Precision Measurement of G E p /G M p 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

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

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

Elastic Scattering  Kinematics  Mott Cross Section  Form Factor  Dipole Form Factor

Rosenbluth Separation  Elastic e-p cross section  At fixed Q 2, fit dσ/dΩ vs. tan 2 (θ/2) »Measurement of absolute cross section »Dominated by either G E or G M

Unpolarized World 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

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, preliminary results expected soon

Super Rosenbluth Separation

Theory†  Direct QCD calculations »pQCD scaling at high Q 2 »Lattice QCD  Meson Degrees of Freedom »Vector Meson Dominance (VMD), Lomon 2002 »Dispersion analysis, Höhler et al »VMD + Chiral Perturbation Theory, Mergel et al  QCD based quark models »CQM, Frank et al »Soliton Model, Holzwarth 1996 »Cloudy bag, Lu et al † Nucleon Electromagnetic Form Factors, Haiyan Gao, Int. J. of Mod. Phys. E, 12, No. 1, 1-40(Review) (2003)

QCD Calculations  Perturbative QCD »diverges at low Q 2 »F 2 /F 1 scaling  Lattice QCD »must extrapolate to physical pion mass »quenched calculations

Meson Based Models  Vector Meson Dominance  Dispersion Analysis

Constituent Quark Models  Relativistic CQM  Soliton Model  Cloudy Bag Model  Models in closest agreement with recent JLab results:

Form Factor BATES  New technique: polarized beam and target »exploits unique features of BLAST »different systematics »insensitive to P b and P t  Q 2 = 0.07 – 0.9 (GeV/c) 2 »overlap with JLab data and RpEX (future exp. at Bates to measure r p )

Asymmetry Super-ratio Method  Polarized cross section  Super-ratio

W.H. Bates Accelerator Facility

BLAST Collaboration R. Alarcon, E. Geis, J. Prince, B. Tonguc, A. Young Arizona State University J. Althouse, C. D’Andrea, 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

Polarized Beam and Target  Stored electron beam (80 mA) E b : 0.27–1.1 GeV P b : 0.70  1 H / 2 D target (ABS) L : 1.0×10 32 /cm 2 s P t : 0.50  3 He target L : 1.2×10 33 /cm 2 s P t : 0.50

Compton Polarimeter  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.  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.

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:

ABS Layout

ABS Specifications  Cell geometry: cylindrical 15mm × 400mm  Cell coating: Drifilm  Cell temperature: T=80K  Target thickness: t=4.4×10 13 cm -2 (H)  Polarization: P z = 0.59 (H), 0.78 (D)  Holding field: B=3mT (H), 35mT (D)

ABS Enhancements MEASURED FIELD ON THE POLE TIPS: Magnet kG Magnet kG Magnet kG Magnet kG Magnet kG Magnet kG Magnet kG Sextupole Damage BLAST Field Effect

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.

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

Spin-Exchange Optical pumping

LDS Experimental Setup

LDS Performance  Current Status »Flux: 1.1×10 18 atoms/s »Atomic fraction: 0.56 »Polarization: 0.37  Improvements »Diamond coating instead of drifilm »Double dissociator »Electro-Optic Modulator (EOM)

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

Detector Package  BLAST Torroid  TOF Scintillators  Čerenkov Detectors  Wire Chambers  Neutron Bars, LADS  Software

Detector Requirements »definition of the momentum transfer vector »optimize statistics »polarized targets: Atomic Beam & Laser Driven Sources »simultaneous A-measurements »e/p/n/   separation Large , beam current, luminosity, polarization Coil shape  1 m diameter in target region BLAST field = 0 at target B-gradients  50 mG/cm Symmetric Detector (  ) e  2 ,  e   mrad,  z  1 cm PID

BLAST Toroid

Detector Subframe

TOF Scintillators  timing resolution: σ=245 ps  ADC spectrum  coplanarity cuts

Čerenkov Detectors  1 cm thick aerogel tiles  Refractive index  White reflective paint  % efficiency  5" PMT's, 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 200μ  signal to noise 20:1

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

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

Tracking Resolution

Radiative Corrections  MASCARAD code »A. Afanasev et al., Phys.Rev.D 64, »Covariant calculation with no cutoff parameter »small corrections (<1%) to asymmetry

Cross Section

Projected Results  Statistics »A 1, A 2  Systematics »θ * 1, θ * 2 Δp, Δθ, Δβ Errors are minimized as a function of β (target spin angle)

Conclusion  The super-ratio method exploits unique characteristics of the BLAST detector  This is the first measurement of μG E p /G M p with polarized beam and target  An important complement to JLab data at higher Q 2 values  If in doubt, take a RATIO…