 

W ± 

u 

32 

Ar (18p+14n) 

e 

μ - 

W ± 

e 

neutron (β - ) decay 

32 

Ar (β + ) decay muon decay 

Universality: If we neglect phase space, nuclear structure, 

and spin states, all decays are equal 


Coupling Constants of the Weak Interaction 

Coupling Constants in Neutron Decay 

p = (udu) 

u 

g V , g A 

W ± e - ν e 

p 

Primordial Nucleosynthesis 

W ± e - 

p 

W ± e + 

ν e 

d 

n = (ddu) 

n 

ν e 

n + ν e ↔ p + e - 

n 

n + e + ↔ p + ν e 

n → p + e - + ν e 

Solar cycle 

Start of Big Bang Nucleosynthesis, 

Primordial 4 He abundance 

Neutrino Detection (SNO, CC) 

2 

H + 

2 

H + 

ν e 

p + p 

e - 

e + ν e 

W ± 

W ± 

p + p 

p + p 

W ± e - 

p + p → 2 H + + e + + ν e p + e - + p → 2 H + + ν e 

Start of Solar Cycle, determines amount of 

Solar Neutrinos 

G. Konrad, University of Mainz The aSPECT Experiment, 7th Int’l UCN Workshop Saint -Petersburg 11 June 2009 

2 

H + 

ν e 

ν e + 2 H + → p + p + e - 

Efficiency of Neutrino Detectors

The Beta Decay Hamiltonian 

Task: 

Construction of Parity-Violating Hamiltonian, 

which couples leptons and hadrons 

60 

Co 

I I e - 

≠ 

60 

Co 

e - 

5 ' 

H 

weak,elementary 

GF 

u γ 

 

γ γ d e γμ 

γμγ5 

ν 

 

V 

 

A 

Properties: Helicity of (relativistic) fermions is -1, of antifermions is 1 

e 

Complication: Nucleons aren‘t elementary particles: 

H G V p γ γ γ n e γ γγ ν 

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 


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 

Spivak 88 

Neutron lifetime [s] 

895 

Nesvizhevsky 92 

Byrne 96 

890 

Mampe 89 

885 

Arzumanov 00 

Mampe 93 

880 

Dewey 03 

PDG2006 

MAMBO 

Serebrov 05 

875 

1988 1992 1996 2000 2004 

Year of Publication 

Many new attempts planned, mostly with magnetic bottles 


The Unitarity of the CKM Matrix 

Cabibbo-Kobayashi-Maskawa-Matrix 

d' 

Vud 

Vus 

Vub 

d 

 

s' V V V s 

cd cs cb 

b ' Vtd 

Vtd 

V 

tb 

b 

 

V 

Unitarity Condition 

ud 

2 

 

V 

us 

2 

 

V 

ub 

2 

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 


Primordial Nucleosynthesis 

Before Phase Transition: 

Equilibrium 

p 

W ± e - 

p 

ν e 

p 

W ± e - 

After Phase Transition (about 1 min after Big Bang): 

n 

ν e 

W ± e + 

n + ν e ↔ p + 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 

“After about three minutes, the cosmic 

hydrogen to helium ratio was fixed…. 

...Nothing of interest has happened since” 

n 

n 

ν e 

n ↔ p + e - + ν e 

….. 

Steven Weinberg 

Lecture, Harvard University, 1975 


The Beta Asymmetry 

Electron Detector (Plastic Scintillator) 

p + 

n 

e 

e - 

Decay Electrons 

v 

dw 1 

A cos pe, 

 

n 

 

c 

A 

N 

N 

up 

up 

 

N 

N 

down 

down 

 

 

 

 

Polarized Neutrons 

Split Pair Magnet 

Magnetic Field 

PERKEO II 

Beam time Result Publication 

1995 A = -0.1189(12) H. Abele, S. B. et al., Phys. Lett. B 407, 212 (1997) 

1997 A = -0.1189(7) H. Abele, S. B. et al., PRL 88, 211801 (2002) 

2004 A = -0.1198(5) (preliminary) 

1995 – 2004 

combined 

A = -0.11933(34) 

(preliminary) 

H. Abele, Prog. Part. Nucl. Phys. 60, 1 (2008) 


Determination of the Coupling Constants 

n 

Fermi-Decay: 

g V = G F·V ud 

Gamow-Teller-Decay: 

p 

p 

1 

2 

1 

2 

 

 

 

 

 

 

ν e 

 

 

 

e - e - ν e 

ν e 

ν e 

 

e - e - a = 1 

a = 1 

g A = G F·V ud·λ 

p e - ν e 

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: 

n 885 s 

2 

1 

a 

13 

2 

~ 0.1 

 

g 

A 

gV 

a = -1 

v 

dw 1a cos pe, 

p 

c 

 

e 

 

 

 

 


The proton detector 

Type: Silicon Drift Detector (SDD) 

Principle: 

p-doped backside, p-doped rings 

Potential Valley 

e - drift to a small anode in the 

middle 

G. Lutz, Semiconductor Radiation Detectors 

P. Lechner et. al. , XRS 2004 33 256-261 

Proton 


Detector features 

Chip size 14x34 mm² 

3 Pads of 10x10 mm² 

Thickness 450 µm 

Entrance window 

30 nm aluminium 

Suitable for UHV 

Integrated temperature diode 

First amplifying FET on chip 

Preamp based on 3 Amptek 

A250 


Investigations of systematic effects 

Background 

‣ Additional neutron shutter 

‣ Different „trap cleaner“ and „beam shifter“ ExB settings 

Adiabatic transmission function 

‣ Different magnetic field settings 

‣ Measurement of the magnetic field at the beam position, before and after the 

beam time 

‣ Different electric field settings 

Edge effect 

‣ Different neutron beam profiles 

‣ Monte Carlo simulations are currectly on the way 

Detector efficiency 

‣ Different trigger settings 

‣ Different post-acceleration voltages 

‣ Tests at a proton source + SRIM calculations are currently on the way 


Typical Proton Event 

Proton Pulse Shape 

ADC value 

2000 

1900 

Pulse Height Energy 

1800 

1700 

1600 

1500 

Event Length about 3μs 

0 20 40 60 80 100 

Time [ x50ns] 


Data analysis 

Fit function: 

xx 

 

t 

y y0 A 1e e 

 

 

p 

xx 

t 

0 0 

1 2

Data analysis 

Fit function: 

xx 

 

t 

y y0 A 1e e 

 

 

p 

xx 

t 

0 0 

1 2

Further on-going improvements 

• Study of surface potential; Kelvin probe 

In collaboration with 

Prof. I. Baikie, 

KP Technologies 

Height [mm] 

40 

35 

30 

25 

20 

15 

10 

5 

0 

0 100 200 300 

KP Technologies Azimuth angle [°] 

4900 

4950 

5000 

5050 

5100 

Work Function [meV] 

Possible solution 

Other surface 

coatings 

‣ evaporated gold? 

‣ chromium? 

‣ colloidal carbon? 

• On-line monitoring of the magnetic field ratio; NMR with polarized 3 He 

Amplitude V 

4 

2 

0 

-2 

-4 

0 5 10 15 20 25 

Time ms 

ν = γ He-3 B 0 

γ He-3 ≈ 32.436 [MHz/ T] 

• Calibration source; electron impact ionization 


Kelvin Probe 

V 

Material 1 Material 2 Material 1 Material 2 Material 1 Material 2 

Φ 2 

E F,2 

Φ 1 Φ 2 Φ 1 

Φ 2 

V C 

Φ 1 

E F,1 

E F,1 

E 

_ 

F,2 E F,1 

+ 

_ 

+ 

_ 

+ 

E F,2 

+ 

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

The Neutrino Electron Correlation Coefficient in Free Neutron Decay ...

You also want an ePaper? Increase the reach of your titles

Delete template?

Save as template?