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