Transcript Studies of Neutron Beta Decay Stefan Baeßler d e- d u u u d p n e How to discover new particles? High Energy Physics Experiments Example: • Production of W-Boson • Search.

Studies of Neutron Beta Decay
Stefan Baeßler
d
e-
d
u u
u d
p
n
e
How to discover new particles?
High Energy Physics Experiments
Example:
• Production of W-Boson
• Search for extra (e.g., righthanded) W bosons
Low Energy Precision Experiments
Example:
• Study of Neutron Beta
• Search for abnormal properties of decay products
Precision measurements in Astronomy
1. Discovery of Neptune:
Urbain Le Verrier,
1811-1877
•
•
Theoretical prediction (Le Verrier, Adams, 1845)
Idea: Explain distortions in orbit of Uranus
Discovery (Galle, 1846)
John Couch
Adams,1819-1892
Neptune
Uranus
Later: Similar story for Pluto
Sun
Distortions of Uranus orbits known since decades
Precision measurements
2. Non-Discovery of Vulcan:
Vulcan
•
•
•
Idea: Explain extra perihelion precession of Mercury
by presence of Vulcan
Convincing observation failed
But failure is more interesting than
a success would have been:
Extra precession (43 arcsec/100 y)
explained in General Relativity
Neptune
Uranus
Perihelion Precession of a planet:
For Mercury, perihelion precession
angle is 1.5 deg/100 y
Sun
Precision measurements
3. Modern Example: Lunar Laser Ranging to search (among other things) for violation of the
Equivalence principle:
Fgrav µ mEarth,g
Sun
Earth
Fgrav µ mMoon,g
Fcentr µ mEarth,i
Vulcan
Moon
Fcentr µ mMoon,i
Lessons:
1. Discoveries can be made with precision measurements
2. The discovered item might be unexpected
3. Even with high precision, a discovery is not guaranteed
Neptune
Sun
Observables in Neutron Beta Decay
e
n
Jackson et al., PR 106, 517 (1957):
Observables in Neutron beta decay, as a function of
generally possible coupling constants (assuming only
Lorentz-Invariance)
ep
n
e

pe  p
me
dW    Ee   1  a
b
Ee E
Ee

 p
p
p p
p  e
+ n   A e  B   N e  D e   ...  R e
E
Ee E
Ee
 Ee
 

 
  Re 
2
Beta-Asymmetry A  2
1 3 
2
Neutrino-Electron-Correlation a 
Neutron lifetime
1
n  G V
2
2
F ud
1 3      E 
2
e
1 
2
1 3 
2
The Standard Model Parameters Vud and λ
Fermi-Decay:
gV = GF·Vud
p
1 

2
e- νe

e- νe



AS ==0,0mS = 0
p
1 

2
e- νe

e- νe



SA==1,0mS = 0
n
Gamow-Teller-Decay:
gA = GF·Vud·λ
p
e- νe
SA==
1, -1
mS = 1
v


dw  1  A cos  pe ,  n  
c


Two unknown parameters, gA and gV, need to be determined in 2 experiments
1. Neutron-Lifetime:
n 1   g V2  3 g A2 
2. Beta-Asymmetry:
2  
A  2
 0.1
2
1  3
n  885 s
gA

gV
Neutron Lifetime Measurements
Decrease of Neutron Counts N with storage time t: N(t) = N(0)exp{-t/τeff}
1/ τeff = 1/τβ+1/τwall losses
895
Spivak 88
Neutron lifetime [s]
Nesvizh.92
890
Byrne 96
Nico 05
Mampe 89
885
PDG2008
Arzumanov
00
Mampe 93
880
Serebrov
2008
Serebrov 05
875
1988
MAMBO
1992
1996
2000
2004
2008
2012
Year of Publication
Many new attempts underway, mostly with magnetic bottles:
Under (at least) construction: Ezhov et al. (ILL, PNPI Gratchina), Bowman et al. (LANL),
Paul et al. (TUM, PSI)
see K.W. Schelhammer, 10:30 h
The Beta Asymmetry: PERKEO II
Electron Detector (Plastic Scintillator)
ep+
Decay Electrons
e
n
Polarized Neutrons
v


dw  1  A cos  pe ,  n  
c


A
Split Pair Magnet
Nup  Ndown
Nup  Ndown
Magnetic Field
Beam time
Result
Publication
1995
A = -0.1189(12)
Phys. Lett. B 407, 212 (1997)
1997
A = -0.1189(7)
Phys. Rev. Lett. 88, 211801 (2002)
2004
A = -0.1198(5) (preliminary)
PERKEO II
Possible Tests of the Standard Model
Multiple determinations (nuclear physics, other correlation coefficients)
overconstrain problem, enable:
1. Search for Right-handed Currents
WR?
2. Search for Scalar and Tensor interactions
Leptoquarks? Charged Higgs Bosons?
3. Search for Supersymmetric Particles
(Loop corrections to Beta Decay change Coupling Constants)
4. Test of the Unitarity of the Cabbibo-Kobayashi-Maskawa-Matrix
 d '   Vud Vus Vub   d 
  
  
s
'

V
V
V
cs
cb    s 
   cd
 b '  V
  
   td Vtd Vtb   b 
Vud  Vus  Vub  1
2
2
2
Unitarity: Situation 2004
0.980
Unitarity
of the CKM Matrix
Vud  1  Vus  Vub
2
2
0.975
Vud
Neutron Measurements needed:
0 +→ 0 +
• Neutron lifetime τn

 n 1  GF2Vud2 1  3 
0.970
2

; λ = gA/gV
• Beta Asymmetry A(λ)
τn [PDG2006]
A [PERKEO II]
0.965
-1.25
-1.26
-1.27
Fermi-Transition:
2
A  2
-1.28
λ = gA/gV
  Re 
1 3 
2
• Neutrino-Electron-Correlation a(λ)
gV  GF Vud
Gamow-Teller-Transition: gA  GF Vud 
a
1 
2
1 3 
2
Unitarity 2008
Vud  1  Vus  Vub
2
2
Neutron Measurements needed:
0.980
• Neutron lifetime τn

Unitarity
of the CKM Matrix
 n 1  GF2Vud2 1  3 
Vud
0.975
2

; λ = gA/gV
• Beta Asymmetry A(λ)
  Re 
2
Nuclear 0+→ 0+ decays
A  2
0.970
1 3 
2
• Neutrino-Electron-Correlation a(λ)
0.965
A [PERKEO II]
-1.25
-1.26
Fermi-Transition:
-1.27
= gA/gV
a
1 
2
1 3 
2
-1.28
gV  GF Vud
Gamow-Teller-Transition: gA  GF Vud 
Neutron lifetime discrepancies have to be
sorted out.
To make A not limiting for neutron-based
determination: ΔA/A < 0.2% needed.
Uncertainty Budget PERKEO II, last run
Error Analysis
Correction
Statistical uncertainty
Uncertainty
PERKEO II
0.26%
Background
0.1%
0.1%
Neutron beam
polarization
0.3 %
0.1%
0%
0.1%
Magnetic mirror effect
0.11%
0.01%
Edge Effect
-0.22%
0.05%
Detector response
0.26%
0.26%
Spin flip efficiency
…
H. Abele, 2006, preliminary
All newer spectrometers use the same principle as PERKEO II
New attempts: UCNA (ultracold neutrons)
Superconducting solenoidal
magnet (1.0 T)
Detector housing
Be coated
mylar foil
MWPC
Plastic scintillator
A0
-0.05
Field Expansion
Region
Polarizer /
Spin flipper
Diamond-coated
quartz tube
UCN source
Short test run: A0=-0.1138(46)(21)
A. Young (NCSU), A. Saunders (LANL), et al.
-0.1
PMT
0.5 Light guide
-0.15
Rate (1/50 keV·s)
Decay volume
A0<P> = -0.1138±0.0046
0.4
Neutron
absorber
0.3
Signal
Background
0.2
0.1
0
0
200
400
600
Energy (keV)
800
1000
Next generation: PERKEO III
Advantages:
• very high countrate w/o pulsing
• reduced background through pulsing
• no edge effect
detector
(plastic scintillator)
detector
(plastic scintillator)
decay volume, 150 mT
cold
neutron beam
beam dump
velocity
selector
chopper
2m
B. Maerkisch, D. Dubbers (Heidelberg), H. Abele (Vienna), T. Soldner (ILL) et al.
New attempts at SNS: abBA / Nab / PANDA
Proton Beam 60 Hz
Spectrometer
Shutter
Choppers
LH2
Flux
Monitor
Adiabatic
Spin Flipper
Neutron Guide
Mercury
Spallation Biological Shield
Target
3He
Collimator
Polarizer
Fast, segmented silicon detector:
Jπ = 1-,2- 21.2 MeV
20.5 MeV
3He+n
Jπ = 0+ 20.1 MeV
Γ = 0.27 MeV
19.8 MeV
p+t
S. Wilburn (LANL),
Jπ = 0+
D. Bowman (ORNL) et al.
4He
Determination of the Coupling Constants
Fermi-Decay:
gV = GF·Vud
p
1 

2
e- νe

e- νe



a=1
p
1 

2
e- νe

e- ν e



a=1
n
Gamow-Teller-Decay:
gA = GF·Vud·λ
p
e- νe
a = -1
v


dw  1  a cos pe , pe 
c



Two unknown parameters, gA and gV, need to be determined in 2 experiments
1. Neutron-Lifetime:
n 1   g V2  3 g A2 
2b. Neutrino-Electron-Correlation a:
n  885 s
1  2
a
~ 0.1
2
1  3
gA

gV

Determination of λ = gA/gV
-1.255
-1.26
-1.265
PERKEO, 1986
λ
Stratowa, 1978
Yerozolimskii, 1997
UCNA, 2009
Liaud, 1997
-1.27
Byrne, 2002
-1.275
-1.28
PERKEO II, 1997
PERKEO II, 2002
PERKEO II, ?
• A measurement of a is independent of possible unknown errors in A,
systematics are entirely different.
• Present experiments have Δa/a ~ 5%, an order of magnitude improvement
is desirable
aSPECT (Mainz, Munich, ILL, Virginia)
e-
p
n
e
v


w( E )  1  a cos pe , pe 
c




Decay rate w(E)
Proton Detector
Magnetic
field
response function @ U=375V
Spectrum for a = +0.3
… for a = -0.103 (PDG 2008)
0
Analyzing Plane
Electrode
200
400
600
Proton kinetic energy E [eV]
Protons
@ 15 kV
Protons
Neutron Decay
Present best experiments: Δa/a = 5%
Present status of aSPECT: (Δa/a)stat = 2% per day
Final aim: 0.3%
aCORN
p
en
e
v


w( E )  1  a cos pe , pe 
c




Magnetic
field
pp
E  Ee,max  Ee
a = -0.103:
“pυ up”
more likely
Aim: Δa/a ~ 2%, maybe 0.5% after NIST upgrade
Tulane (F. Wietfeldt), Indiana, NIST, et al.
p
pe
The cosθeν spectrometer Nab @ SNS
p
e-
e
n
e
v


dw  1  a cos  e 
c


Kinematics:
• Energy Conservation
E  Ee,max  Ee
• Momentum Conservation
electron and proton phase space
pp2  pe2  p 2  2 pe p cose
cose  1
cut
1.2
1 a
1
pp2 distribution
pp2 (MeV2/c2
1.4
0.8
0.6
cos e  0
0.4
pe
cos e  pp 2 
Ee
cos  e  pp 2   1
cos  e  pp 2   1
cose  1
0.2
Ee = 550 keV
0
0
0.1
0.2
0.3
0.4
0.5
Ee (MeV)
0.6
0.7
0.8
0.0
0.5
pp
1.0
2
[MeV2/c2]
1.5
The cosθeν spectrometer Nab @ SNS
pe
cos e  pp 2 
Ee
pp2 distribution
mp
dz
tp 
pp  cos p ( z)
cos  e  pp 2   1
cos  e  pp
2
  1
Simulated count rate
1 a
107
106
105
Ee = 300 keV
Ee = 500 keV
104
Ee = 700 keV
Ee = 550 keV
0.0
0.5
pp
1.0
2
1.5
103
[MeV2/c2]
Neutron beam
Segmented
Si detector
0.00
0.02
0.04
1/tp
2
0.06
0.08
[1/μs2]
• Spectrometer and detector
shared with abBA
• Will likely be converted in
asymmetric configuration
30 kV
TOF region transition
region
acceleration
region
decay
volume
• Aim: ~0.1%
D. Pocanic, S.B. (Virginia),
D. Bowman (ORNL), et al.
More observables: Fierz Interference Term
e
n
ep
n
Jackson et al., PR 106, 517 (1957):
e

pe  p
me
dW    Ee   1  a
b
Ee E
Ee

 p
p
p p
p  e
+ n   A e  B   N e  D e   ...  R e
E
Ee E
Ee
 Ee
Fierz-Interference Term: b  0
• Signal expected for MSSM: b ~ 10-3
(Ramsey-Musolf, 2007)
• Not measured in neutron beta decay, Nab might be able to.
 

 
More observables: Neutrino Asymmetry
e
n
ep
n
Jackson et al., PR 106, 517 (1957):
e

pe  p
me
dW    Ee   1  a
b
Ee E
Ee

 p
p
p p
p  e
+ n   A e  B   N e  D e   ...  R e
E
Ee E
Ee
 Ee
  Re 
2
Neutrino-Asymmetry B  2
1 3 
2
• Signal expected for MSSM at ΔB ~ 10-3 (Ramsey-Musolf, 2007)
• Last measurements: B = 0.9802(50) (PERKEO II, 2007)
B = 0.9801(46) (Serebrov, 1998)
 

 
More observables: R/N correlation
e
n
ep
n
Jackson et al., PR 106, 517 (1957):
e

pe  p
me
dW    Ee   1  a
b
Ee E
Ee

 p
p
p p
p  e
+ n   A e  B   N e  D e   ...  R e
E
Ee E
Ee
 Ee
 

 
2
v
Electron polarization N  1    A
c
• Standard-Model: NSM = 0.07; RSM = 0.0066 ~ 0
• Scalar or Tensor Interactions lead to deviations (Leptoquarks, charged Higgs, Sleptons in
SUSY)
• Of special interest: R, as it is Time-Reversal violating, measures imaginary part of coupling
constants
R/N correlation
σn
pp
e: N gives
up-down asymmetry
Pb-foil
p
e: R gives
forward-backward asymmetry
Pb-foil
Polarized
n beam
Detection of electron polarization through Mott
scattering in Pb foil: The probability of having
a V track is electron spin dependent.
50 cm
pe
Result:
N exp  0.056(11)(5)
 NSM  0.066 
Rexp  0.008(15)(5)
R
SM( FSI )
 0.00066 
scintillator
MWPC
scintillator
K. Bodek (Cracow), Villigen, CAEN, Leuven, Kattowice,
Accepted in PRL, 2009
Summary
• Rich experimental program with the study of neutron decay
correlations
• New physics might be found with precision measurements.
Maybe soon!
• Main problem: Neutron lifetime disagreement
Thank you for your interest !!