W. M. Snow Physics Department Indiana University/IUCF PAVI06
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Transcript of W. M. Snow Physics Department Indiana University/IUCF PAVI06
W. M. SnowPhysics Department
Indiana University/IUCFPAVI06
Parity Violation in Neutron Spin Rotation Experiments
What is the weak NN interaction? Why measure it?
What is the phenomenon of PV spin rotation?
Example: spin rotation experiment in n+4He
Other possible spin rotation measurements (n+p, n+D)
The Weak NN Interaction: What is it?
|N>=|qqq>+|qqqqq>+…=valence+sea quarks+gluons+… interacts through strong NN force, mediated by mesons |m>=|qq>+…Interactions have long (~1 fm) range, QCD conserves parity
outside=QCD vacuum
weak
If the quarks are close, the weak interaction can act, which violates parity. Relative weak/strong amplitudes: ~[e2/m2
W]/[g2/m2]~10-6
Quark-quark weak interaction induces NN weak interactionVisible using parity violation
q-q weak interaction: an “inside-out” probe of strong QCD
~1 fm
~1/100 fm range
SM Structure of Low-Energy qq Weak Interaction
At low energies Hweak takes a current-current form with charged and neutral weak currents
Hweak~GF(Jc Jc
+ Jn Jn),where
Jc ~u(1+ 5)d´ ; Jn
~u(1+ 5)u-d(1+ 5)d-s(1+ 5)s
-4sin2w JEM
Isospin structure: ∆I = 0, 1, 2
Hweak ∆I = 2~ Jc
I = 1 Jc
I = 1 , charged currents
Hweak ∆I = 1~ Jc
I = 1/2 Jc
I = 1/2+ Jn I = 0
Jn I = 1
neutral current will dominate the ∆I = 1 channel [unless strange sea quarks contribute].
Hweak ∆I = 0~ Jc
I = 0 Jc
I = 0+ Jn I = 0
Jn I = 0 + Jn
I = 1 Jn
I = 1, both charged
and neutral currents Hweak is known,can be used to probe QCD
Weak qq-> Weak NN: what can we learn?
s=1 nonleptonic weak interactions [I=1/2 rule, hyperon decays not understood]
Question: is this problem specific to the strange quark, or is it a general feature in thenonleptonic weak interactions of light quarks?
To answer, we must look at s=0 nonleptonic weak interactions (u,d quarks)
Any such process is dominated by strong interaction->must measure ~1E-7 PV effects
Weak NN interaction is one of the few experimentally feasible systems
Recent Development: perturbation theory for weak NN interaction [Zhu, Holstein, Ramsey-Musolf, et
al,Nucl. Phys. A748 (2005) incorporates chiral symmetry of QCD, Ramsey-Musolf and Page review(hep-ph/0601127): relates coefficients in perturbation theory to experimental weak NN observables in NN and few body systems ]
Longer-term goal: calculate coefficients of weak NN operators from QCD on lattice [Bean & Savage]
NN PT coefficients and observables (Musolf/Page)
Column gives relation between PV observable and weak couplings (A=-0.11mt )
assumes calculations of PV in few body systems are reliable(some need to be done [nD ] or redone/)
EFT coupling (partial wave mixing)
np A
np P nD A n
np
pp Az p Az
mt (3S1- 3P1) -0.11
0 -3.56
-2.68
0 -1.07
mt (3S1- 1P1) 0 0.63 -1.39
1.34 0 -0.54
ms0 (1S0- 3P0) 0 -
0.16-0.95
1.8 4k/mN -0.72
ms1 (1S0- 3P0) 0 0 -
0.24-1.2 4k/mN -
0.48
ms2 (1S0- 3P0) 0 0.32 1.18 0 4k/mN 0
experiment
(10-7 )0.6±2.1
1.8±1.8
42±38
8±14
-0.93/-1.5±0.21
-3.3±0.9
Weak NN: Relations to Other Areas
PV in atomic physics: nuclear anapole moment [seen in 133Cs, searched in Tl, plans in Fr, Yb,…]
PV in p-shell and light s-d shell nuclei with parity doublets [lots of good measurements (14N, 18F, 19F, 21Ne), should be calculable given weak NN]
PV in heavy nuclei [TRIPLE measurements +theoretical analysis with chaotic nuclear wavefunctions gives prediction for effective weak couplings in heavy nuclei]
PV in electron scattering [use Z exchange to extract s portion of proton EM form factors, but need to avoid contamination from q-q weak]
Neutrinoless double beta decay [test of EFT treatment for matrix elements of 4-quark operators similar to weak NN]
Weak NN: What can be learned with Low Energy (meV) Neutrons?
Low energy neutrons: s-wave and p-wave elastic scattering, (n,)
For elastic scattering, Az->0 as k ->0
Hard to flip spin quickly for large polarized targets
MeV gamma polarimeters are inefficient
Easy to flip neutron spin
-> 2 classes of experiments: PV spin rotation [~Re(f)] and reactions with inelastic channels [gamma capture]
Possible experiments: PV spin rotation in n-p and n-4He (and just maybe n-D?), PV gamma asymmetry in n-p and n-D (next talk D. Bowman)
“Slow” Neutrons: MeV to neV
235U
n 2MeV
ν β
γ
γβ
ν
235Un 0.1eV
235Un
n
10-8 10-7 10-6 10-5 10-4 10-3 10-2 10-1 10010-6
10-5
10-4
10-3
10-2
10-1
100
101
102
103
W(En)
En [eV]
T30K T293K
UCN Very cold Cold ThermEpitherm
30K
300K
Nuclear reactor
NIST Cold Neutron
Guide Hall
Reactor makesneutrons, cooled to ~20Kby liquid hydrogen
Neutron mirrors (“guides”)conduct the neutrons ~100meters with small losses.
Neutron Optical Potential
Phase shift (n-1)kLIndex of refraction n=|K|/|k|=√[(Ek-<V>)/Ek]
|k> ei|k>
matter
s-wavefscatt=b
L
|K=nk>
For Ek-<V> negative, neutron reflects
from the optical potential
c
c=√[b/] critical angle
<V>=neutron opticalpotential~spatially-averagedversion of nN potential
Parity Violation in Neutron Spin Rotation
transversely polarized neutrons corkscrew due to weak NN interaction [opposite helicity components of |>y=1/√2(| >z+ |>z) accumulate different phases from sn·kn term in forward scattering amplitude] PV rotation angle per unit length d/dx related to parity-odd amplitude
[=(n-1)kx, n-1=2f/k2, fweak=gk ->d/dx~g] d/dz ~1x10-6 rad/m expected on dimensional grounds
y
z
HelicityComponents
Transverse Polarization
OpticalRotation
φ
Medium withParity Violation
Spin rotation in neutron opticsn
)()0( kfff PVPC
rr⋅+= σ
)))((2
1(2
kffk
kl PVPC
rr⋅++= σφ
PVPC φφφ ±=±
( )zzy −+=2
1
( )zeze PVPCPVPC ii −+ −−+− )()(
2
1 φφφφ
( )2
12 PCPC f
kkl
πρφ += PVPV flφ 2=
Incoming neutron with spin along :
Coherent forward scattering amplitude for low-energy neutrons:
Neutron optical phase shift:
is different for opposite helicity states
krr
⋅σ
PV phase shift is opposite for |+z> and |-z>
The PV rotation of the angle of transverse spin is the accumulated phase difference:
l
medium
σr
kr neutron spin
neutron wave vectorhelicity
PVPVPV flφφ 42 ==−= −+
y
N-4He Spin Rotation Collaboration
C.D. Bass1, B.E. Crawford2, J.M. Dawkins1, K. Gan6, B.R. Heckel5, J.Horton1, C. Huffer1, P.R. Huffman4 ,D. Luo1, D.M. Markoff4, A.M. Micherdzinska1, H.P. Mumm3, J.S. Nico3,A.K. Opper6,
E.Sharapov7 ,M.G. Sarsour1, W.M. Snow1, H.E. Swanson5, V. Zhumabekova8
Indiana University / IUCF2
Gettysburg College2
National Institute of Standards and Technology (NIST)3
North Carolina Central University / TUNL4
University of Washington5
George Washington University6
Joint Institute for Nuclear Research, Dubna, Russia7
Al-Farabi Khazakstan National University8
NSF PHY-0457219
n
Parity Violating d/dz: of order 1E-6 rad/meter
d/dz of low energy neutron due to Larmour precession in external B field of the Earth: ~1 rad/meter!
design of experiment is completely dominated by need to reduce systematic effects from magnetic fields
The Background: Magnetic Fields
extBvr
⋅ns
nsr
nsr PVPC φφ +
nn psrr
⋅
l
x
znpr
polarizer analyzer
n
y
Conceptual Design of Spin Rotation Apparatus
Neutron Spin Rotation apparatus (top view)
n
Neutron Spectrum and Flux n
Neutron flux: ~1E+9 neutrons/cm2 sec over 5 cm x 5 cm guide
How can neutrons be polarized?
B
B gradients (Stern-Gerlach,sextupole magnets)electromagneticF=()B
Reflection from magneticmirror: electromagnetic+strongf=a(strong) +/- a(EM) with | a(strong)|=| a(EM)|f+=2a, f-=0
B
Transmission throughpolarized nuclei: strongσ≠ σ- T ≠ TSpin Filter:T=exp[-σL]
L
Neutrons are polarized through spin-dependent scattering from magnetized mirrors
Polarization: ~98% transmission: ~25%
28 cm
White NeutronBeam
Magnet Box
Plate CurvatureRadius ~ 10m
polarized NeutronBeam
“Supermirror” Neutron Polarizer
n
Permanent magnet box
B
Target/Cryostat Magnetic Shieldingn
Suppressing the magnetic field
~2nT field in the target region still not good enough (causes rotation ~100 times bigger than pv), and still need to oscillate PV signal ->Target Design
n
Nonmagnetic Helium Cryostatn
Target design: Oscillation of PV Signal
3He n-detector
Analyzer
Target Chamber – back position
– Coil
Target Chamber – front position
PolarizerCold Neutron Beam
•
•F+ PV
-F - PV
B-F) - PV
mag - PV
A B
B – A = 2PV•
z
yx
n
B-F)+ PV
mag + PV
Cold Neutron Beam
•
•F
-F
Target design: Noise Suppression
3He n-detector
Analyzer
Target Chamber – back position
– Coil
Target Chamber – front position
Polarizer
Cold Neutron Beam
•
•
A B
n
B – A = 2PV
Output Coil and Polarization Analyzer
↑
+ x
BB
x
y
z
Output coil adiabatically rotates neutron spin by +/- /2, conserving component of neutron spin along B, flipped at 1 Hz
=sinN+-N-
N++N-
Segmented 3He ionization chamber
3He and Ar gas mixture Neutrons detected through n+3He 3H+1H High voltage and grounded charge-
collecting plates produce a current proportional to the neutron flux
4 Detection Regions along beam axis - velocity separation (1/v absorption)
S.D.Penn et al. [NIM A457 332-37 (2001)]
HV platewindow
signal plates
half voltage rings
full voltage plates
n
charge collection plates are divided into 4 quadrants (3" diam) separated L/R and U/D beam
Systematic Effects in PV Spin Rotation
Associated with residual longitudinal B fields (~ 10 nT )
effect estimated size (B~10 nT)
– He diamagnetism (BHe ≠ Bvac) ~10-8 rad/m– He optical potential (VHe ≠ Vvac) ~10-8 rad/m
– small angle/inelastic– scattering ~10-8 rad/m– Internal fields in pi-coil
PV spin rotation independent of neutron energy, B rotation depends on neutron energy
At NIST we will amplify systematic effects from target scattering by increasing B to~1 T in separate measurements.
∫∫⋅
≠⋅
21 vv2211 dBdB l
vvlvv
The Spallation Neutron Source at ORNL
$1.4B--1GeV protons at 2MW, ready in ~2007 (first neutrons recently!
Short (~1 usec) proton pulse– mainly for high TOF resolution Will be brightest spallation neutron source (also JSNS@JPARC)
SNS Fundamental Neutron Physics Beam
30-50 mballisticguide
Summary and plans for PV n spin rotation
1. Apparatus at NIST now, beam/apparatus tests
2. Previous attempt in 1996 ( PV(n,) = ( 8.0 ±14 (stat) ± 2.2 (syst) ) 10-7
rad/m, D. Markoff thesis)
3. NIST goal: PV=310-7 rad/m (3 months) + ~1 month of systematic study. This sensitivity will improve our knowledge of NN weak interaction
4. Letter of Intent approved to perform PV neutron spin rotation in LHe at SNS (sensitivity goal PV=110-7 rad/m in 12 months)
5. n-p spin rotation possible at SNS in a ~20 cm liquid parahydrogen target
6. n-D spin rotation possible at ILL/FRM in a few cm liquid orthodeuterium target
n
n-4He spin rotation in termsof weak couplings (10-6rad/m, Dmitriev)
n-4He orthogonal to 133Cs, p-4He
n+p->D+ asymmetry determines f fDesplanques) plot: =3E-7 rad/m, =5E-9
Status, Near-Term Goals for ∆I = 0, 1 Weak NN
Neutron Spin Rotation apparatus- Filters
n
1) Small wavelength cut-off filer: Be-Bi
Polycrystalline Beryllium => when neutron =< 2dmax (dmax – maximum interplanar spacing in the Be), neutrons will be scattered out of beam by diffraction. Neutrons with 2dmax do not diffract, will pass through. Bismuth crystal – stops gammas. Both filters needs to be cooled to LN temperature. (We have 4” of Be and 4” of Bi)
1) Long wavelength cut-off filter: stack of Ni/Ti supermirrors deposited on~100 Si wafers. c = 3* c(Ni); c(Ni) = 21 mrad/nm and oriented at a small angle
Reflected beam
Transmitted beam ()
Incident beam
(2< < 2 Absorber ()
Supermirrors
Flight of the neutron
Supermirror polarizer passes neutrons with vertical “up” spin
Typical polarization 98%
SM FRONT TARGET BACK TARGET
PI-COIL
MAGNETIC SHIELDING
FLIPPING COILSM
L
R
Top view:
Side view:
L R
Flight of the neutron
Both neutrons experience Larmour precession about residual B-fields Right neutron experiences additional rotation due to weak
interaction with LHe
SM FRONT TARGET BACK TARGET
PI-COIL
MAGNETIC SHIELDING
FLIPPING COILSM
L
R
Top view:
Side View
y
x
R
L
Flight of the neutron
-Coil Larmour precesses the neutron spin 180-degrees about the vertical axis
SM FRONT TARGET BACK TARGET
PI-COIL
MAGNETIC SHIELDING
FLIPPING COILSM
R
L
Top view:
Side view:
y
x
LR
Flight of the neutron
Both neutrons experience Larmour precession about residual B-fields
Left neutron experiences additional rotation due to weak interaction with LHe
SM FRONT TARGET BACK TARGET
PI-COIL
MAGNETIC SHIELDING
FLIPPING COILSM
R
L
Top view:
Side view:
y
x
R L
Flight of the neutron
1. Transverse component of spin rotated into a horizontal B-field2. Spins then rotated into a vertical direction (same as ASM)
(the direction of rotation into vertical reverses at 1Hz rate)
SM FRONT TARGET BACK TARGET
PI-COIL
MAGNETIC SHIELDING
FLIPPING COILSM
R
L
Top view:
Side view:
y
x
or:
Flight of the neutron
Neutron spins are either parallel or antiparallel to the ASM
Parallel spins pass through ASM and enter 3He Ion Chamber detector
Asymmetry of count rate for flipping coil states & target states yields spin rotation
SM FRONT TARGET BACK TARGET
PI-COIL
MAGNETIC SHIELDING
FLIPPING COILSM
−+
−+
+−
=⋅NN
NNAP ϕsin
R
L
Top view:
Eight cold neutron guides, two for fundamental physics (NG-6, NG-7)
NIST Center for Neutron Research (NCNR)
n
Thermal neutrons
Reactor 20 MW (fission neutrons)
Moderation D2O
Cold neutrons
Moderation LH
E<5 meV; T=20K wavelength ( ~ few A
Cold neutrons are conducted by neutron mirrors (guides) over 80 – 100m with small losses to experimental areas.
Signal Modulation via Target Motion Plus Magnetic Field Precession (Top View)
3He n-detectors
Analyzer
Target ChamberBack Position
Target ChamberFront Position
- Coil
Polarizer
Cold Neutron Beam
PNC
−ϕPNC ϕPNC
ϕBKG + ϕPNCϕBKG − ϕPNC
•
•
3He n-detectors
Analyzer
Target ChamberBack Position
Target ChamberFront Position
π- Coil
Polarizer
Cold Neutron Beam
ϕPNC
−ϕPNC ϕPNC
ϕBKG + ϕPNCϕBKG − ϕPNC
•
•
Target is split into 2sections A,B along neutron beam with “ coil” between
Front full: +PNC in A, + -> - in coil, B empty
Back full: 0 in A, 0 incoil, +PNC in B
Difference=2 PNC
Theory needed at nucleon and quark levels
Nucleon level: need new calculations of relationship between NN P-odd observables and NN weak couplings, especially for n+D->3H+g asymmetry, n+4He spin rotation, n+D spin rotation
This should be doable: add VPNC as perturbation to strong calculation
in potential models, EFT approaches,…
Quark level: need to calculate weak couplings in QCD models, use measured couplings to discriminate, or lattice+chiral extrapolation
Models will need to treat q-q correlations in nucleon correctly in strong regime to get the physics right
Cold Neutron Guide Hall at NCNR
n
n
Scientific Impact of Measurement of Weak NN Couplings
Parts of the weak NN interaction needed for nuclear and atomic systems will essentially be determined (hr2 missed)
New probe of nuclear structure using existing PV measurements in medium and heavy nuclei
Resolve present inconsistencies in data from previous experiments
Test internal consistency of meson exchange model and/or cPT
Ambitious but foreseeable goal for a SM+QCD calculation (lattice gauge theory+chiral extrapolation)
Sensitivity to aspects of QCD dynamics (qq correlations) which cannot be “faked” by single-particle model approaches
Executive Committee of SNS Instrument Development Team
(IDT)
David Bowman, LANLVince Cianciolo, ORNLMartin Cooper, LANLJohn Doyle, HarvardChris Gould, NC State/TUNLGeoff Greene, Tennessee/ORNLPaul Huffman, NC State/TUNLMike Snow, Indiana/IUCF, chair
SNS: open user facility,
peer review, scheduling
Join the IDT! www.phy.ornl.gov/nuclear/neutrons
QCD vs Electroweak: which is more “fundamental” ?
LQCD=-1/4 F F+q(iD-m)q (QCD=0)
LEW= LV + LF+ LHiggs+ Lint
LV = =-1/4 F F+mW2(W+
2+ W-2)/2 +mZ
2 Z2/2LF= q(i∂-m)q Lhiggs= ∂µ ∂µ
-mH22
Lint= LVF + LHV+ LHF+ LHH
LVF =g/2(W+µ J+µ +W-
µ J-µ) +e Aµ Jµ,EM+g/cosW ZµJµ,neut
LHV=[mW2(W+
2+ W-2)/2+mZ
2 Z2/2] /(1+ /2)LHF= qMq /LHH=- 3/6 - 4/24
Strong Lagrangian
Electroweak Lagrangian
3He ionization chamber neutron detector
Neutrons detected through the following reaction:
n+3He 3H+1H
Charged reaction-products ionize the gas mixture
High voltage and grounded charge-collecting plates produce a current proportional to the neutron flux
S.D.Penn et al. [NIM A457 332-37 (2001)]
HV platewindow
signal plates
half voltage rings
full voltage plates
ORIGINAL DESIGN
Ionization Chamber3He and Ar gas mixture
4 Detection Regions along beam axis velocity separation (1/v absorption)
Gas pressure so that transverse rangeof the proton < 0.3 cm
What We Have Done BeforeSegmented Ionization
Chamber Detector for n-4Hennn
Note region size increases for approximately equal countrates: 30% of beam in regions 1, 2, 3+4
ORIGINAL DESIGN
Ionization Chambern + 3He → p + t
Collect charged proton and tritonon charge collection plates.
Divide charge collection plates into 4 quadrants (3" diam) separated L/R and U/D beam
What We Have Done BeforeSegmented Ionization
Chamber Detector for n-4He
nnn
0.5 atm 3He, 3 atm Ar gas mixture
4 detection regions along axis4 quadrants per region 16 channels with coarse position sensitivity and large energy bins
Count rate: 107 n/sec – current mode~ 7×105 n/sec/channel
(Allows measurement of rotations from magnetic fields ~ 40 G)
(1996 digital picture shows 4-region, quadrant detector)
Penn et al. NIM 457, 332 (2001)(Includes DM Markoff in author list.)
What We Have Done BeforeSegmented Ionization
Chamber Detector for n-4Hennn
qq weak processes hidden in the weak NN vertex
Non-negligible amplitudes from sea quarks possible
Sign cancellations among different contributions
Renormalization group calculations evolve from weak scale to strong scale
“factorization” amplitude[<N| Hweak|Nm>~<0|qq|m><N|q 5q|N>]
P-odd admixture into N
sea quarks
Valencequarks
DDH
+…
What might the Weak NN Interaction do for QCD?
Chiral symmetry breaking seems to dominate ground-state dynamics of light hadrons such as protons and neutrons
Mechanism of chiral symmetry breaking in QCD |vacuum> is not (yet?) understood (instantons?) but it can induce q-q correlations
For strong QCD physics we need to understand q-q
correlations)
weak qq interaction range~1/100 size of nucleon-> sensitive to short-range part of q-q correlations, an “inside-out” probe
QCD |vacuum>: 2 (distinct?) phenomena:Chiral symmetry breaking +quark confinement
psp
smq~few MeV mqeff~300 MeV
<>=0 =helicity-flip process
Meson Exchange Model (DDH) and other QCD Models
Barton’s theorem [CP invariance forbids coupling between S=0 neutral mesons and on-shell nucleons] -> pp parity violation blind to weak pion exchange [need np system to probe Hweak
∆I = 1]
fnow calculated to be ~3E-7 with QCD sum rules (Hwang+Henley, Lobov) and SU(3) soliton model (Meissner+Weigel), calculation in chiral quark model in progress (Lee et al).
N
N
PV
PC
,,ω
assumes , , and ω exchange dominate the low energy PNC NN potential as they do for strong NN
Weak meson-nucleon couplings f, h
0, h1, h
2, hω0, hω
1 to be determined by experiment