a precision measurement of g e p /g m p with blast

A Precision Measurement of GE

p/GMp with BLAST

Chris Crawford

MIT Laboratory for Nuclear Science

for the BLAST collaboration

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

QED Lamb shift

World Data

Unpolarized Data» CEA, Bonn, DESY, SLAC» Christy et al. (JLab 2004)» Arrington et al. (JLab )

Polarization Transfer» Milbrath et al. (BATES) 1999» Jones et al. (JLab), 2000» Dieterich et al. (MAMI), 2001 » Gayou et al. (JLab), 2002

Theory» quark orbital momentum» 2 effects

World Data

Unpolarized Data» CEA, Bonn, DESY, SLAC» Christy et al. (JLab 2004)» Arrington et al. (JLab )

Polarization Transfer» Milbrath et al. (BATES) 1999» Jones et al. (JLab), 2000» Dieterich et al. (MAMI), 2001 » Gayou et al. (JLab), 2002

Theory» quark orbital momentum» 2 effects

Unpolarized Data» CEA, Bonn, DESY, SLAC» Christy et al. (JLab 2004)» Arrington et al. (JLab )

Polarization Transfer» Milbrath et al. (BATES) 1999» Jones et al. (JLab), 2000» Dieterich et al. (MAMI), 2001 » Gayou et al. (JLab), 2002

Theory» quark orbital momentum» 2 effects

World Data

MIT-Bates Exp. 01-01

new technique

precision testof theoriesat low Q2

meson cloud structure

Spokespersons: H. Gao, J. Calarco, H. Kolster

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 transport

Q2 = 0.1 – 0.8 (GeV/c) 2 » overlap with JLab data

Asymmetry Super-ratio Method

Helicity Asymmetry

Super-ratio

±

Polarized Beam and Target

Storage Ring» E = 850 MeV» Iinj=175 mA» Pb = 0.65

Internal ABS Target» 40 cm storage cell» t = 2.71013 cm-2 » Pt = 0.40

isotopically pure internal targethigh polarization, fast spin reversal L = 1.6 1031 cm-2s-1

Detector Package

BLAST Coils

Backward-Angle TOF

TOF Scintillators

Drift Chambers

Čerenkov Detectors

Neutron Detectors

LADS Detectors

BLAST

Data Sets

1H run 1, December 2003» reversed BLAST field – electrons out-bending

» 20 kC beam (3.4 pb-1) Pz=.45 480k elastic events

1H run 2, April 2004» nominal BLAST field – electrons in-bending

» 57 kC beam (9.6 pb-1) Pz=.40 950k elastic events

2D run, May – October 2004 » 450 kC beam (76 pb-1) Pz=.75

1H run 3, November 2004» ~250 kC (42 pb-1) planned

TOF Scintillator Cuts

TOF paddle, electron

TO

F p

add

le,

pro

ton

Resolution and Yields

TOF paddle #

preliminary

Experimental Spin Asymmetry

GE/GM by Super-ratio Method

GE/GM by Fit Method

fit for:» take advantage of large acceptance

» optional: also fit for spin angle)

super-ratio method: » in each Q2 bin

» redundant measurement of polarization

AL(0.16) AR(0.16)

AL(0.26) AR(0.28)

AL(0.41) AR(0.41)

AL(0.54) AR(0.54)

Preliminary Results

» combineddatasets(77 kC)

» preliminary results based on ¼ of the projected data

Preliminary Results

» combineddatasets(77 kC)

» preliminary results based on ¼ of the projected data

Conclusion

new precision measurement of GEp/GM

p

exploits unique capabilities of BLAST 2 – 4% projected errors at low Q2