The Strange Form Factors of the Proton and the G 0 Experiment Jeff Martin University of Winnipeg Collaborating Institutions Caltech, Carnegie-Mellon, William&Mary,

The Strange Form Factorsof the Proton

and the G0 Experiment

Jeff Martin

University of Winnipeg

Collaborating InstitutionsCaltech, Carnegie-Mellon, William&Mary, Grinnell College, Hampton,

IPN-Orsay, LPSC-Grenoble, JLab, Kentucky, LaTech, NMSU, TRIUMF, UConn,

UIUC, U Manitoba, U Maryland, UMass, UNBC, U Winnipeg, VPI, Yerevan

Z

N e

eA

ZM

ZE

Z GGGNJN ,, ||

neutral weak form factors,axial form factor

• Projected precision of NW form factors in 0.1 - 1 GeV2 Q2 range ~ 10% from the current generation of experiments

Nucleon form factors measured in elastic e-N scatteringNucleon form factors

•From scattering theory: form factor ~ Fourier transform of density•Well defined experimental observables• provide an important benchmark for testing non-perturbative QCD structure of the nucleon

N e

ME GGNJN , ||

electromagnetic form factors

• Measured precision of EM form factors in 0.1 - 1 GeV2 Q2 range ~ 2 - 4%

Flavour Decompositionof the Form Factors

e p

Species Charge Weak Charge

u

d

s

W2sin

3

81

W2sin

3

41

W2sin

3

41

3

2

3

1

3

1

psMEW

pdMEW

puMEW

pZME

psME

pdME

puME

pME

GGGG

GGGG

,,

2,,

2,,

2,,

,,

,,

,,

,,

sin3

41sin

3

41sin

3

81

3

1

3

1

3

2

0.0002 0.2312sin

angle mixing weak theis 2

W

W

exchange:

Z0 exchange:

Goal: Isolate GMs and GE

s by isolating the Z0 exchange graph

G0 Program

At a given Q2, full decomposition of GsE, Gs

M, GeA requires 3 new

parity-violation measurements:

Forward angle e + p (elastic) ← ← data-taking completedata-taking completeBackward angle e + p (elastic) ← ← data-taking 2005-2007data-taking 2005-2007

Backward angle e + d (quasi-elastic) ← ← data-taking 2005-2007data-taking 2005-2007

G0 will perform theseparation at threedifferent Q2 values -

0.3, 0.5, 0.8 (GeV/c)2

)()()()sin41(

)()()(

)()()(

222

222

22

QGQGA

QGQGQA

QGQGA

eAMWA

ZMMM

ZEEE

Parity-ViolatingElastic Scattering

2

e e pp

LR

LRA

unpol

AMEF AAAQG

224

2

where:

Scatter polarized electrons from unpolarized protons, and measure the asymmetry:

prevalent at forward angles

prevalent at backward angles

Note: A ~ 10-6 = 1 ppm

Experiment

General requirements:• High statistics (1013-1014) events

– high current, high polarization beam– large acceptance– high rate capability

• Systematics– control of helicity-correlated beam properties– background characterization and rejection

Forward-Angle Measurements

• Elastic proton detection• Toroidal focusing spectrometer• Time-of-flight distinguishes elastic protons from

pions, inelastic protons

G0 beammonitoring girder

superconducting magnet (SMS)

detectors (Ferris wheel)

cryogenic supply

target service module

G0 Forward-Angle Configurationat Jefferson Laboratory

Beam

Estimate of background yield under the elastic peak is performed by using

- data with full and empty (gas H2) targets, different pressures

- data with dummy entrance and exit windows (Al)

- Data with W radiator and dummy windows (electro/photo production)

Unfold backgrounds from target windows and inelastic LH2 processes

pions

elastic protons

inelastics

Fractional background in elastic cut

Dominant Systematic: Backgrounds

Yield

asymmetry

Detector 8

• For smaller detectors, yields and asymmetries are measured on each side of the elastic peak and smooth interpolation is reasonable.

• For larger detectors, background asymmetry is large and varies across the elastic peak.

Small increase of uncertainty in elastic asymmetry

due to background for small detectors.

Yield

asymmetry

Detector 13

Effect of backgrounds will likely dominate systematic uncertainty for larger detectors.

Background Correction

For preliminary result, used linear interpolation

Progress Background Correction

Constrain yield with Monte Carlo

motivated fit.

Constrain asymmetry by enforcing

constant elastic asymmetry.

ppm

Yield – Ring 8 Asymmetry – Ring 8

Forward angle asymmetries

- full statistics

- detectors 13 to 15 not shown.

- preliminary background corr.

- 25% blinding factor applied

Asy

mm

etr

y (

ppm

)

Increasing Q2

Detector number

Preliminary Results

Systematic uncertainties included for preliminary result:

- Deadtime corr 2%

- Beam parameters corr 0.01 ppm

- Leakage beam corr 0.10 ppm

- Beam Polarization 2%

- Background corr not included

- Q2 determination 1%

- Rad. Corr., EM FF’s small

BLINDING FACTOR APPLIED

G0 Daily NewsBox Has Been OpenedBlinding Factor RevealedAnalysis complete,

preparing for public release of results

Next Phase of G0: Backward-Angles

Electron detectionAdd Cryostat Exit Detectors (“CED’s”) to define electron trajectoryAdd aerogel Čerenkov counter to reject -

Measurements on H and D to separate GMs,

GAe

Ebeam (MeV) Q2 (GeV2)

424 0.3

585 0.5

799 0.8

beam

target

magnet

FPD #1

FPD #16 CED#9

CED#1

Čerenkov

inelastic e-

or photo -

elastic e-

Canadian Contributions to Backward-Angle Expt.

• Čerenkov counters built by:– U Manitoba, TRIUMF, UNBC– Grenoble (France)

• Čerenkov/CED support structure (TRIUMF) and CED construction.

• Electronics for Canadian Čerenkov counters:– U Winnipeg

• Acquisition and Analysis of Data– Pion asymmetries (U Winnipeg)

First run starts

Dec. 3, 2005

Summary

• Forward angle production run complete.• Analysis of forward angle data nearing

completion.• Preparations for back-angle run underway:

– additional scintillators, Čerenkov counters being tested.

– new electronics being tested.– new analysis software in preparation.

• Back angle running will begin in December 2005, will continue throughout 2006, 2007.

By 2008:Separated Form Factors

Q2 dependence of the strange electricand magnetic form factors below Q2 = 1 GeV2