Status Report of the measurement of GE

n via 3He(e,e´ n)

Robert J. FeuerbachThe College of William and Mary

for the E02-013 Collaboration

E02-013 Status January 4, 2006 Hall A Collaboration Meeting

Elastic EM Form Factors

For an extended spin-1/2 particle, the general vertex term is:

Elastic cross-section:

Or in terms of the Sachs Form factors:

Dominate at large Q2

E02-013 Status January 4, 2006 Hall A Collaboration Meeting

Double Polarization Measurement

E02-013 Status January 4, 2006 Hall A Collaboration Meeting

Exclusive QE scattering: 3He(e,e’n)

e’

n

veto

scintillator

e

E02-013 Setup

~85% polarization~50%

E02-013 Status January 4, 2006 Hall A Collaboration Meeting

Running conditions• 75 msr BB,

~100 msr ND

• Ltotal ~ 5·1036/(cm2 s) Len ~ 5·1035/(cm2 s)

• BB trigger ~ 2.3 kHzND trigger ~ 2.5 MHz

• MWDC ~ 2.6 MHz/uA 20 MHz per plane

• Target polarization ~50%• Beam polarization ~84%

• Electronic LT ~ 82%• DAQ LT ~ 84%

• Average 4 extra hits per plane affected tracking (See Seamus’s talk Saturday)

E02-013 Status January 4, 2006 Hall A Collaboration Meeting

AssignmentsAnalyze Q2=1.8 GeV2 dataset All the students

Calibrate other datasets ND timing (Jon), BB scint. (Ameya), Shower/preshower (Sergey),beam Raster + DC offsets (Brandon)

BB optics (Seamus), Target (Aidan)

Analysis of all datasets: shared Each student has 1-2 datapoints as primary responsibility, and one more as a “check”: each Q2 has 3 students on it.

Veto efficiency ( proton mis-ID) Jon

ND stability over time Tim

EPICs and Target conditions database Brandon

Monte-Carlo for background studies (pion production, etc.)

Gregg Franklin

Overall Analysis software Rob

Finite acceptance effects (understood) Gregg

ND documentation (~ done) Tim/Albert(geom), Rob(algorithms)

BB geom and algorithms (~ done) Seamus

Electronics config. doc. (~~ done) Sergey(BB), Alexandre(ND)

E02-013 Status January 4, 2006 Hall A Collaboration Meeting

Electron arm reconstruction

• Preshower/shower selects electrons well

• Momentum σ ~.9%@ 1.3 GeV/c on average

• VertexZ σ ~ 4mm• “super-elastics” due to

imperfect optics

• Scintillator σt ~ 360ps

E02-013 Status January 4, 2006 Hall A Collaboration Meeting

Neutron Detector• Data shown are from an analysis of the 1.8 GeV2 dataset.• Neutron Detector timing resolution of σ ~ 400ps• Neutrons/Charged particles differentiated by matching hits in Veto to

clusters in the ND.• 1/β calculated from time of hit at ND and Rf-corrected time from BB

scintillators.

3He(e, e’ A) – all charged ND-particle events

protons

Fast pions/muons?

E02-013 Status January 4, 2006 Hall A Collaboration Meeting

QE Event Selection

• Use missing 3-momentum to select QE events

E02-013 Status January 4, 2006 Hall A Collaboration Meeting

QE Event selection: ProtonsNuclear effects (FSI) can distort the observed asymmetry … Pperp < 150 MeV/c

Momentum distributionof nucleons in 3He

E02-013 Status January 4, 2006 Hall A Collaboration Meeting

QE Event selection: Neutrons

Neutrons

AccidentalBackground

E02-013 Status January 4, 2006 Hall A Collaboration Meeting

QE Event selection: Neutrons

Accidental Background

E02-013 Status January 4, 2006 Hall A Collaboration Meeting

Rough Asymmetries @ Q2 = 1.8 GeV2 Integrated Asymmetries (NOT corrected for many several %-size effects) for different Beam ½-wave plate settings and Target polarization direction changes. Yet need to

account for distortion of the asymmetry from

•False asymmetries: Electronic LT (~.5%) Tracking eff (~.2%?) DAQ livetimes(<.1%) Beam Charge (<.02%)

•Finite acceptance

•Dilution factors from N2

proton mis-ID

E02-013 Status January 4, 2006 Hall A Collaboration Meeting

Projected Results

Systematic uncertainties for highest Q2 point still smaller than shown statistical uncertainty.

Projected error bars assumeresults match theGalster parameterization.

E02-013 Status January 4, 2006 Hall A Collaboration Meeting

The Future: GEn @ 4.5 GeV2?

• E0=3.7 GeV/c, E’=1.3 GeV/c, θBB=59° to measure GE

n/GMn @ 4.5 GeV2 (limit of CLAS data on GMn).

• Add shielding along beamline for the ND: reduces its rate by factor of 2.

• Larger cell size to remove halo effects: drop overall rate factor 3.

• Triple Ib to 24uA Len~1.5 1036 /(s cm2).

• A measurement with the 20% relative statistical uncertainty (vs. 14% at 3.4 GeV2) could be done in 500hrs (25 days).

E02-013 Status January 4, 2006 Hall A Collaboration Meeting

Summary• Electron and neutron detector analysis is

well understood, though still being “tweaked”

• Can identify quasi-elastic proton and neutron events to extract the asymmetry and GE

n.• Presently analyzing the 1.8 GeV2 dataset to

identify problem areas: so far found electronic deadtime and tracking eff. (surprises), and proton mis-ID to be leading corrections.

• Plan to complete 1.8 GeV2 analysis this spring, and results from all datasets by late 2007/early 2008 (my estimate).

• Investigating measurement at 4.5 GeV2 ?

E02-013 Status January 4, 2006 Hall A Collaboration Meeting

Backup slides

E02-013 Status January 4, 2006 Hall A Collaboration Meeting

Proton Asymmetry

E02-013 Status January 4, 2006 Hall A Collaboration Meeting

Background Asymmetry

E02-013 Status January 4, 2006 Hall A Collaboration Meeting

Data Quality

• The experiment ran for 3 months in Spring 2006.• Four datasets (Q2=1.4, 1.8, 2.4, 3.4 GeV2) were taken.• Commissioned BigBite precision detector package, BigHand, hybrid

cell and advanced optics system for the target. • Novel holding field magnet and compass to control and understand

field gradients at the target.

Beam polarization ~84%

(Q2~3.5 GeV runs)

Target polarization ~50% IN BEAM

E02-013 Status January 4, 2006 Hall A Collaboration Meeting

World data on GEn

E02-013 Status January 4, 2006 Hall A Collaboration Meeting

Data Analysis• “Results” shown are from an advanced preliminary analysis of the 1.8 GeV2

dataset.• “Neutrons” are ALL clusters in BigHand without matching veto hits (very

loose criteria for now).• Achieved momentum resolution from BB of dp/p~0.8%, BigHand timing

resolution of 400ps (sigma)3He(e, e’ N)