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