A Precision Measurement of GE
p/GMp with BLAST
Chris Crawford
MIT Laboratory for Nuclear Science
May 18, 2005
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
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
Form Factors of the Nucleon
Form Factor definition
Nucleon current
Breit frame
Elastic Cross Section
= target spin angle w/r to the beam line
World DataWorld Unpolarized Data
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
GE/GM — World Data
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)
Models Consistent with Polarized Data
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
Asymmetry Super-ratio Method
Beam-Target Double Spin Asymmetry
Super-ratio
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
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:
Laser Driven Source (LDS)
Optical pumping& Spin Exchange
Spincell designTarget and
PolarimeterResults
Spin-Exchange Optical pumping
LDS Experimental Setup
Pictures of the LDS
Atomic Dissociation
Atomic Polarization
Comparison of Polarized Targets
BLAST Detector PackageDetector Requirements
Definition of qe 2, e .°, z 1
cm e/p/n/ separation
PID: t 1, Čerenkov Optimize statistics
Large Acceptance Asymmetry Super-ratios
Symmetric Detector Polarized targets
1 m diameter in target regionZero field at targetB-gradients 50 mG/cm
TOF Scintillators
timing resolution: σ=350 psvelocity resolution: σ= 1%
ADC spectrum coplanarity cuts
Cosmics TOF Calibration
L 15
L 12
L 9
L 6
L 3
L 0
R 0 R 3 R 6 channels
channels
TOF Scintillator Cuts
TOF paddle, electron
TO
F p
add
le,
pro
ton
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
Drift Chambers 2 sectors × 3 chambers 954 sense wires 200μm wire resolution signal to noise ratio 20:1
NSED (Online Display)
Reconstruction
Scintillators» timing, calibration
Wire chamber» hits, stubs, segments
» link, track fit
PID, DST
Newton-Rhapson Track Fitter
Hyperbolic timedist function
D
TDC
Linear T2D Calibration
28 MeV
12 MeV
p (GeV/c)
~ 1mm resolution
2
72
33
Wire Chamber Efficiency
WC Offsets/Resolution/Cuts
pe e e ze
pp p p zp
Tracking Efficiency
Comparison of Yields with MC
Experimental Spin Asymmetry
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
Extractions of GE/GM
Single AsymmetryExtraction
Systematic Errors
Q2 (1.8%)» comparison of e and p
» difference between left and right sectorsmost problematic
» appeal to TOF timing !
(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°
GE/GM Results
Extraction of GE and GM
GE and GM ResultsBLAST + World Data
Q2 Corrections from TOF
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 cloudis dip in GE around Q2=0.3 GeV2 real?
systematic errors are being reduced
Top Related