status report of the measurement of g e n via 3 he(e,e´ n) robert j. feuerbach the college of...
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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)
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