11

The G0 ExperimentThe G0 Experiment

Fatiha BenmokhtarFatiha Benmokhtar

Carnegie Mellon University

Jlab Users Group Meeting, June 17th 2008

fatiha@ernest.phys.cmu.edufatiha@ernest.phys.cmu.edu

22

OutlineOutline

• Strange quark contribution to the nucleon properties Strange quark contribution to the nucleon properties (special interest on the electromagnetic properties). (special interest on the electromagnetic properties).

• G0 experiment: Forward and Backward angle.G0 experiment: Forward and Backward angle.

• Status of the G0Backward angle experiment analysis.Status of the G0Backward angle experiment analysis.

• Expected results (Soon!!! Expected results (Soon!!! ) )

33

Nucleon constituents Nucleon constituents

- The sea contains all flavors, but • the u and d sea can’t be distinguished from the valence• the heavier quarks (c,b,t) are too heavy to contribute much• Strange quark is the natural candidate to study the sea.

With how much do virtual strange pairs contribute With how much do virtual strange pairs contribute to the structure of the nucleon ?to the structure of the nucleon ?

.....+ gssdduuuudp

« sea= virtual pairs »valence

44

Strange Quark Contribution to the Strange Quark Contribution to the Nucleon PropertiesNucleon Properties

• Spin:Spin:

NssNσ s Hyp ->

-N -> 0 to 30 % with big theoretical 0 to 30 % with big theoretical uncertainties. uncertainties.

DI -Nucleon scattering (NuTeV)

. For x <0.1 %~dx))x(s)x(s(x 421

0

• Mass:Mass:

• Longitudinal Momentum:Longitudinal Momentum:

09.015.0 s positive and 0 ~s

N|ss|N 5

-p elastic scattering (E734 BNL)

• Polarized Semi-Inclusive DIS (HERMES)

• PQCD predicts -0.1

55

Strangeness Contribution to Strangeness Contribution to the Nucleon the Nucleon

Electromagnetic PropertiesElectromagnetic Properties

??NssN s

M

s

E GG ,

Goal: Determine the contributions of the strange quark sea ( ) to the charge and current distributions in the nucleon :

“strange form factors” GsE and Gs

M

ss

How do we measure them?

66

Strange Form Factors Strange Form Factors

sME

dME

uMEME GGGG ,,,, 3

1

3

1

3

2

nspsnupdndpu GGGGGG ,,,,,, ;;

Z

sM,EW

dM,EW

uM,EW

p,ZM,E GsinGsinGsinG

222

3

41

3

41

3

81

pZnpW

sMEMEMEME GGGG ,,,2

,,,, sin41

sin2W =.2312 ± 0.00015

Charge symmetry ->

measured

?

77

Parity Violation AsymmetryParity Violation Asymmetry

2Z

2

EM

NCPV

LR

LRPV

M

Q~

M

M~

σσ

σσA

• Scatter polarized electrons off unpolarized target,

• Asymmetries of the order of ppm

Electric Magnetic Axial

5

Z

10M

M,

• Proton target

• Deuteron target

(static case)

• Helium target

)(2

sin2

22

nE

pE

sE

WF

PV GG

GQGA

-Complete calculation by Schiavilla et al., is available. - Enhanced sensitivity to Axial form factor.

p

AME2

FPV

σ

AAA

24π

QGA

sEG

np

nnppd

AAA

eAM

eAWA

sMM

ZMM

sEE

ZEE

GGG)sin(A

GGGA

GGG)(A

241

88

PVPV. ExperimentsExperiments

H, DHH, 4HeHH,DTarget

F/BF/BFFBAngle

Ges, GM

s , GA(p+n)GE

s, GMs GE

s , GMsGE

s + 0.4 GMsGM

s, GA(p+n) Separation

0.12 - 1.0 (0.23, 062)

0.1, 0.23(0.48)

0.10.480.04, 0.1Q2 (GeV/c)2

G0(JLab)

2003-2007

PVA4(MAMI)

2002-2008

HAPPEX II(JLab)

2004-2005

HAPPEX I(Jlab)

1998-2002

SAMPLE(MIT-Bates)1998-2002

Beam

LH2 Target

SuperconductingCoils Particle Detectors

• HAPPEX-III 2009 at 0.6 GeV2

eAM

eAWA

sMM

ZMM

sEE

ZEE

GGG)sin(A

GGGA

GGG)(A

241

Forward

Backward

)1()1(

tan)1(21

M4

Q

2

12

2

2

2

99

G0 ProgramG0 Programin Hall C of Jefferson Labin Hall C of Jefferson Lab

e

A

s

M

s

E

G

G

G• Forward angle e + p …04

• Backward angle e + p ...06-07

• Backward angle e + d …06-07

1010

G0 Forward AngleG0 Forward Angle(2004)(2004)

Low Q2

Focal Plane Detectors (FPD)

One measurement on LH2

Ee = 3.045 GeV, 31 MHz beam str.

Recoil proton detection (52o < p <76o)

0.12 ≤ Q2 ≤ 1.0 GeV2

Counting experiment

Time-of-flight electronics

Target

Collimators

High Q2

e-

PionsInelastic protons

Elastic cut

1111

Results from the Forward angleResults from the Forward angle

Compared to ANVS (“No VectorStrange”) EM form factors : Kelly PRC 70 (2004) 068202

D.S. Armstrong, et al., PRL 95, 092001 (2005)

G0 Backward

NVSphysV

pE

pM

pE

F

sM

sE AA

RG

GG

QGGG

)0(

22

2 1

24

Examining full data set, probability thatGE

s+GMs ≠ 0 is 89%

1212

• R. Young et al., Phys. Rev. Lett. 99, 122003 (2007)

• Global fit of all the existing forward angle data

- If SAMPLE result is used for the axial form factor GAe

GEs = 0.002 +- 0.018 & GM

s = -0.01 +- 0.25

- If GAe is taken from Zhu et al., PRD 62 (2000)

033008

GEs = -0.011 +- 0.016 & GM

s = 0.22 +- 0.2

More measurements of the axial form factor are needed.

G0Backward angle! G0Backward angle!

Global Analysis at QGlobal Analysis at Q22 ~0.1 ~0.1 GeVGeV22

)T(eAG 1

NVSA

• Contains the term

1313

G0 Backward Angle G0 Backward Angle ExperimentExperiment

(06-07)(06-07)

1414

The G0 Backward CollaborationThe G0 Backward Collaboration

G0 Spokesperson: Doug Beck (UIUC)

California Institute of Technology, Carnegie-Mellon University, College of William and Mary, Grinnell College, IPN Orsay, JLab,

LPSC Grenoble, Louisiana Tech. Univ., New Mexico State University, Ohio University, TRIUMF, University of Illinois,

University of Kentucky, University of Manitoba, University of Maryland, University of Winnipeg, Virginia Tech, Yerevan

Physics Institute, University of Zagreb

Analysis Coordinator: Fatiha Benmokhtar (CMU)

Students:Carissa Capuano (W&M), Alexandre Coppens (Manitoba), Colleen

Ellis (Maryland) , Juliette Mammei (VaTech), Mathew Muether (UIUC), John Schaub (NMSU), Maud Versteegen (LPSC),

Stephanie Bailey (Ph.D. W&M, Jan ’07)

1515

G0 Backward AngleG0 Backward Angle• Electron Beam: 362 and 687 MeV -> Q2: 0.23 and 0.62 GeV2

• Turn-around of the magnet, change polarity

• Electron off LH2 or LD2 target, electron detection : Θ = 108°.

• Add Cryostat Exit Detectors (9 CEDs per Octant)-> separate elastic

and inelastic electrons in the CED*FPD space.

• Aerogel Cerenkov detector per octant for /e separation. (p < 380 MeV/c)

Cerenkov

e Beam

CED

FPD

1616

G0 beammonitoring

Spokesman

Target service module

Detectors(Mini-Ferris wheel)

CED+Cherenkov

Sup. Cond. Magnet (SMS)

Detectors(Ferris Wheel) FPD

1717

Beam SpecificationsBeam Specifications

Acor Ameas -1

2Yi

YPi

Pi

• Helicity correlated beam properties

-> false asymmetry.

Correction : linear regression:

• 2ns beam structure

• 86 % longitudinal polarization

• Helicity changed every 1/30 sec (MPS).

• Form Asym. from a pseudo-random quartet structure in helicity (+--+ or -++-).

• 2 half wave plate states IN/OUT

1010-6-6< 0.3 x 10< 0.3 x 10-6-6Beam haloBeam halo

34 eV34 eV 2.5 +/- 0.52.5 +/- 0.5Energy differenceEnergy difference

4 nrad4 nrad 0.0 +/- 0.10.0 +/- 0.1y angle differencey angle difference

4 nrad4 nrad -0.8 +/- 0.2-0.8 +/- 0.2x angle differencex angle difference

40 nm40 nm -17 +/- 2-17 +/- 2y position y position differencedifference

40 nm40 nm -19 +/- 3-19 +/- 3x position x position differencedifference

2 ppm2 ppm 0.09 +/- 0.080.09 +/- 0.08Charge asymmetryCharge asymmetry

““Specs”Specs”Achieved (IN-Achieved (IN-OUT)/2OUT)/2Beam ParameterBeam Parameter

Very small in

our case (ppb)

1818

Simplified Electronics SchemeSimplified Electronics Scheme(for one Octant)(for one Octant)

• G0 backward uses 2 kinds of Trigger:

electrons

pions

FPD

FPD

CE

DC

ED9 CED

14 FPDAltera

FBGA256

Coincidence

with the BPO CEDi*FPDj

coincidences

Scalers

Scalers

AlteraFBGA100

Cerenkov

Scalers

AlteraFBGA256

pions

electronsYES

NO

Scalers

Scalers

. Read Scalers for each CED*FPD combination -> Build the coincidence matrix.

• Electron trigger: CED*FPD*Cerenkov

• Pion trigger: CED*FPD no Cerenkov

1919

Electron Yield Electron Yield (Hz/(Hz/μμA/Oct)A/Oct)

45 C45 C

LH2, 687 MeVLH2, 687 MeV

LD2, 687 MeVLD2, 687 MeV

LH2, 362 MeVLH2, 362 MeV

LD2, 362 MeVLD2, 362 MeV

9090 CC 120 C120 C

70 C70 C 45 C

2020

Blinding Factor(Mult.)

LH2, LD2 Raw Asymmetries, Ameas

687 & 362 MeV

Instrumental & Beam corrections:Rate corrections from electronics

Helicity-correlated beam propertiesBeam Polarization correction

Background corrections: Corrections from inelastic electrons

Background from target wallsPion asymmetry contamination

LH2 Aphys

GEs GM

s GAe

Unblinding

LD2 Aphys

Q2 Determination

4 separate blinding factors

G0Back Analysis StrategyG0Back Analysis Strategy

EM radiative corrections

Previous experiments GE

s+ GMs

2121

. . Pass 1: Pass 1:

Uncorrected yields & blinded asymmetries

. . Pass 2: Pass 2:

Scaler counting correction ( the famous ‘problem’! )

. . Pass 3:Pass 3:

Electronic correction (deadtime, randoms, contamination)

. . Pass 4: Pass 4:

Linear regression correction

Four Passes per Analysis Replay

G0 AnalysisG0 Analysis

Analyzer Database

2222

NA Scaler Counting ‘Problem’NA Scaler Counting ‘Problem’ • An occasional bit drop in a North American scaler

was traced down to trigger electronics. Was noticed at high rates: LD2 target at 362 MeV. This was fixed during the run (Jan07)

• Problem blind to helicity. Problem blind to helicity.

Exaggerated re-production with a Pulser

-Pulser data-Simulation

5 cut removes ~1% of our data for 362MeV LD2,which is the worst case!

Uncut 7 6 5 4 3

Cut on yield

• Test by cutting data; compare with

French octants.

• Confirmed by unchanged asymmetry after fix

Effect on Asymmetry

2323

• Dead Time and Randoms in CEDxFPD coincidences and cerenkov electronics.

• All the electronics chain was simulated.

• Current scan runs

A

LD

2-3

62

Me

V

(H

z/A

)

Corrected

Measured

Electron Measured-Corrected Yield Electron Measured-Corrected Yield

LD

2-3

62

Me

V

(H

z/A

)

A

Measured

Corrected

No Dead Time

No Dead Time

Global correction: ~ 2-4% to the asymmetry in the elastic locus.

Rate Correction for Electronic Rate Correction for Electronic Related EffectsRelated Effects

2424

Four Pass AsymmetriesFour Pass Asymmetries (LH2 687 MeV)(LH2 687 MeV)

1-Raw asymmetries 2-Scaler counting correction

3-Electronic corrections

4-Linear regression correction

Asy

mm

etry

(pp

m)

Asy

mm

etry

(pp

m)

Asy

mm

etry

(pp

m)

Asy

mm

etry

(pp

m)

2525

LH2 687 Field ScanLH2 687 Field Scan

1meas back

el

A fAA

f

fitbcknomrate ,

fdata,tot

nomrate

\

Gaus + linear + “elastic” fit to dataGaus extracted from fit (background)

Data

Elastic simulation with radiation

Total simulation

Inelastic + pi0 simulation (no radiation)

% 5 f

Gaus + linear + “elastic” fit to dataGaus extracted from fit (background)

Data

Elastic simulation with radiationTotal simulation

Inelastic + pi0 simulation (no radiation)

We do understand the backgrounds and work is in progress.

1meas back

el

A fAA

f

fitbcknomrate ,

fdata,tot

nomrate

-Nominal field current set at 3500 A.

- Vary the field and study the CED*FPD yield.

2626

• Helicity-Correlated Beam Properties: Linear Regression:Helicity-Correlated Beam Properties: Linear Regression:

• Sensitivities to helicity-correlated beam motion smaller at backward angles. Elastic Electron False Asymmetries: < 4ppb

Other CorrectionsOther Corrections

• Electromagnetic Radiative CorrectionsElectromagnetic Radiative Corrections

• Simulation program for radiative corrections is under development:

R ~ 4% (preliminary.)

• Transverse Beam Asymmetry: Transverse Beam Asymmetry: Very small: under study.Very small: under study.

• Many other small checks were done. Many other small checks were done.

2727

From Asymmetries to Strange Form From Asymmetries to Strange Form FactorsFactors

e

ApMW

VsM

sE

pE

nV

nM

pM

nE

pE

pV

pM

pEWp

MpE

F

GG

RGGGRGGGG

RGGGG

QGA

2

0

22222

2

41

11

1411

24

sin

sin

)(

Ref:Diaconescu, Schiavilla & van Kolck, PRC 63 (2001) 044007

(Addition of the 2body currents corrections)

See: J. Liu PhD. thesis, UMD 2006.

• Proton

TTLL

ATTW

VTT

VLLF

RvRv

qRvqRvqRvQGA

),(')sin(),(),( 22 41

22• Deuteron

2828

From Asymmetries to Strange Form From Asymmetries to Strange Form FactorsFactors

sA

)T(eA

sM

sEe GaGaGaGaa),,q(A 4

13210

Isoscalar sff

(Estimated) to be extracted

- We will use the best and up to date inputs for the quantities going into the ai coefficients.

- Solve the system for the G0 Forward ( proton target) and G0 Backward kinematics (proton and deuteron targets)

2929

Expected G0 ResultsExpected G0 Results

- Error bars dominated by statistics.- Systematic experimental: backgrounds Small- Systematic from the nucleon form factors

3030

G0 SummaryG0 Summary

• G0 Forward angleG0 Forward angle

– GEs+nGM

s from Q2=0.12 to 1 GeV2

– Strange quark contribution non-zero at 89% confidence level

– Nicely consistent with emerging picture at Q2=0.1 GeV2

– Gave some clues about where to look next (HAPPEX-III)

• G0 Backward angleG0 Backward angle

– Provide clean separation of GEs, GM

s, and GA at Q2=0.23 and 0.6 GeV2

– Data Analysis almost at the end, complete separation coming soon!

3131

Additional Program to theAdditional Program to theG0 ExperimentG0 Experiment

• Parity-violation in electro and photo excitation of Parity-violation in electro and photo excitation of the Delta resonance: - inelastic electronthe Delta resonance: - inelastic electron

- photopion asymmetries).- photopion asymmetries).

- - Analysis in good progressAnalysis in good progress

• Beam normal asymmetries and two-photon Beam normal asymmetries and two-photon exchange. - -exchange. - -

- - Analysis in good progress.Analysis in good progress.

3232

Backup SlidesBackup Slides

3333

EEbeambeam = 680 MeV, = 680 MeV, ee = 100° = 100° (Deuteron)(Deuteron)

xsection

a0

a1

a2 a3 a4

3434

At 685 MeV: vs scattering angle via At 685 MeV: vs scattering angle via interpolationinterpolation

3535

0sinˆ

nenm AnpAA

)()F~

F~

Im()F~

F~

Im(GA 25453Mn

)(

1)1(22

22

1

2

EM

e

GGQ

m

iF~

-contains intermediate hadronic state information

magnitude of transverse asymmetry

depends on direction of transverse beam polarization

Transverse Polarization DataTransverse Polarization Data (G0 backward) (G0 backward)

3636

G0: N → G0: N → • Measurement:Measurement: Parity-violating Parity-violating

asymmetry of electrons scattered asymmetry of electrons scattered inelasticallyinelastically– AANNΔΔ gives direct access to gives direct access to GGAA

NNΔΔ

• Directly measure the axial (intrinsic spin) Directly measure the axial (intrinsic spin) response during response during N →N →ΔΔ++ transition transition

– Will find Will find GGAANNΔΔ over a range of over a range of QQ22

• 0.05 GeV/c0.05 GeV/c22 < Q < Q22 < 0.5 GeV/c < 0.5 GeV/c22..– First measurement in neutral current First measurement in neutral current

processprocess

• Data: Data: Inelastic electrons measured Inelastic electrons measured by G0by G0– Scattered from both LH2 and LD2, each at Scattered from both LH2 and LD2, each at

two energies (362MeV & 687MeV)two energies (362MeV & 687MeV)

Elast

ic R

egio

n: G0

Inel

astic

Reg

ion:

Inel

astic

Reg

ion:

NN

Raw Asymmetry (averaged over inelastic region)

Asymmetry (ppm) vs Octant (LH2 @ 687MeV)

Asy

mm

etry

(pp

m)

• BLINDEDBLINDED OUT

IN

3737

π)(

π)(

π)(

Finel Δ+Δ+Δ

πα

QG=A 321

2

24

(1) = 2(1sin2W) = 1 (Standard Model)

(2) = non-resonant contrib. (small)

(3) = 2(14sin2W) F(Q2,s) (N- resonance)

)(QGs),F(Q A

NΔ

22 At tree-level:

expected precision

•F contains kinematic information & all weak transition form factors→Extract GA

N from F

Measurement of theMeasurement of the Parity Violating Asymmetry Parity Violating Asymmetry

in the in the N N →→ΔΔ Transition Transition

BLINDEDBLINDED

Asymmetry (ppm) vs Octant (LH2 @ 687MeV)

Raw Blinded Asymmetry (averaged over inelastic region)

Q2 Range of G0 Measurement

22   )  ( QvsQG AN

3838

Longitudinal – corrected for transverse :

Pion AsymmetriesLH2 687 MeV

BLINDED

Blue data points : Half Wave plate “in”Red data points : Half Wave plate “out”

3939

BLAST Results BLAST Results

C. Crawford et al., Phys. Rev. Lett. 98, 052301 (2007)

4040

Nucleon Electromagnetic Nucleon Electromagnetic Form Factors Form Factors

Accessible via parity violating amplitudes, but how?

. Measured with precision over wide range of Q2 .

(10~15% for neutron electric F.F. at low Q2 )-A lot of improvement over the last decade with double polarization exp.

. Preliminary results from blast

didn’t improve Gen much.

4141

Different Nucleon EM FF ParametrizationsDifferent Nucleon EM FF Parametrizations

4242

Some examples of correctionsSome examples of corrections

LH2, 687 MeV

LH2, 362 MeV

LH2, 687 MeV, 60 A ~ 7%LH2, 362 MeV, 60 A ~ 6% LD2, 687 MeV, 20 A ~ 9%LD2, 362 MeV, 35 A ~13%

(work in progress )

• Deadtime correctionsDeadtime corrections to the yield to the yield– Simulated the complete electronics Simulated the complete electronics

chain:chain:

Deadtimes (%)

• Randoms correctionsRandoms corrections– LH2 randoms smallLH2 randoms small– LD2 randoms significant, esp. LD2 randoms significant, esp.

inelastic el.inelastic el.

• measure directly! measure directly!

4343

Cerenkov EfficienciesCerenkov Efficiencies

• Electron detection efficiencyElectron detection efficiency• Determined using three different techniquesDetermined using three different techniques• Does not change asymmetryDoes not change asymmetry

Four CerenkovDetectors

CED/FPD

Coincidence

electron

pion

4444

Quartz PMTsQuartz PMTs• Aerogel Cerenkov counters for Aerogel Cerenkov counters for /e separation /e separation

(LD2)(LD2)

boroscilicate quartz

Note difference in vertical scales

– 4 - 5 in. PMTs each4 - 5 in. PMTs each– boroscilicate glass very boroscilicate glass very

sensitive to neutronssensitive to neutrons– replace with quartzreplace with quartz

– beam current for LD2 limited beam current for LD2 limited by by

high real high real rates rates (neutron) Ch. accidentals(neutron) Ch. accidentals

• quartz tubes allow increase quartz tubes allow increase in effective electron in effective electron efficiency by ~x2efficiency by ~x2– current limits 20 current limits 20 A (35 A (35 A) A)

at 687 (362) MeVat 687 (362) MeV

• final tubes installed over final tubes installed over Xmas breakXmas break

4545

Four Pass Asymmetries (LD2 362 MeV)Four Pass Asymmetries (LD2 362 MeV)Four Pass Asymmetries (LD2 362 MeV)Four Pass Asymmetries (LD2 362 MeV)

4646

Data Quality (LDData Quality (LD22 362MeV) 362MeV)

YieldYield

FPD#

Asym.

Asym.

YieldYield

Asym

.

Asym

.

Yield (Hz/uA)

Co

un

ts

(lo

g s

ca

le)

CE

D#

BUT NA Scaler Data at high rates Most of the Data is good quality

Co

un

ts

(lo

g s

ca

le)

Co

un

ts

(lo

g s

ca

le)

Co

un

ts

(lo

g s

ca

le)

Asymmetry (ppm) Asymmetry (ppm)

Yield (Hz/uA)

4747

•1- R. Young et al., Phys. Rev. Lett. 97, 102002 (2006)

•2- R. Young et al., Phys. Rev. Lett. 99, 122003 (2007) - Added the latest Happex point. Ges = 0.002 +- 0.018 & Gms = -0.01 +- 0.25

If Zhu et al. is used:

Ges = -0.011 +- 0.016 & Gm^s = 0.22 +- 0.2

• J. Liu et al, Phys. Rev. C 76, 025202 (2007) - Using Zhu’s model value for GeA.

Ges = -0.006 +- 0.016 GMs =0.33 +- 0.21

68% CL

95%

CL

Global Analysis at QGlobal Analysis at Q22 ~0.1 ~0.1 (GeV/(GeV/cc))22

Leinweber et al.

There are two other fits one from K. Paschke And one from F. Maas..

144.0004.0 sMG

4848

Strange Form Factors Calculations Strange Form Factors Calculations at Qat Q22=0=0

0

2

22

0 2

)(4

)(

)0(

Q

sE

N

sE

s

sMs

dQ

QdGM

d

dG

G

79.23

131

32 sdup an

d

If s is negative, then s and

sbar would make (+) overall contribution to p.

convention: positive charge radius negative radius s

GEs < 0 s-quark on the outside

see R. Jaffe, PLB 229 (1989) 275 or Geiger & Isgur, PRD 55 (1997) 299

Hannelius, Riska + Glozman, Nucl. Phys. A 665 (2000) 353

Λ

4949

Electromagnetic Radiative CorrectionsElectromagnetic Radiative Corrections

Target Energy A0 rc A0 tree Rccorrection

LD2 687 -46.6 -48.43 3.7%LD2 362 -13.64 -14.17 3.9%LH2 687 -36.81 -38.22 3.8%LH2 362 -10.1 -10.49 3.9%

LH2 687 With Radiative Effects

LH2 687 WithoutRadiative Effects

Geant Strategy:•Generate electron at random point in target •Assign random scattering direction and energy to the electron•Calculate cross section and asymmetry for that scattering process

Follow process of Tsai [SLAC=PUB-848]

1971. Compute asymmetry [ ] based on the kinematics at the reaction vertex after the radiative emission.

This is compared to Born asymmetry

calculation [ ] with

Both and are calculated including

ionization losses in the target prior to

scattering.

5050

Nucleon constituents Nucleon constituents

- The sea contains all flavors, but • the u and d sea can’t be distinguished from the valence• the heavier quarks (c,b,t) are too heavy to contribute much• Strange quark is the natural candidate to study the sea.

With how much do virtual pairs contribute With how much do virtual pairs contribute to the structure of the nucleon ?to the structure of the nucleon ?

.....+ gssdduuuudp

« sea= virtual pairs »valence

5151

G0 Forward AngleG0 Forward Angle(2004)(2004)

Low Q2

Focal Plane Detectors (FPD)

One measurement on LH2

Ee = 3.045 GeV, 31 MHz beam str.

Recoil proton detection (52o < p <76o)

0.12 ≤ Q2 ≤ 1.0 GeV2

Counting experiment

Time-of-flight electronics

Target

Collimators

High Q2

e-

PionsInelastic protons

Elastic cut