Precision Measurement of GEp/GMp with BLAST

1
Precision Measurement of GEp/GMp with BLAST

• Chris Crawford
• MIT Laboratory for Nuclear Science
• Ricardo Alarcon, John Calarco, Ben Clasie, Haiyan
  Gao, Hauke Kolster, Jason Seely, Tim Smith,
  Vitaliy Ziskin, and the BLAST Collaboration

2
Outline

• Introduction and Motivation
• Theoretical calculations
• Existing Measurements
• Rosenbluth technique
• Recoil proton polarization (FPP)
• Super Rosenbluth
• BLAST Experiment
• Asymmetry super-ratio method
• Polarized beam, polarized targets, detectors
• Projected Results

3
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

4
Elastic Scattering

• Kinematics
• Mott Cross Section

• Form Factor
• Dipole Form Factor

5
Rosenbluth Separation

• Elastic e-p cross section
• At fixed Q2, fit ds/dO vs. tan2(?/2)
• Measurement of absolute cross section
• Dominated by either GE or GM

6
Unpolarized World Data
7
Polarization Transfer

• Recoil proton polarization
• Focal Plane Polarimeter
• recoil proton scatters off secondary 12C target
• Pt, Pl measured fromf distribution
• Pb, and analyzing powercancel out in ratio

8
World Data

• Unpolarized Data
• Polarization Transfer
• Milbrath et al. (BATES) 1999
• Jones et al. (JLAB), 2000
• Dieterich et al. (MAMI), 2001
• Gayou et al. (JLAB), 2002
• Super-Rosenbluth
• JLab Hall A, preliminaryresults expected soon

9
Super Rosenbluth Separation
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Theory

• Direct QCD calculations
• pQCD scaling at high Q2
• Lattice QCD
• Meson Degrees of Freedom
• Vector Meson Dominance (VMD), Lomon 2002
• Dispersion analysis, Höhler et al. 1976
• VMD Chiral Perturbation Theory, Mergel et al.
  1996
• QCD based quark models
• CQM, Frank et al. 1996
• Soliton Model, Holzwarth 1996
• Cloudy bag, Lu et al. 1998

Nucleon Electromagnetic Form Factors, Haiyan
Gao, Int. J. of Mod. Phys. E, 12, No. 1,
1-40(Review) (2003)
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QCD Calculations

• Lattice QCD
• must extrapolate tophysical pion mass
• quenched calculations

• Perturbative QCD
• diverges at low Q2
• F2/F1 scaling

12
Meson Based Models

• Vector Meson Dominance

• Dispersion Analysis

13
Constituent Quark Models

• Relativistic CQM
• Soliton Model
• Cloudy Bag Model

• Models in closest agreement with recent JLab
  results

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Form Factor Ratio _at_ BATES

• New technique polarized beam and target
• exploits unique features of BLAST
• different systematics
• insensitive to Pb and Pt
• Q2 0.07 0.9 (GeV/c) 2
• overlap with JLab dataand RpEX (future exp.at
  Bates to measure rp)

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Asymmetry Super-ratio Method

• Polarized cross section
• Super-ratio

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W.H. Bates Accelerator Facility
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BLAST Collaboration

• R. Alarcon, E. Geis, J. Prince, B. Tonguc, A.
  Young
• Arizona State University
•  J. Althouse, C. DAndrea, A. Goodhue,
  J. Pavel, T. Smith,
• Dartmouth College
• T. Akdogan, W. Bertozzi, T. Botto, M. Chtangeev,
  B. Clasie, C. Crawford, A. Degrush, K. Dow,
  M. Farkhondeh, W. Franklin, S. Gilad,
  D. Hasell, E. Ilhoff, J. Kelsey, H. Kolster,
  A. Maschinot, J. Matthews, N.
  Meitanis, R. Milner, R. Redwine, J.
  Seely, S.Sobczynski, C. Tschalaer, E.
  Tsentalovich, W. Turchinetz, Y. Xiao,
  H. Xiang, C. Zhang, V. Ziskin, T.
  Zwart
• Massachusetts Institute of Technology
• Bates Linear Accelerator Center

• D. Dutta, H. Gao, W. Xu
• Duke University
• J. Calarco, W. Hersman, M. Holtrop,
  O. Filoti, P. Karpius, A. Sindile, T. Lee
• University of New Hampshire 
• J. Rapaport
• Ohio University 
• K. McIlhany, A. Mosser
• United States Naval Academy
•  J. F. J. van den Brand, H. J. Bulten,
  H. R. Poolman
• Vrije Universitaet and NIKHEF 
• W. Haeberli, T. Wise
• University of Wisconsin

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Polarized Beam and Target

• Stored electron beam (80 mA) Eb 0.271.1 GeV
  Pb 0.70
• 1H / 2D target (ABS) L 1.01032/cm2 s Pt
  0.50
• 3He target L 1.21033/cm2 s Pt 0.50

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Compton Polarimeter

• Full photon energy spectrum measured as function
  of laser helicity and for background
• Polarization measurements made at currents up to
  130 mA. Signal to background ratio worsens at
  high currents but still tractable.

• Polarization about 0.70 typical
• Statistical precision of measurements governed
  mostly by signal-to-background ratio. Typical
  precision of 1-2 per hour.
• Systematic errors estimated at 5 level
  presently. Working on reducing these through
  improved analysis of energy spectrum.

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Atomic Beam Source

• Standard technology
• Dissociator nozzle
• 2 sextupole systems
• 3 RF transitions

Spin State Selection
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ABS Layout
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ABS Specifications

• Cell geometry cylindrical 15mm 400mm
• Cell coating Drifilm
• Cell temperature T80K
• Target thickness t4.41013 cm-2 (H)
• Polarization Pz 0.59 (H), 0.78 (D)
• Holding field B3mT (H), 35mT (D)

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ABS Enhancements
Sextupole Damage
MEASURED FIELD ON THE POLE TIPS Magnet 1 8.3
kG Magnet 2 9.1 kG Magnet 3 9.9 kG Magnet 4
11.6 kG Magnet 5 15.6 kG Magnet 6 15.4
kG Magnet 7 15.2 kG
BLAST Field Effect
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Ion polarimeter
Ions produced by electron beam inside the storage
cell are extracted and accelerated by
electrostatic lenses. The spherical deflector
directs ions into the polarimeter arm. The Wien
Filter provides mass separation, and nuclear
reaction with large analyzing power is used to
measure nuclear polarization. Currently, the
tritium target is not installed yet, and Ion
Polarimeter is used as a mass spectrometer.
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Laser Driven Source (LDS)

• Optical pumping Spin Exchange
• Spincell design
• Target and Polarimeter
• Results

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Spin-Exchange Optical pumping
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LDS Experimental Setup
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LDS Performance

• Current Status
• Flux 1.11018 atoms/s
• Atomic fraction 0.56
• Polarization 0.37
• Improvements
• Diamond coating instead of drifilm
• Double dissociator
• Electro-Optic Modulator (EOM)

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Comparison LDS vs ABS

• ABS well established technology
• High polarization
• deuterium tensor
• nuclear vector
• Pure atomic species

• LDS Advantages
• Higher FOM
• Higher target thickness
• Compact design
• LDS Disadvantages
• Deterioration of the coating over time due to
  alkali vapor after operating 100 hrs
• Low D tensor polarization
• Additional dilution from the pumping alkali

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Detector Requirements

• Definition of the momentum transfer vector
• (????)e ? 2 ?, ??e ? ? mrad, ?z ? 1 cm
• Optimize statistics
• Large ?, luminosity, polarization
• Polarized targets Atomic Beam Laser Driven
  Sources
• Coil shape ? 1 m diameter in target region
• BLAST field 0 at target
• B-gradients ? 50 mG/cm
• Simultaneous A-measurements
• Symmetric Detector
• e/p/n/?? separation
• PID

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Detector Package

• BLAST Torroid
• TOF Scintillators
• Cerenkov Detectors
• Wire Chambers
• Neutron Bars, LADS
• Software

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BLAST Toroid
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Detector Subframe
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TOF Scintillators

• timing resolution s245 ps
• ADC spectrum
• coplanarity cuts

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Cerenkov Detectors

• 1 cm thick aerogel tiles
• Refractive index 1.02-1.03
• White reflective paint
• 80-90 efficiency

• 5" PMT's, 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 201

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Software

• BLASTmc Monte Carlo using Geant321
• BlastLib2 recon library based on ROOT
• integrated on-line display
• and offline reconstruction
• CODA data acquisition
• EPICS slow controls

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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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Tracking Resolution
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Radiative Corrections

• MASCARAD code
• A. Afanasev et al., Phys.Rev.D 64,113009
• Covariant calculation with no cutoff parameter
• small corrections (lt1) to asymmetry

42
Cross Section
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Projected Results

• Statistics
• A1, A2
• Systematics
• ?1, ?2 ?p, ??, ?ß

• Errors are minimized as a function of ß (target
  spin angle)

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Conclusion

• The super-ratio method exploits unique
  characteristics of the BLAST detector
• This is the first measurement of µGEp/GMp with
  polarized beam and target
• An important complement to JLab data at higher Q2
  values
• If in doubt, take a RATIO