w. m. snow physics department indiana university npss, bar harbor
DESCRIPTION
Neutron Physics. W. M. Snow Physics Department Indiana University NPSS, Bar Harbor. 5 lectures: 1. Physics/Technology of Cold and Ultracold Neutrons 2. Electroweak Standard Model Tests [neutron beta decay] 3. Nuclear physics/QCD [weak interaction between nucleons] - PowerPoint PPT PresentationTRANSCRIPT
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W. M. SnowPhysics DepartmentIndiana UniversityNPSS, Bar Harbor
Neutron Physics
5 lectures:
1. Physics/Technology of Cold and Ultracold Neutrons2. Electroweak Standard Model Tests [neutron beta decay]3. Nuclear physics/QCD [weak interaction between nucleons]4. Physics Beyond the Standard Model [EDM/T violation]5. Other interesting stuff that neutrons can do [NNN interaction, searches for extra dimensions,…]
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SM Tests with Neutron Decay
1. Some facts about the weak interaction2. Connection with Big Bang Theory3. Neutron Decay: description4. Lifetime and T-even correlation coefficients5. Searches for T-odd correlations
Thanks for slides to: K. Bodek (PSI), H. Abele (Heidelberg), Chen-Yu Liu (LANL), Paul Huffman (NC State), Takeyasu Ito (Tennessee/ORNL)
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Neutron -decay
Clean extraction of fundamental parameters at the charged current sector of the electroweak theory.
Combine: Neutron lifetime + -asymmetry + lifetime GF, Vud, gA
Weak decay rate of K,B mesons Unitarity of CKM matrix
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Why is neutron decay interesting forCosmology?
t~1 sec after Big Bang, neutrons and protons are free (no nuclei). Relative number~Boltzmann factor, kept in equilibrium by weak interactions.
Universe expands and cools. Weak reaction rates fall below expansion rate->neutrons start to decay, proton # goes up
t~few minutes, universe cool enough to bind the deuteron->neutrons are safe again
Nuclear reactions quickly guide almost all neutrons into 4He
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BBN Predictionsfor lightElements inEarly Universe(pre-WMAP)
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PredictionsFor 4Hewith differentneutino #Width due toneutronlifetime
Lopez/Turner 01
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Neutron/Nuclear Beta Decay: What is it Good For?
Now gives the best/comparable constraints on certain forms of:
(1) new T-even V,A charged currents (from L-R symmetric, exotic fermion, leptoquark, R-parity-violating SUSY, and composite models(2) New T-odd V,A charged current interactions (from leptoquark models)
Can soon give the best/comparable constraints on:
new T-odd scalar charged current interactions (from extra Higgs, leptoquark, composite, and some SUSY models)
Can soon give the best measurement of Vud
P. Herczeg, Prog. Part. Nucl. Phys 46 (2001).
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The weak interaction: just like EM, except for a few details…
3 “weak photons” [W+, W-, Z0], can change quark type
e- e-
e- e-
one EM photon
e- e-
e-
Z0
e-
u d
u
W+-
d
V(r)=e2/r, m ‘V’(r)≈[e2/r]exp(-Mr) , MZ,W≈ 80-90 GeV
“Empty” space (vacuum) is a weak interaction superconductor |B|
vacuum superconductor
penetration depth
r
weak field
our “vacuum”
1/ MZ,W
r
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The weak interaction violates mirror symmetry and changes quark type
u e
W+-
d
Only the weak interaction breaks mirror symmetry: not understood
weakinteraction = eigenstates
[CKM] quark mass eigenstates
Vud in n decayMatrix must be unitary
r->-r in mirror, but s->+s
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The Quark Mixing CKM MatrixThe Quark Mixing CKM Matrix
Parametrization: 3 angles, and a phase
A, , are real
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The Quark Mixing CKM Matrix
bsd
Ubsd
CKM
VVVVVVVVV
tbtstd
cbcscd
ubusud
U CKMdWVud
e |Vud|2 + |Vus|2 + |Vub|2 = 1-
Vud from
•Nuclear beta decay Vud=0.9740(5), 2.3 sigma•Pi beta decay Vud=0.9717(56)•Neutron beta decay
Vus from
•Hyperon decays•K decays
ud
d
u ud
u u
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)1()1(8gT 522
2
5
2
fi ew
wduud mk
mkkg
V
Matrix element for d-u transition:
lhud
eduud
JJV
V
2G
)1()1(2
GT
F
55F
fi
nPp
TAp
nsp
MVp
kkigkm
kgkgiA
kkigkm
kgkgiV
])(2
)()([
])(2
)()([
52
5
2
52
22
2
vector- and axial vector currents:
V
Aud
F
enp
npp
Vint
ggaAVVG
km
GL
).)((22
1
)1()2
)1((22
155
v
Lagrange function for neutron decay:
Formalism
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CKM Unitarity/Standard Model Tests
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Vud from Neutron and Nuclear beta decay
=GA/GV
Perkeo result:A0 = -0.1189(7) = -1.2739(19)
n = (885.7 0.7) sworld average
n = (878.5 0.7st 0.3syst) s“Gravitrap” result
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Withc: Coulomb (isospin)
correctionR: nucleus-dependent
radiative correctionR: nucleus-independent
radiative correction
Superallowed -transitions
Ft0+0+=3072.3(9)s Vud=0.9740(5) Towner, Hardy 4 Sept 2002 PDG:Vud=0.9740(10
)
)1(2
)1)(1(
2200
00
RudF
Rc
VGkFt
ft
2.5 sigma deviation from unitarity !!Nucl-th/0209014
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Pion -decay
x
ex ex
Br = 1.025(34) .10-8
=2.6033(50) .10-6s
)1()1(2)νeππ()2ln/(
21
02
RRF
eud fffG
BrKV
Vus=0.9670±0.0160Br ± 0.0009=0.967 ± 0.016
CKM Workshop, HD, September 2002:Br ~ 1.044 ± 0.007syst± 0.009systx 10-8 PIBETA : Vud = 0.9771(51) (Pocanic, Ritt)
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Vus
Kaon semileptonic decays– K+0l+l
– K0L-l+l sul+l
= (2.12±0.08%), = -2.0% for K+ and 0.5% for K0
)1)(1()0(π192
21
253
2
RkusF IfCm
VG
Vus = 0.2196 ± 0.0017exp ± 0.0018th = 0.2196 ± 0.0026 (PDG 2002)
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70000 events
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Vud from neutron -decay
)1()31( 221R
Rud fVC
)8(0240.0),15(71335.1
,101613.1)2/( 14322
RR
eF
f
smGC
)13(9717.0
)19(2739.1),7(7.885
udV
Wilkinson 1982, CKM Workshop September 2002:Marciano et Sirlin
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Radiative CorrectionR
R = /(2)[4ln(mz/mp) + ln(mp/mA) + 2Cborn] + ...
R = (2.12 - 0.03 + 0.20 + 0.1)%= 2.40(9)%
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Neutron -decay lifetime
Cold Neutron beam experiments:– Absolute measurements of the neutron number and the decay
particle flux. Bottled UCN:
– Ratio of the neutrons stored for different periods. It is a relative measurement.
– Material bottle -- Mampe (887.6 3 s)• Wall loss depends strongly on the UCN spectrum.• Systematically limited.
– Magnetic bottle -- hexapole bottle (876.7 10 s), NIST bottle.• Statistically limited.
N0 /Nd
N(T) N0e T / T
ln(N0 /N(T))
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The best results for neutron lifetime
N beam 889.2±4.8 (Sussex-ILL,
1995) 886.8±1.2±3.2 (NIST,
2004)
Particle data (2003 without PNPI-ILL,2003 & NIST,2004):
n = (885.70.8) s
UCN storage 878.5±0.7± 0.3 (PNPI-
ILL,2004)
885.4±0.9±0.4 (KI-ILL, 1997) 882.6±2.7 (KI-ILL, 1997) 888.4±3.1±1.1 (PNPI, 1992) 887.6±3.0 (ILL, 1989)
`
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In-Beam Neutron Lifetime Experiment
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In-Beam Lifetime Apparatus
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Neutron Lifetime Using UCN Magnetic Trap in Superfluid 4He
Goal: 0.1 second precision, 1 order of magnitude improvement
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Neutron lifetime usingmagnetictrapping
Huffmann et al., Nature
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Expression for Neutron Decay Correlation Coefficients
)](1[
)( 20
ee
ee
e
e
ee
en
e
e
e
e
eeeeeee
EpR
EEppD
EpB
EpA
Emb
EEppa
dddEEEEpddWdE
11% -11% 97% SM: 0
correlation asymmet
rytriple
correlationasymmetry
Triplecorrelation
SM: 0
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Neutron decay A Coefficient Neutron spin – electron momentum
angular correlation Sensitive to GA/GV= Important input for determining CKM
element Vud from neutron
Rn
VRF
VRF
Vud
fK
G
GGV
)31()1(1
)1(
22
2
22
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T-even Angular correlations for Polarized neutrons
Electron
Proton
Neutrino
Neutron SpinA
B
C Observables in neutron decay:
Lifetime SpinMomenta of decay particles
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A Correlation
Coefficient A and lifetime determine Vud and
Electron
Neutron SpinA
Electron Neutron SpinA
W()={1+v/cPAcos()}
231)1(
2
A
NNNNAexp
on flipper spin with spectrum electron
off flipper spin with spectrum electron
:N:N
)31(sec44908V 2
2ud
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Principle of A-coefficient Measurement
B fieldDetector 1 Detector 2
Polarized neutron Decay electron
cos)()()()()(
21
21exp AP
ENENENENEA
(End point energy = 782 keV)
n
e
dW=[1+PAcos]d(E)
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Decay Asymmetry Apparatus PERKEO (Abele et al)
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UCNA Experiment: Beta asymmetry
Goal: measure A to 0.2% or better with UCN.
R R0(1 v /c)PA (E)cos
-asymmetry = A(E) in angular distribution of decaying e-
from polarized neutrons
A 2 ( 1)1 32 0.11620.0013
gAgV
1.26700.0035
T. J. Bowles, A. R. Young, et al.
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Neutron Polarization Using UCN Can obtain > 99.9% polarization using •B potential
wall for wrong spin UCN
A number of methods to measure “depolarization” — only modest accuracy is needed when the polarization is high
Polarization goal: >99.9%
B
n
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Experiment DesignNeutron Absorber
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Angular correlations in neutron decay with T Violation
Angular distribution with explicit dependence on electron spin contains 4 T-odd observables (lowest order):
D : T-odd P-evenR : T-odd P-oddV : T-odd P-oddL : T-odd P-evenN : T-even P-even
e
pep
Pp
Jn
nepeenepneenνe
eeeeeee
JσPσpJσPJσpJpp NEE
LE
VE
REE
D
dddEEEEpdddEW
eeee1
)( 20
T-invariance +neglect of FSE D, R, V, L = 0
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Angular correlations in neutron decay with T Violation
D and R are sensitive to distinct aspects of T-violation:
2'2'2222'2'222
*''**''*
*''*2
'*''*'**
Im21
Im211
Im21
ATATGTVSVSF
TVTVASASGTF
AATGT
AVTSAVTSGTF
CCCCMCCCCM
RCCCCCCCCI
IMM
CCCCI
MR
DCCCCCCCCI
IMMD
T
FSI
FSI
D is primarily sensitive to the relative phase between V and A couplings.R is sensitive to the linear combination of imaginary parts of scalar and tensor couplings.
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The R-correlation for neutron decay
Transverse electron polarization component contained in the plane perpendicular to the parent polarization.
Not measured for the decay of free neutron yet !
A
TT
A
SS
CCCT
CCCS
''Im;Im
TSR 33.028.0
1,,,,26.1Re,1Re,3,1
'''
'
TTSSAAA
VVVGTF
CCCCCCCCCCMM
22
'*'**
32Im
AV
SSATTAV
CCCCCCCCCR
One obtains finally:
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T-violation in n -decay may arise from:– semileptonic interaction (due-e)– nonleptonic interactions
SM-contributions for D- and R-correlations:– Mixing phase CKM gives contribution which is 2nd order in weak
interactions: < 10-10
-term contributes through induced NN PVTV interactions:< 10-9
Candidate models for scalar contributions (at tree-level) are:– Charged Higgs exchange– Slepton exchange (R-parity violating super symmetric models)– Leptoquark exchange
The only candidate model for tree-level tensor contribution (in renormalizable gauge theories) is:– Spin-zero leptoquark exchange.
Where could it come from?
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R-correlation measurements in Nuclei
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1st order “Final State Effect” contribution
In the SM:
A = -0.1173(13)
= 0 :
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Anticipated accuracy: R (neutron) 510-3
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Needed:– Intense source of highly polarized, free neutrons– Efficient polarimetry for low energy electrons (200-800 keV)
Best combination:– Polarized cold neutron beam: Ndecay 2 cm-3s-1
– SMott 0.4 0.5
Experiment
Difficulties:o Weak decay source in presence of high background due to
neutron capture.o Depolarization of electrons due to multiple Coulomb scattering in
detectors and Mott target.
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Principle of measurement Tracking of electrons in
low-mass, low-Z MWPCs
Identification of Mott- scattering vertex.
R-correlation: asymmetry for events in the plane parallel ( = 0) to the neutron polarization. Frequent neutron spin
flipping. „Foil-in” and „foil-out”
measurements.
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Current Situation in Neutron Decay
Lots of experimental activity to measure Vud in n decay, Vus in K decay with higher accuracy to test CKMunitarity
Many correlation coefficients are accessible experimentally,can be used to search for beyond SM physics
New experimental techniques/sources are available