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A Precision Measurement of GE
p/GMp with BLAST
Chris Crawford
Thesis Defense
April 29, 2005
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Outline
Introduction» Formalism
» World Data
» Experiment overview
Experimental Setup» LDS polarized target
» BLAST detector
» Calibrations
Analysis» Cuts & yields
» Asymmetry
» Extraction of GE/GM
» Systematic errors
Conclusion» Results: GE/GM
» Separation of GE, GM
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Introduction
GE,GM fundamental quantities describing charge/magnetization in the nucleon
Test of QCD based calculations and models
Provide basis for understanding more complex systems in terms of quarks and gluons
Probe the pion cloud QED Lamb shift
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Form Factors of the Nucleon
Form Factor definition
Nucleon current
Breit frame
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Elastic Cross Section
= target spin angle w/r to the beam line
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World DataWorld Unpolarized Data
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Polarization Transfer
Recoil proton polarization
Focal Plane Polarimeter» recoil proton scatters
off secondary 12C target» Pt, Pl measured from
φ distribution » Pb, and analyzing power
cancel out in ratio
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GE/GM — World Data
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Theory and ModelsDirect QCD calculations
» pQCD scaling at high Q2
» Lattice QCDMeson Degrees of Freedom
» Dispersion analysis, Höhler et al. 1976» Soliton Model, Holzwarth 1996» VMD + Chiral Perturbation Theory, Kubis et al. 2000» Vector Meson Dominance (VMD), Lomon 2002
QCD based constituent quark models (CQM)» LF quark-diquark spectator, Ma 2002» LFCQM + CBM, Miller 2002
†Nucleon Electromagnetic Form Factors, Haiyan Gao, Int. J. of Mod. Phys. E, 12, No. 1, 1-40(Review) (2003)
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Models Consistent with Polarized Data
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Form Factor Ratio @ BATES
Exploits unique features of BLAST» internal target: low dilution, fast spin reversal» large acceptance: simultaneously measure all Q2 points » symmetric detector: ratio measurement
Different systematics » also insensitive to Pb and Pt
» no spin transportQ2 = 0.1 – 0.9 (GeV/c) 2
» input for P.V. experiments» structure of pion cloud
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Asymmetry Super-ratio Method
Beam-Target Double Spin Asymmetry
Super-ratio
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Polarized Beam and Target
Storage Ring» E = 850 MeV» Imax=225 mA» Pb = 0.65
Internal ABS Target» 60 cm storage cell» t = 4.91013 cm-2 » Pt = 0.80
isotopically pure internal targethigh polarization, fast spin reversal L = 3.1 1031 cm-2s-1
H2: 98 pb-1 D2: 126 pb-1+2005 run
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Atomic Beam Source
Standard technologyDissociator & nozzle2 sextupole systems3 RF transitions
1
3
2
4nozzle
6-pole
1
2MFT (2->3)
1
3
6-pole
1Spin State Selection:
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Laser Driven Source (LDS)
Optical pumping& Spin Exchange
Spincell designTarget and
PolarimeterResults
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Spin-Exchange Optical pumping
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LDS Experimental Setup
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Comparison of Polarized Targets
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BLAST Detector PackageDetector Requirements
Definition of qe 2, e .°, z 1
cm e/p/n/ separation
PID: t 1, Cerenkov Optimize statistics
Large Acceptance Asymmetry Super-ratios
Symmetric Detector Polarized targets
1 m diameter in target regionZero field at targetB-gradients 50 mG/cm
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TOF Scintillators
timing resolution: σ=350 psvelocity resolution: σ= 1%
ADC spectrum coplanarity cuts
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Cosmics TOF Calibration
L 15
L 12
L 9
L 6
L 3
L 0
R 0 R 3 R 6 channels
channels
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TOF Efficiency
green: efficiency
magenta: non-bias
red: misses
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TOF Scintillator Cuts
TOF paddle, electron
TO
F p
add
le,
pro
ton
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Čerenkov Detectors
1 cm thick aerogel tiles Refractive index 1.02-1.03 White reflective paint 80-90 % efficiency
5" PMTs, sensitive to 0.5 Gauss Initial problems with B field Required additional shielding 50% efficiency without shielding
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Wire Chambers
2 sectors × 3 chambers 954 sense wires resolution 200μm signal to noise 20:1
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Reconstruction
Scintillators» timing, calibration
Wire chamber» hits, stubs, segments
» link, track fit
PID, DST
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Newton-Rhapson Track Fitter
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Hyperbolic timedist function
D
TDC
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Linear T2D Calibration
28 MeV
12 MeV
p (GeV/c)
~ 1mm resolution
2
72
33
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Wire Chamber Efficiency
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Tracking Efficiency
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WC Offsets/Resolution/Cuts
pe-pe(e) pp-pp(e) p-p(e)
p-p(e) zp- zp(ze)
pe e e ze
pp p p zp
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Resolution and Yields
TOF paddle #
preliminary
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Experimental Spin Asymmetry
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Single-asymmetry Method measure P first,
use to calculate R» model-dependent
Super-ratio Method2 equations in P, R
in each Q2 bin j
» independent measure of polarization in each bin!
» 2n parameters Pj, Rj
Global Fit Methodfit for P, R1, R2, … from
all Aij together» model independent
» better statistics
» n+1 parameters
» can also fit for
i = left,right sector
j = Q2 bin (1..n)
= spin angle
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Extractions of GE/GM
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Systematic Errors
Q2 (1.8%)» comparison of e and p
» difference between left/rightsector errors mostsignificant
» TOF timing will help
(0.8%) » fieldmap: 47.1° ± 1°
» Hohler: 47.5° ± 0.8°
» Fit Method: 42° ± 3°
» (1st 7 bins) 48° ± 4°
» T20 analysis: 46.5° ± 3°
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GE/GM Results
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Extraction of GE and GM
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GE and GM ResultsBLAST + World Data
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Conclusion
1st measurement of GE/GM using double spin asymmetry
2 – 3.5× improvement in precision of GE/GM at Q2 = 0.1– 0.5 GeV2
sensitive to the pion cloudnarrow dip structure observed in GE
around Q2=0.3 GeV2 ?systematic errors are being reduced