Nucleon Structure fromParity-Violating Electron Scattering

Doug BeckUIUC

28 Sept. 2006

Outline:

1. Selected history2. Physics motivation3. Experiments4. Present situation

Selected History• Zel’dovich 1959

– speculation about neutral current analog of weak interaction causing β-decay

– also PV electron scattering– also size of effect

• Cahn & Gilman 1978– neutral weak form factors– first record of strange quark

currents

Selected History• Pioneering measurement of Prescott, et

al. 1978– first measurement of neutral weak current– first parity-violating electron scattering

experiment

12C: A = -0.6 ppm9Be

• Bates 12C (1990), Mainz 9Be (1989) expts

Selected History• Kaplan & Manohar 1988

– flavor singlet contribution to neutral current → strange quarks

Selected History• McKeown & DHB 1989

– parity-violating electron scattering applied to Kaplan and Manohar– s contribution to charge form factor

• Jaffe 1989

The nucleon matrix elements of the operators

are estimated using dispersion theory fits to the nucleon isoscalar form factor, together with a standard treatment of φ−ω mixing and some mild assumptions on the asymptotic behavior (at large q2) of nucleon form factors. The results indicate a significant strange quark content in the nucleon.

( ) ( )

ssx

xsxxsr

s

s

γμ ×≡

≡ +

21

22

Physics Motivation• Proton is both ordinary and extraordinary object

– 50% of mass of visible universe– masses of constituents ~ 1% of total mass

q

q

• What is it made of?– 3 valence quarks: “full-time”

• carry baryon number– sea of gluons (force carriers)– and associated quark-antiquark

pairs• analog of Lamb shift physics

– very complicated because• strong coupling ●• gluons interact with each other

→many-body physics with virtual particles

Quark Currents in the Nucleon• Measure

– e.g.

– note

then

:,, ,,, npZp GGG γγ NqqeNG iii

i μΓ∑~

( )psME

pdME

puME

pME GGGG ,

,,,

,,

,, 3

132

+−=γ

nsps

nupd

ndpu

GGGGGG

,,

,,

,,

=

=

=charge symmetry(see G. A. Miller PRC 57 (98) 1492.)

( )( )( ) pZ

MEnME

pMEW

sME

pZME

nME

pMEW

dME

pZME

pMEW

uME

GGGG

GGGG

GGG

,,

,,

,,

2,

,,

,,

,,

2,

,,

,,

2,

sin41

sin42

sin43

−−−=

−+−=

−−=

γγ

γγ

γ

θ

θ

θ

dropping the p superscripts on the left

• spin current– spin triplet: moments cancel– spin singlet: zero net moment, zero

net convection– also requires separation

. , Non-Zero?• charge distribution

– if s, s are separated, non-zero net contribution

• convection current– if s, s are separated, non-zero net

contribution

s

μs

21

zs +=

μs

21

zs −=

s

rs

rs

s

s

GEs GM

s

Parity-Violating Electron Scattering

• Interference term violates parity: use

where

pZG ,

( )e,e ′r

• contributes to electron scattering

- interference term: large x small

2ZMM +∝ γσ

γM ZM

( ) ( )22

2

5

24

10~

γγ τεπα

σσσσ

ME

AMEF

LR

LRPV

GGAAAQG

A

+

++−=

+−

≡ −

( )( ) ( ) e

AMWA

ZMMM

ZEEE

GGA

GGAGGAγ

γγ

θεθ

τθε′−−===

2sin41

,

e p

Z

e p

γ

( ) ( ) ( )[ ]

( ) ( )( )2

2

2

12

11

,4

,2/tan121

εττθε

τ

θτθε

−+=′

=

++=−

pMQ

2. Experiment

Summary of PV Electron Scattering Experiments

K. Kumar

published, running

published x2, running

published (ing)published (ing)

2008

2006

SAMPLE Experiment

Ebeam = 200 MeVIbeam = 40 μAPbeam = 35%Δθ = 130 - 170o

ΔΩ = 1.5 srltarget = 40 cmL = 4.3 x 1038 cm-2 s-1

A ~ -7 ppm

• Measure GM (Q2 = 0.1 GeV2) for 1H,2H

Caltech, Illinois, Kentucky, LaTech, Maryland, MIT, Virginia Tech, W&M

Z

The Axial Current Contribution

• Recall

– “unknown form factor” GAe(Q2)

– related to form factor measured in neutrino scattering

– also contains “anapole” form factor– determine isovector piece by combining proton

and neutron (deuteron) measurements

unp

AMEPV AAAAσ2

++∝

( )( ) ( ) e

AMWA

ZMMM

ZEEE

GGsinA

GGA,GGAγ

γγ

θεθ

τθε

′−−=

==241

e p

Z

γ

“box”

e p

Zγ

“mixing”

e pγ

“quark pair”

( ) )(2

)()(

2)()()'()('

5

2

52

2

2

222

12

puqMQFiqqq

MQF

qMQFiQFpupQJp

EA⎟⎟⎠

⎞−/−

⎜⎜⎝

⎛+−=

γσγγ

σγ

νμνμμ

νμνμ

γμ

What is the Anapole Moment?• As first noted by Zel’dovich (Sov. Phys. JETP 6 (58) 1184), a parity-

violating coupling of the photon can occur

where FA and FE are the anapole (parity-violating, time-reversal conserving) and electric dipole (parity- and time-reversal- violating) moments, respectively

• At low Q2 the corresponding interaction energy is (Musolf and Holstein,

Phys. Rev. D 43 (91) 2956)

• The classical analogy of the anapole moment is that property of a toroidal magnetic field that leads to a torque in an external current field

jMFej

MFeL AA

anapole

vv ⋅−−= σψγψγ μμ 22

522 ~

jxjU

a

avvv

vv

σσ

=Γ

⋅−=

aσv

jv

SAMPLE Results

• GMs using calculation for GA

e

– GMs = 0.37 ± 0.20 ± 0.27 ± 0.07

– Phys. Lett. B583 (2004) 79

• GAe

– third experiment: Q2 = 0.03• T. Ito, et al. PRL 92 (2004) 102003

– consistent with theoretical calculation

• Zhu, et al. Phys. Rev. D 62 (2000) 033008

Where Were We (Spring 05)?• HAPPEx (Q2 = 0.5 GeV2),

Mainz (Q2 = 0.1, 0.23 GeV2)

• New HAPPEx H and 4He measurements– Q2 ≈ 0.1 GeV2

Forward angle H data

• New G0 measurements– 0.1 ≤ Q2 ≤ 1 GeV2

• New HAPPEx H and 4He measurements– higher statistics

( ) 22 , QGGEQ p

E

pM

i ≅=ετη

• Important ‘kinematic’ variable: η(Q2, Ei)– for forward measurement, + ηGE

s GM ∝ Aexpt - ANVSs

for G0, HAPPEx H

Strange Quark Contribution to Proton

. , Data @ Q2 = 0.1 GeV2GEs GM

s

HAPPEx He

• HAPPEx calculation: Q2 = 0.1 GeV2

GM = 0.28 ± 0.20s

GE = -0.006 ± 0.016s

Fitting All Data• What are the experimental ranges for GE(Q2), GM(Q2)?

• Can fit all present data to assumed forms for GE, GM

ss

s s

( ) ( ) ( )( ) ( )

( )222

22

222

0,2

1

0

16.51

sM

sMs

M

sE

sEs

E

Q

QGQG

Q

GQG

Λ+

==

Λ++=

ττ

Galster

dipole

Λ=Λ=Λ sM

sE

with

Simple Fit to All Data

• Fit

( ) 22.039.00

95.080.02

0,

±==

±−=

QG

GsM

sE 22/8.192 =νχ

HAPPEx He

Backward Angle Data• In order to more cleanly separate GE and GM, need to

measure at backward angles– GA also comes in: requires 2H measurement

• Mainz: PVA4– starting 1H, 2H at Q2 = 0.23 GeV2

• JLab: G0– data for 1H, 2H: through 02/07– Q2 = 0.23, 0.63 GeV2

Electron EnergyC

ount

s

s s

G0: Q2 = 0.23 GeV2 On-line Data• Very clean hydrogen elastic signal

– all backgrounds total ~ 5-10%• Blinded, on-line results

– no corrections for h.c. beam parameters, deadtime, …

Deuterium test run (May)

Rat

e (k

Hz/μA

)

Hydrogen data (Aug.)

FPD

(quasi) elastic electrons

CED

G0: Q2 = 0.23 GeV2 On-line DataElastic

Background

P R E L

I M I N

A R Y

Octant

Octant

G0 Q2 = 0.23 GeV2 On-lineTransverse Asymmetry

• BLINDED online results – no corrections for h.c. beam parameters, deadtime, …

Transverse: Elastic

P R E L I M I N A R Y

Octant

Expected Results for G0 Separation

GEsGM

s GAe

Summary• Direct view of nucleon sea with electroweak probe

• First ‘nucleon’ measurement performed at Bates– SAMPLE experiment suggests positive value for μs and

significant modification of axial current – anapole moment

• New data from HAPPEx, G0, PVA4– best information on GM ~ +0.30 at 1-2 sigma

• Need more leverage: new G0 and PVA4 backward angle measurements– difficult to make Q2 separation with only SAMPLE point– measurements underway

s