Precision Measurement of GE

p/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

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

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

Elastic Scattering

Kinematics

Mott Cross Section

Form Factor

Dipole Form Factor

Rosenbluth Separation

Elastic e-p cross section

At fixed Q2, fit dσ/dΩ vs. tan2(θ/2)» Measurement of absolute cross section

» Dominated by either GE or GM

Unpolarized World 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

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 Q2

» Lattice QCDMeson 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)

QCD Calculations

Perturbative QCD» diverges at low Q2

» F2/F1 scaling

Lattice QCD» must extrapolate to

physical pion mass

» quenched calculations

Meson Based Models

Vector Meson Dominance Dispersion Analysis

Constituent Quark Models

Relativistic CQMSoliton ModelCloudy Bag Model

Models in closest agreement with recent JLab results:

Form Factor Ratio @ 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 data

and RpEX (future exp.at Bates to measure rp)

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 TechnologyBates 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) Eb: 0.27–1.1 GeV Pb: 0.70

1H / 2D target (ABS) L: 1.0×1032/cm2 s Pt: 0.50

3He target L: 1.2×1033/cm2 s Pt: 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 technologyDissociator & nozzle2 sextupole systems3 RF transitions

1

3

2

4nozzle

6-pole

1

2MFT (2->3)

1

3

6-pole

1Spin State Selection:

ABS Layout

ABS Specifications

Cell geometry: cylindrical 15mm × 400mmCell coating: DrifilmCell temperature: T=80KTarget thickness: t=4.4×1013 cm-2 (H) Polarization: Pz = 0.59 (H), 0.78 (D)

Holding field: B=3mT (H), 35mT (D)

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 designTarget and

PolarimeterResults

Spin-Exchange Optical pumping

LDS Experimental Setup

LDS Performance

Current Status» Flux: 1.1×1018 atoms/s» Atomic fraction: 0.56» Polarization: 0.37

Improvements» Diamond coating instead of drifilm» Double dissociator» Electro-Optic Modulator (EOM)

Detector Package

BLAST TorroidTOF ScintillatorsČerenkov DetectorsWire ChambersNeutron Bars, LADSSoftware

Detector Requirements» definition of the momentum transfer vector

» optimize statistics

» polarized targets: Atomic Beam & Laser Driven Sources

» simultaneous A-measurements

» e/p/n/ separation

NeL

Large , beam current, luminosity, polarization

Coil shape 1 m diameter in target region BLAST field = 0 at targetB-gradients 50 mG/cm

Symmetric Detector

()e 2 , e mrad, z 1 cm

PID

BLAST Toroid

Detector Subframe

TOF Scintillators

timing resolution: σ=245 psADC spectrumcoplanarity cuts

Čerenkov 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

Wire Chambers

2 sectors × 3 chambers 954 sense wires resolution 200μ signal to noise 20:1

Software

BLASTmc – Monte Carlo using Geant321BlastLib2 – recon library based on ROOT

» integrated on-line display » and offline reconstruction

CODA – data acquisitionEPICS – 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 64,113009

» Covariant calculation with no cutoff parameter

» small corrections (<1%) to asymmetry

Cross Section

Projected Results

Statistics» A1, A2

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 μGEp/GM

p with polarized beam and target

An important complement to JLab data at higher Q2 values

If in doubt, take a RATIO…