1. Studies of Neutron Beta Decay
Stefan Baeßler p n e- d u

2. How to discover new particles?
High Energy Physics Experiments
Low Energy Precision Experiments
Example:
Production of W-Boson
Search for extra (e.g., righthanded) W bosons
Example:
Study of Neutron Beta
Search for abnormal properties of decay products

3. Precision measurements in Astronomy
Discovery of Neptune:
Urbain Le Verrier,
John Couch Adams,
Uranus
Neptune
Sun
Theoretical prediction (Le Verrier, Adams, 1845)
Idea: Explain distortions in orbit of Uranus
Discovery (Galle, 1846)
Later: Similar story for Pluto
Distortions of Uranus orbits known since decades

4. Precision measurements
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

5. Precision measurements
Modern Example: Lunar Laser Ranging to search (among other things) for violation of the Equivalence principle:
Sun
Earth
Vulcan
Moon
Neptune
Lessons:
Discoveries can be made with precision measurements
The discovered item might be unexpected
Even with high precision, a discovery is not guaranteed
Sun

6. Observables in Neutron Beta Decay
Jackson et al., PR 106, 517 (1957):
Observables in Neutron beta decay, as a function of generally possible coupling constants (assuming only Lorentz-Invariance) p n e-
Beta-Asymmetry
Neutrino-Electron-Correlation
Neutron lifetime

7. The Standard Model Parameters Vud and λ
Fermi-Decay:
gV = GF·Vud p e- νe
A = 0
A = -1
2. Beta-Asymmetry: νe e-
S = 0, mS = 0
S = 1, mS = 0
S = 1, mS = 1 n p e- νe νe e-
Gamow-Teller-Decay:
gA = GF·Vud·λ e- νe p
Two unknown parameters, gA and gV, need to be determined in 2 experiments
1. Neutron-Lifetime:

8. 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
MAMBO
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

9. The Beta Asymmetry: PERKEO II
Electron Detector (Plastic Scintillator) p+ n e-
Decay Electrons
Polarized Neutrons
Split Pair Magnet
PERKEO II
Magnetic Field
Beam time
Result
Publication
1995
A = (12)
Phys. Lett. B 407, 212 (1997)
1997
A = (7)
Phys. Rev. Lett. 88, (2002)
2004
A = (5) (preliminary)

10. Possible Tests of the Standard Model
Multiple determinations (nuclear physics, other correlation coefficients) overconstrain problem, enable:
Search for Right-handed Currents
WR?
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 Cabbibo-Kobayashi-Maskawa-Matrix

11. Unitarity: Situation 2004 Neutron Measurements needed:. 9 8
τn [PDG2006]
A [PERKEO II]
Unitarity
of the CKM Matrix . 9 7 5
Neutron Measurements needed:
Neutron lifetime τn
Beta Asymmetry A(λ)
Neutrino-Electron-Correlation a(λ) d V u
0+→ 0+ . 9 7
; λ = gA/gV . 9 6 5 - 1 . 2 5 - 1 . 2 6 - 1 . 2 7 - 1 . 2 8
λ = gA/gV 12

12. Unitarity 2008 Neutron Measurements needed: Neutron lifetime τn
Beta Asymmetry A(λ)
Neutrino-Electron-Correlation a(λ)
; λ = gA/gV . 9 8
Unitarity
of the CKM Matrix . 9 7 5
τn [Serebrov 2005]
Vud
Nuclear 0+→ 0+ decays . 9 7
τn [PDG2006] . 9 6 5 A [ P E R K E O I I ] - 1 . 2 5 - 1 . 2 6 - 1 . 2 7 - 1 . 2 8 l = g / g A V
Neutron lifetime discrepancies have to be sorted out.
To make A not limiting for neutron-based determination: ΔA/A < 0.2% needed.

13. Uncertainty Budget PERKEO II, last run
Error Analysis
Correction
Uncertainty
PERKEO II
Statistical uncertainty
0.26%
Background
0.1%
Neutron beam polarization
0.3 %
Spin flip efficiency 0%
Magnetic mirror effect
0.11%
0.01%
Edge Effect
-0.22%
0.05%
Detector response …
H. Abele, 2006, preliminary
All newer spectrometers use the same principle as PERKEO II

14. New attempts: UCNA (ultracold neutrons)
Superconducting solenoidal
magnet (1.0 T)
Be coated
mylar foil
MWPC
Detector housing
Plastic scintillator
PMT
Decay volume
Polarizer / Spin flipper
Light guide
Field Expansion Region
Neutron absorber
Diamond-coated
quartz tube
UCN source
Short test run: A0= (46)(21)
A. Young (NCSU), A. Saunders (LANL), et al.

15. 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
velocity
selector
chopper
cold
beam dump
neutron beam
2 m
B. Maerkisch, D. Dubbers (Heidelberg), H. Abele (Vienna), T. Soldner (ILL) et al.

16. New attempts at SNS: abBA / Nab / PANDA
Proton Beam 60 Hz
Spectrometer
Shutter
Choppers
Flux
Adiabatic
LH2
Monitor
Spin Flipper
Neutron Guide
Collimator
3He
Mercury
Polarizer
Biological Shield
Spallation
Target
Fast, segmented silicon detector:
Jπ = 1-,2-
21.2 MeV
20.5 MeV
19.8 MeV
Jπ = 0+
20.1 MeV
3He+n
Γ = 0.27 MeV
p+t
S. Wilburn (LANL),
D. Bowman (ORNL) et al.
Jπ = 0+
4He

17. Determination of the Coupling Constants
Fermi-Decay:
gV = GF·Vud p e- νe
a = 1
a = -1
2b. Neutrino-Electron-Correlation a: νe e- n p e- νe νe e-
Gamow-Teller-Decay:
gA = GF·Vud·λ e- νe p
Two unknown parameters, gA and gV, need to be determined in 2 experiments
1. Neutron-Lifetime:

18. Determination of λ = gA/gV
Stratowa, 1978
Yerozolimskii, 1997
UCNA, 2009
PERKEO, 1986
Liaud, 1997
Byrne, 2002
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

19. aSPECT (Mainz, Munich, ILL, Virginia)
Proton Detector
Magnetic
field
Decay rate w(E)
response U=375V
Spectrum for a = +0.3
… for a = (PDG 2008)
200
400
600
Analyzing Plane
Electrode
Proton kinetic energy E [eV]
Protons
15 kV
Neutron Decay
Present best experiments: Δa/a = 5%
Present status of aSPECT: (Δa/a)stat = 2% per day
Final aim: 0.3%

20. aCORN e- p n Tulane (F. Wietfeldt), Indiana, NIST, et al. a = -0.103:
Magnetic
field
Transverse proton- and electron momentum restricted by collimator
Neutrino-direction inferred from proton-TOF
Asymmetry I/II ,measured
a = :
“pυ up” more likely
Aim: Δa/a ~ 2%, maybe 0.5% after NIST upgrade
Tulane (F. Wietfeldt), Indiana, NIST, et al. 21

21. The cosθeν spectrometer Nab @ SNS
Kinematics:
Energy Conservation
Momentum Conservation n
electron and proton phase space
cut
1.4 2 c /
1.2
pp2 [MeV2/c2]
pp2 distribution
0.0
0.5
1.0
1.5
Ee = 550 keV 2 V e 1 M (
0.8 2 p p
0.6
0.4
0.2
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8 E
(MeV) e

22. The cosθeν spectrometer Nab @ SNS. 2 4 6 8
1/tp2 [1/μs2]
103
Simulated count rate
Ee = 300 keV
104
105
106
107
Ee = 500 keV
Ee = 700 keV
pp2 [MeV2/c2]
pp2 distribution
0.0
0.5
1.0
1.5
Ee = 550 keV
Segmented
Si detector
Neutron beam
decay
volume
TOF region
transition
region
acceleration
30 kV
Spectrometer and detector shared with abBA
Will likely be converted in asymmetric configuration
Aim: ~0.1%
D. Pocanic, S.B. (Virginia),
D. Bowman (ORNL), et al.

23. More observables: Fierz Interference Termp n e-
Jackson et al., PR 106, 517 (1957):
Fierz-Interference Term:
Signal expected for MSSM: b ~ (Ramsey-Musolf, 2007)
Not measured in neutron beta decay, Nab might be able to.

24. More observables: Neutrino Asymmetryp n e-
Jackson et al., PR 106, 517 (1957):
Neutrino-Asymmetry
Signal expected for MSSM at ΔB ~ (Ramsey-Musolf, 2007)
Last measurements: B = (50) (PERKEO II, 2007)
B = (46) (Serebrov, 1998)

25. More observables: R/N correlationp n e-
Jackson et al., PR 106, 517 (1957):
Electron polarization
Standard-Model: NSM = 0.07; RSM = ~ 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 27

26. R/N correlation σn pp
e: N gives
up-down asymmetry
50 cm
Pb-foil pe p
e: R gives
forward-backward asymmetry
Polarized
n beam
Pb-foil
Detection of electron polarization through Mott scattering in Pb foil: The probability of having a V track is electron spin dependent.
Result:
Bodek
scintillator
MWPC
scintillator
K. Bodek (Cracow), Villigen, CAEN, Leuven, Kattowice,
Accepted in PRL, 2009

27. Thank you for your interest !!
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 !!
Größer