Stefan Baeßler

Studies of Neutron Beta Decay

pn

e-

e

du d

du u

How to discover new particles?

Example:• Production of W-Boson• Search for extra (e.g., righthanded) W bosons

High Energy Physics Experiments Low Energy Precision Experiments

Example:• Study of Neutron Beta• Search for abnormal properties of decay products

1. Discovery of Neptune:

Urbain Le Verrier,1811-1877

John Couch Adams,1819-1892

• Theoretical prediction (Le Verrier, Adams, 1845)Idea: Explain distortions in orbit of Uranus

• Discovery (Galle, 1846)

Later: Similar story for Pluto

Uranus

Neptune

Sun

Distortions of Uranus orbits known since decades

Precision measurements in Astronomy

Precision measurements

2. Non-Discovery of 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

Uranus

Neptune

Sun

Vulcan

Perihelion Precession of a planet:For Mercury, perihelion precession angle is 1.5 deg/100 y

Precision measurements

3. Modern Example: Lunar Laser Ranging to search (among other things) for violation of the Equivalence principle:

Neptune

Sun

Vulcan

Sun

Moon

Earth

grav Moon,gF mµ

centr Moon,iF mµ

centr Earth,iF mµ

grav Earth,gF mµ

Lessons:1. Discoveries can be made with precision measurements2. The discovered item might be unexpected3. Even with high precision, a discovery is not guaranteed

e

e

e e e en

e

ee e e

ee

1

+ ...

p p

E E

p

a

A

m

p p pB N

dW E bE

Dp

E E ER

E E

Observables in Neutron Beta Decay

ud1 2

n e

22 1 3FVG E

pn

e-

e

n

e

Neutron lifetime

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

Observables in Neutron beta decay, as a function of generally possible coupling constants (assuming only Lorentz-Invariance)

Beta-Asymmetry

Neutrino-Electron-Correlation

2

2

Re2

1 3A

2

2

1

1 3a

The Standard Model Parameters Vud and λ

νe

n

2

1p

Fermi-Decay:

gV = GF·Vud

Gamow-Teller-Decay:

gA = GF·Vud·λ

p

p

νee- e-

2

1

e- νe

νe νee- e-

Two unknown parameters, gA and gV, need to be determined in 2 experiments

1. Neutron-Lifetime: 1 2 2n V A3g g n 885 s

A = 0

A = 0

A = -1

2

22 0.1

1 3A

A

V

g

g2. Beta-Asymmetry:

n1 cos ,e

vdw A p

c

S = 0, mS = 0

S = 1, mS = 0

S = 1, mS = 1

Decrease of Neutron Counts N with storage time t: N(t) = N(0)exp{-t/τeff}

1/ τeff = 1/τβ+1/τwall losses

Neutron Lifetime Measurements

MAMBO

see K.W. Schelhammer, 10:30 h

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)

Electron Detector (Plastic Scintillator)

Polarized Neutrons

Split Pair Magnet

Decay Electrons

n1 cos ,e

vdw A p

c

p+

n

e-

e

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)

The Beta Asymmetry: PERKEO II

PERKEO II

up down

up down

N NA

N N

cd cs cb

td

ud

td tb

us ubd ' d

s ' s

b ' b

V V V

V V V

V V V

12

ub

2

us

2

ud VVV

Possible Tests of the Standard Model

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

Multiple determinations (nuclear physics, other correlation coefficients) overconstrain problem, enable:

Unitarity: Situation 2004

-1.28-1.27-1.26-1.25

λ = gA/gV

0.965

0.970

0.975

0.980

Vud

τn [PDG2006]A [PERKEO II]

0+→ 0+

ud u

2

u

2

s b1V V V Unitarityof the CKM Matrix

Neutron Measurements needed:

• Neutron lifetime τn

• Beta Asymmetry A(λ)

• Neutrino-Electron-Correlation a(λ)

21 2n ud

2 1 3FVG

2

2

Re2

1 3A

2

2

1

1 3a

; λ = gA/gV

uV

udA F

dFFermi-Transition:

Gamow-Teller-Transition:

g G

g

V

G V

Neutron Measurements needed:

• Neutron lifetime τn

• Beta Asymmetry A(λ)

• Neutrino-Electron-Correlation a(λ)

21 2n ud

2 1 3FVG

2

2

Re2

1 3A

2

2

1

1 3a

; λ = gA/gV

uV

udA F

dFFermi-Transition:

Gamow-Teller-Transition:

g G

g

V

G V

ud u

2

u

2

s b1V V V

Unitarityof the CKM Matrix

τn [PD

G2006]

Vud Nuclear 0+→ 0+ decays

τn [Serebrov 2005]

Unitarity 2008

-1.28-1.27-1.26-1.25= gA/gV

0.965

0.970

0.975

0.980

A [PERKEO II]

To make A not limiting for neutron-based determination: ΔA/A < 0.2% needed.

Neutron lifetime discrepancies have to be sorted out.

Error Analysis Correction UncertaintyPERKEO II

Statistical uncertainty 0.26%

Background 0.1% 0.1%

Neutron beam polarization

0.3 % 0.1%

Spin flip efficiency 0% 0.1%

Magnetic mirror effect 0.11% 0.01%

Edge Effect -0.22% 0.05%

Detector response 0.26% 0.26%

…

Uncertainty Budget PERKEO II, last run

H. Abele, 2006, preliminary

All newer spectrometers use the same principle as PERKEO II

UCN source

Polarizer / Spin flipper

Diamond-coated quartz tube

MWPCPlastic scintillator

Light guide

Superconducting solenoidal magnet (1.0 T)

Decay volumeField Expansion Region

Detector housing

PMT

Neutron absorber

New attempts: UCNA (ultracold neutrons)

Short test run: A0=-0.1138(46)(21)

A. Young (NCSU), A. Saunders (LANL), et al.

A0

-0 .1 5

-0 .1

-0 .0 5

E n erg y (k eV )

Rat

e (1

/50

keV

·s)

00 2 0 0 4 0 0 6 0 0 8 0 0 1 0 0 0

0 .1

0 .2

0 .3

0 .4

0 .5S ig n a l

A 0< P > = -0 .11 3 8 ± 0 .0 0 4 6

B ack g ro u n d

Be coatedmylar foil

Next generation: PERKEO III

detector(plastic scintillator)

decay volume, 150 mT

beam dump

2 m

velocityselector

chopper

B. Maerkisch, D. Dubbers (Heidelberg), H. Abele (Vienna), T. Soldner (ILL) et al.

Advantages:

• very high countrate w/o pulsing

• reduced background through pulsing

• no edge effect

coldneutron beam

detector(plastic scintillator)

New attempts at SNS: abBA / Nab / PANDA

3HePolarizer

Spin FlipperAdiabatic

Proton Beam 60 Hz

Biological Shield

ShutterChoppers Flux

Monitor

Neutron Guide

Spectrometer

MercurySpallationTarget

Collimator

LH2

19.8 MeV

4He

3He+n p+t

20.5 MeV20.1 MeVJπ = 0+

Jπ = 0+

Jπ = 1-,2- 21.2 MeV

Γ = 0.27 MeV

Fast, segmented silicon detector:

S. Wilburn (LANL),

D. Bowman (ORNL) et al.

Determination of the Coupling Constants

νe

n

2

1p

Fermi-Decay:

gV = GF·Vud

Gamow-Teller-Decay:

gA = GF·Vud·λ

p

p

νee- e-

2

1

e- νe

νeνee- e-

Two unknown parameters, gA and gV, need to be determined in 2 experiments

1. Neutron-Lifetime: 1 2 2n V A3g g n 885 s

a = 1

a = 1

a = -1

A

V

g

g2b. Neutrino-Electron-Correlation a:

2

2

1~ 0.1

1 3

a

1 cos ,ee

vdw a p p

c

Determination of λ = gA/gV

PERKEO II, 1997

PERKEO II, 2002

Yerozolimskii, 1997

PERKEO, 1986Liaud, 1997

Stratowa, 1978

Byrne, 2002

PERKEO II, ?

UCNA, 2009

• 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

Analyzing Plane Electrode

Proton Detector

Neutron Decay

Protons

Magnetic field

0 200 400 600

… for a = -0.103 (PDG 2008)

Proton kinetic energy E [eV]

Dec

ay r

ate

w(E

)

Spectrum for a = +0.3

aSPECT (Mainz, Munich, ILL, Virginia)

pn

e-

e ( ) 1 cos ,

ee

vw E a p p

c

response function @ U=375V

Present best experiments: Δa/a = 5%Present status of aSPECT: (Δa/a)stat = 2% per dayFinal aim: 0.3%

Protons @ 15 kV

aCORN

Tulane (F. Wietfeldt), Indiana, NIST, et al.

pn

e-

e ( ) 1 cos ,

ee

vw E a p p

c

epp

pp

e,max eEE E

Magneticfield

a = -0.103:

“pυ up” more likely

Aim: Δa/a ~ 2%, maybe 0.5% after NIST upgrade

Ee (MeV)

p p2 (

MeV

2 /c2

electron and proton phase space

0

0.2

0.4

0.6

0.8

1

1.2

1.4

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

The cosθeν spectrometer Nab @ SNS

p

n

e-

e1 cos e

vdw a

c

e

pp2 [MeV2/c2]

p p2 di

stri

buti

on

0.0 0.5 1.0 1.5

2ep

e

1 cos e

pa p

E

Ee = 550 keV

2pcos 1e p

2pcos 1e p

Kinematics:

• Energy Conservation

• Momentum Conservation2 2 2

e ep 2 cos ep p p p p

e,max eEE E

cos 1e

cos 0e

cos 1e

cut

The cosθeν spectrometer Nab @ SNS

SegmentedSi detectorNeutron beam

decayvolume

TOF region transitionregionacceleration

region

30 kV

0.00 0.02 0.04 0.06 0.08

1/tp2 [1/μs2]

103

Sim

ulat

ed c

ount

rat

e

Ee = 300 keV

104

105

106

107

Ee = 500 keV

Ee = 700 keV

pp2 [MeV2/c2]

p p2 di

stri

buti

on

0.0 0.5 1.0 1.5

2ep

e

1 cos e

pa p

E

Ee = 550 keV

2pcos 1e p

2pcos 1e p

D. Pocanic, S.B. (Virginia),

D. Bowman (ORNL), et al.

• Spectrometer and detector shared with abBA

• Will likely be converted in asymmetric configuration

• Aim: ~0.1%

pp

p pcos ( )

m dzt

p z

More observables: Fierz Interference Term

pn

e-

e

n

e

Fierz-Interference Term:

• Signal expected for MSSM: b ~ 10-3 (Ramsey-Musolf, 2007)

• Not measured in neutron beta decay, Nab might be able to.

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

0b

e

e

e e e en

e

ee e e

ee

1

+ ...

p p

E E

p

b

B N D R

mdW E a

p p p p

E E

E

E E EA

e

e

e e e en

e

ee e e

ee

1

+ ...

p p

E E

p

b

B N D R

mdW E a

p p p p

E E

E

E E EA

More observables: Neutrino Asymmetry

pn

e-

e

n

e

Neutrino-Asymmetry

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

2

2

Re2

1 3B

• 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)

e

e

e e e en

e

ee e e

ee

1

+ ...

p p

E E

p

b

B N D R

mdW E a

p p p p

E E

E

E E EA

More observables: R/N correlation

pn

e-

e

n

e

Electron polarization

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

2

1 ANv

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

e: N gives up-down asymmetry

pep

pp

σn

MWPCscintillator scintillator

Pb-foil

Pb-foil

50

cm

R/N correlation

Polarizedn beam

exp

SM

exp

SM( )

0.056(11)(5)

0.066

0.008(15)(5)

0.00066FSI

N

N

R

R

Detection of electron polarization through Mott scattering in Pb foil: The probability of having a V track is electron spin dependent.

Result:

K. Bodek (Cracow), Villigen, CAEN, Leuven, Kattowice,

Accepted in PRL, 2009

e: R gives forward-backward asymmetry

• 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 !!

Summary