A Precision Measurement of G E p /G M p with BLAST Chris Crawford Thesis Defense April 29, 2005

Outline  Introduction »Formalism »World Data »Experiment overview  Experimental Setup »LDS polarized target »BLAST detector »Calibrations  Analysis »Cuts & yields »Asymmetry »Extraction of  G E /G M »Systematic errors  Conclusion »Results:  G E /G M »Separation of G E, G M

Introduction  G E,G M 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 Data World Unpolarized Data

Polarization Transfer  Recoil proton polarization  Focal Plane Polarimeter »recoil proton scatters off secondary 12 C target »P t, P l measured from φ distribution »P b, and analyzing power cancel out in ratio

 G E /G M — World Data

Theory and Models  Direct QCD calculations »pQCD scaling at high Q 2 »Lattice QCD  Meson Degrees of Freedom »Dispersion analysis, Höhler et al »Soliton Model, Holzwarth 1996 »VMD + Chiral Perturbation Theory, Kubis et al »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 BATES  Exploits unique features of BLAST »internal target: low dilution, fast spin reversal »large acceptance: simultaneously measure all Q 2 points »symmetric detector: ratio measurement  Different systematics »also insensitive to P b and P t »no spin transport  Q 2 = 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 » I max =225 mA » P b = 0.65  Internal ABS Target » 60 cm storage cell » t = 4.9  cm -2 » P t = 0.80  isotopically pure internal target  high polarization, fast spin reversal  L = 3.1  cm -2 s -1  H 2 : 98 pb -1 D 2 : 126 pb run

Atomic Beam Source  Standard technology  Dissociator & nozzle  2 sextupole systems  3 RF transitions nozzle 6-pole 1 2 MFT (2->3) pole 1 Spin State Selection:

Laser Driven Source (LDS)  Optical pumping & Spin Exchange  Spincell design  Target and Polarimeter  Results

Spin-Exchange Optical pumping

LDS Experimental Setup

Comparison of Polarized Targets

BLAST Detector Package Detector Requirements  Definition of q  e  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 region Zero field at target B-gradients  50 mG/cm

TOF Scintillators  timing resolution: σ=350 ps  velocity resolution: σ= 1% ADC spectrum coplanarity cuts

Cosmics TOF Calibration L 15 L 12 L 9 L 6 L 3 L 0 R 0R 3R 6 channels

TOF Efficiency green: efficiency magenta: non-bias red: misses

TOF Scintillator Cuts TOF paddle, electron TOF paddle, proton

Čerenkov Detectors  1 cm thick aerogel tiles  Refractive index  White reflective paint  % efficiency  5" PMTs, sensitive to 0.5 Gauss  Initial problems with B field  Required additional shielding  50% efficiency without shielding

Wire Chambers  2 sectors × 3 chambers  954 sense wires  resolution 200μm  signal to noise 20:1

Reconstruction  Scintillators »timing, calibration  Wire chamber »hits, stubs, segments »link, track fit  PID, DST

Newton-Rhapson Track Fitter

Hyperbolic time  dist function D TDC

Linear T2D Calibration 28 MeV 12 MeV  p (GeV/c) ~ 1mm resolution 2

Wire Chamber Efficiency

Tracking Efficiency

WC Offsets/Resolution/Cuts p e -p e (  e )p p -p p (  e ) p-p(e)p-p(e) p-p(e)p-p(e)z  p - z  p (z e ) p e  e  e z e p p  p  p z p

Resolution and Yields TOF paddle # preliminary

Experimental Spin Asymmetry

Single-asymmetry Method  measure P first, use to calculate R »model-dependent Super-ratio Method  2 equations in P, R in each Q 2 bin j »independent measure of polarization in each bin! »2n parameters P j, R j Global Fit Method  fit for P, R 1, R 2, … from all A ij together »model independent »better statistics »n+1 parameters »can also fit for  i = left,right sector j = Q 2 bin (1..n)  = spin angle

Extractions of  G E /G M

Systematic Errors   Q 2 (1.8%) »comparison of  e and  p »difference between left/right sector errors most significant »TOF timing will help   (0.8%) »fieldmap: 47.1° ± 1° »Hohler: 47.5° ± 0.8° »Fit Method: 42° ± 3° » (1 st 7 bins) 48° ± 4° »T 20 analysis: 46.5° ± 3°

 G E /G M Results

Extraction of G E and G M

G E and G M Results BLAST + World Data

Conclusion  1 st measurement of  G E /G M using double spin asymmetry  2 – 3.5× improvement in precision of  G E /G M at Q 2 = 0.1– 0.5 GeV 2  sensitive to the pion cloud  narrow dip structure observed in G E around Q 2 =0.3 GeV 2 ?  systematic errors are being reduced