Recent Results from BLAST at MITBates

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Recent Results from BLAST at MIT-Bates
Workshop on Electron-Nucleus Scattering X, Isola
dElba, Italy, June 23-27, 2008
June L. Matthews

• Department of Physics and Laboratory for Nuclear
  Science
• Massachusetts Institute of Technology
• Cambridge, MA USA

with special thanks to M. Kohl, D. Hasell, C.
Crawford
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• Electromagnetic structure of nucleons and light
  nucleiwith spin-dependent electron
  scatteringfrom internal polarized targets at Q2
  lt 1(GeV/c)2
• Longitudinally polarized electron beam (h ?1)
• Polarized (windowless) internal target in storage
  ring isotopically pure, background free
• Detector with large angular and energy
  acceptance simultaneous measurement of all
  reaction channels over complete Q2 range
• Exploit field free region at target to allow
  orientation of target polarization in any
  direction
• Exploit single and double polarization
  observables to keep systematic errors low

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• Nucleon Form FactorsProton and neutron
  electric and magnetic form factors
• Deuteron Structure Charge, quadrupole, and
  magnetic form factorsPolarized quasi-elastic
  electrodisintegration
• Pion electroproductionN ? D(1232) transition
  in inclusive and exclusive processes

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• Stored Beam 850 MeV, gt200 mA, Pe ? 65
• Internal Target Polarized hydrogen,
  vector/tensor polarized deuterium
• Flow 2.2 x 1016 atoms/s
  Density
  6 x 1013 atoms/cm2
• Luminosity 6 x 1031cm-2s-1
• Polarization PH/D ? 80
• Detector Bates Large Acceptance Spectrometer
  Toroid
• Left-right symmetric
• q 20o 80o, f -15o 15o
• 0.1 lt Q2 lt 0.8 (GeV/c)2
• Simultaneous detection of e?, p?, p, n, d

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Monitoring of electronbeam polarization
Injection with longitudinal spinat internal
target
Siberian snake to restorelongitudinal
polarization
In-plane spin transport
Pe 0.65 0.04
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• Compton (g e-) scattering in highly
  relativistic frame
• Angular distribution compressed into narrow
  kinematic cone
• Photon frequencies shifted from visible to gamma
  region
• Detect backscattered photons with compact
  detector at ?180o
• Compton scattering cross section
• Well known theoretically
• Depends on electron spin and photon helicity
• Can extract electron beam polarization by
  measuring asymmetries in scattering rates for
  circularly polarized laser light

• ds/dEg (ds0/dEg)1 PlPeAz(Eg)
• (ds0/dEg) is unpolarized cross section
• (Klein-Nishina)
• Eg is energy of back-scattered photon
• Pl is circular polarization of incident
• photons (l ?1)
• Pe is longitudinal polarization of
• electron beam
• Az(Eg) is longitudinal asymmetry function

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• Design Considerations
• Based on NIKHEF Compton polarimeter
• Located upstream of BLAST target to reduce
  background
• Measures longitudinal projection of electron
  polarization
• Back-scattered gamma trajectory defined by
  electron momentum
• Polarimeter Layout
• Laser in shielded hut 18 m optical path
• Interaction with electron beam in 4 m straight
  section
• Remotely movable mirrors
• CsI gamma detector 10 m from interaction region

Laser exit
SHR Injection Line
Ring Dipole
Interaction Region
Electron beam
Remotely controlled mirrors
Scattered photons
Laser line
CsI detector
Laser hut
BLAST

W. Franklin, T. Akdogan, JLM, T. Zwart et al.
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• Laser
• Solid-state continuous-wave, very stable
• 5W output at 532 nm (green)
• Optical Transport
• Simple, robust lens arrangement for transport to
  IR and focusing
• Mechanical chopper wheel (rotating at 9 Hz)
  allowed background measurements by blocking
  laser beam during time intervals
• Circular polarization state produced by Pockels
  Cell for rapid helicity reversal (during
  background measurements)
• Phase-compensated mirror arrangement
• Interaction Region
• 4 degrees of freedom for laser beam scans
• Laser beam position and angle scanned to
  maximize count rate
• Laser beam intercepts stored electron beam at lt
  2 mrad

Y angle
X angle

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• Polarization measurement performed for each fill
  of ring
• Database of polarization for BLAST experiment in
  blocks of 4 hrs
• Polarization stable within few percent as a
  function of time
• Changes usually correlated with electron beam
  properties
• Mean polarization (2004) 0.654
• Long term errors dominated by systematics

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• Separately prepare mI ½, -½ (hydrogen) and
  with sextupoles and RF transitions
• Switch between states every 5 minutes
• R. Milner, students et al.

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• Separately prepare mI ½, -½ (hydrogen) and
  mI 1, 0, -1 (deuterium)with sextupoles and
  RF transitions
• Switch between states every 5 minutes

R. Milner, students et al.
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• Left-right symmetric
• Large acceptance0.1 lt Q2/(GeV/c)2 lt 0.820o lt q
  lt 80o, -15o lt ? lt 15o
• COILS Bmax 3.8 kG
• DRIFT CHAMBERSTracking, charge
  selectiondp/p3, dq 0.5o
• CERENKOV COUNTERSe/p separation
• SCINTILLATORSTrigger, ToF, PID (p/p)
• NEUTRON COUNTERSNeutron tracking (ToF)

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The BLAST Toroid (Bates)

• 8 copper coils
• to minimize gradients
• at target
• coil positions adjusted
• to minimize target field
• field mapped (3D)
• 1 of calculated field
• 6730 A, 3700 G
• 3 momentum resolution

K. Dow et al.
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Drift Chambers (MIT)

• 954 sense wires 200µm wire resolution signal to
  noise ratio 201

• 3 chambers per sector
• single gas volume
• 2 superlayers per
• chamber (?10o stereo)
• 3 sense layers per superlayer
• 18 layers total tracking
• momentum analysis
• scattering angles
• event vertex
• particle charge

D. Hasell, R. Redwine students
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Cerenkov Detectors (ASU)

• e, ? discrimination
• 1 cm thick aerogel
• n 1.021.03
• 80-90 efficiency

R. Alarcon students
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Time-of-Flight Scintillators (UNH)
350 ps timing resolution 1 velocity resolution
top vs. bottom
elastic timing peak
J. Calarco students
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Neutron Scintillators (Ohio U., Bates)

• 20 detection efficiency
• LADS (PSI, JLab) detectors added on beam-right
  for increased sensitivity in Gen measurement
• TOF scintillators, drift chambers provided good
  charged particle veto

J. Rapaport, M, Kohl
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ABS allows free choice of target spin angle in
horizontal plane 32o (2004) / 47o (2005)
e- left ? q ? 90o Target spin perpendicular to
momentum transfer q
e- right ? q ? 0o Target spin parallel to
momentum transfer q
32o
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• Charge /-
• Coplanarity
• Kinematics
• Timing

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• Very clean quasielastic 2H(e,en) spectra
• Highly efficient proton veto (drift chambers
  TOF)

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TOF (ns)
p
ADC
Cerenkov detector information discriminates p /
e and p- / e- events.
Time correlation for candidate e' p events,
corrected for path-length
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• General definition of the nucleon form factor
• Sachs Form Factors
• In the one-photon exchange approximation the
  above form factors are observables of elastic
  electron- nucleon scattering

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• Double polarization observables in elastic ep
  scatteringwith recoil polarization or polarized
  target
• Polarized cross section
• Double spin asymmetry
• Target polarization components Px sin q
  cos f , Pz cos q
• Measured asymmetry Aexp PePt Aphys
• Scattered electron can be detected in either the
  left (AL) or the right (AR) sector of BLAST
• Super-ratio (AL/AR)exp (AL/AR)phys,
  independent of Pe and Pt

?
?
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• Recall that electron in left (right) sector
  corresponds to target spin perpendicular q
  90o (parallel q 0o) to q
• zL,R, xL,R are kinematic factors

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• Beam and target asymmetries also evaluated
  individually no significant false asymmetries
  detected
• Aphys fit with Höhler parameterization of form
  factors to extract Pb Pt 51.8 ?0.3, 51.9
  ?0.2
• Agreement ?? Confidence in target spin angle as
  determined from measurement of target holding
  field angle
• Value of target spin angle agrees with that
  determined from analysis of T20 in e d scattering
• Radiative corrections small
• 300 kC integrated e- flux
• 90 pb-1 integrated luminosity

Data with electron detected in left and right
sectors
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C.B. Crawford et al., PRL 98 (2007) 052301

• Impact of BLAST data combined with cross
  sections on separation of GpE and GpM
• Errors factor 2 smaller
• Reduced correlation
• Deviation from dipole at low Q2!

Ph.D. work of C. Crawford (MIT) and A. Sindile
(UNH)
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Proton Form Factor Ratio
Jefferson Lab
Dramatic discrepancy!

• All Rosenbluth data from SLAC and JLab in
  agreement
• Dramatic discrepancy between results of
  Rosenbluth and recoil polarization
• Multi-photon exchange considered probable
  explanation

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• Must account for FSI, MEC ,RC, IC
• Perform full Monte Carlo simulation of BLAST
  acceptance using deuteron electrodisintegrationmo
  del of H. Arenhövel
• Spin-perpendicular beam-target vector asymmetry
  AVed shows high sensitivity to GnE
• Compare measured AVedwith simulation, with GnE
  as a free parameter
• Use measured tensor asymmetry to control FSI

Ph.D. work of V. Ziskin (MIT) and E. Geis (ASU)
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E. Geis et al., nucl-ex/0803.3872v2 accepted by
Phys. Rev. Lett.
Ph.D. work of V. Ziskin (MIT) and E. Geis (ASU)
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• Must account for FSI, MEC, RC, IC
• Perform full Monte Carlo simulation of BLAST
  acceptance using deuteron electrodisintegration
  model of H. Arenhövel
• Beam-target vector asymmetry AVed in both
  spin-parallel and spin- perpendicular kinematics
  shows sensitivity to GnM
• Enhanced sensitivity in super-ratio

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Neutron Magnetic Form Factor GnM

• Pre-polarization era
• GnM world data fromunpolarized experiments
• Cross section ratioquasielastic
• CLAS preliminary
• Polarization era
• GnM world data 3He

PRELIMINARY
BLAST preliminary
Ph.D. work of N. Meitanis (MIT) and B. ONeill
(ASU)
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Elastic Electron-Deuteron Scattering

• Spin 1 ? three elastic form factors
  GdC, GdQ, GdM
• Quadrupole momentM2dQd GdQ(0) 25.83
• GdQ ? Tensor force, D-wave
• Unpolarized elastic cross section
• Polarized cross section

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PRELIMINARY
PRELIMINARY
Final result expected soon!
Ph.D. work of C. Zhang (MIT)
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Ph.D. work of C. Zhang (MIT)
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Final result expected soon!
PRELIMINARY
Ph.D. work of C. Zhang (MIT)
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• Vector-polarized elastic ed scattering
• AVed ? T10, T11
• Ph.D. work of P. Karpius (UNH)
• Electrodisintegration D(e,e'p)
• Beam-vector asymmetry as function of pmiss
• Effect of d-state AV changes sign (seen in data)
• Quasielastic tensor asymmetry
• Ph.D. work of A. Maschinot and A. DeGrush (MIT)

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H(e,e')D, H(e,e'p)n, H(e,e'p)p0

• Trigger 1 (Charged)

e-
p
p

• Trigger 7(Inclusive)

• Trigger 2(Neutral)

p n
e-
e-
n
p0,p
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H(e,e')D inclusive

BLAST MAID Sato/Lee
PRELIMINARY
Ph.D. work of O. Filoti (UNH)
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Pion Production Asymmetries

• Dilution factors are determined from elastic
  analysis and the compton polarimeter
• Single Asymmetry, Ah.
• Single Asymmetry, ASz.
• Double Asymmetry, AhSz

Y event yield Q electron charge h electron
helicity Sz target spin state
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H(e,e'p)n and H(e,e'p)p0 exclusive

Double Asymmetry AhSz
PRELIMINARY
p channel
PRELIMINARY
p0 channel
Analysis by A. Shinozaki (MIT) Ph.D. work of Y.
Xiao (MIT)
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D(e,e'p)nn,pp Double Asymmetries

D(e,ep) channel Models p from free p
PRELIMINARY
PRELIMINARY
D(e,ep-) channel Models p- from free n
Analysis by A. Shinozaki (MIT)
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Charge distributions
Neutron
Proton
The Frontiers of Science A Long Range Plan (Dec
2007)
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Isospin and Quark Distributions
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Pre-BLAST Charge Distributions
Nuclear Physics The Core of Matter, The Fuel of
Stars National Research Council (1999)
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• Proton, neutron, and deuteron spin observables
  measured with polarized electron beam
• ? High precision, excellent control
  of systematics
• Nucleon structure
• Deuteron structure
• Pion production from H and D

• Consistent and precise determination of elastic
  nucleon form factors at low momentum transfer?
  Structure at low Q2 beyond dipole form factor

• Precision measurement of T20 allows separation of
  GdC and GdQ
• First measurement of T11 allows determination of
  GdM at low Q2
• Asymmetries in electrodisintegration probes
  d-state in deuteron
• wave function

• Single and double spin asymmetries in N? D
  transition (H)
• Double and tensor asymmetries in pion production
  on D

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• BLAST A GREAT SUCCESS!!!
• First-class single and double polarization data
  on H and D in elastic, quasielastic and Delta
  region
• Produced 9 Ph.D.s 3 more to come

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Future of BLAST?
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Proton Form Factor Ratio
Jefferson Lab
Dramatic discrepancy!

• All Rosenbluth data from SLAC and JLab in
  agreement
• Dramatic discrepancy between results of
  Rosenbluth and recoil polarization
• Multi-photon exchange considered probable
  explanation

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One- and two-photon amplitudes will interfere
interference term has opposite sign for e and e-
scattering
Ratio of cross sections for positron-proton
and electron-proton elastic scattering (P.
Blunden) as a function of virtual photon
polarization
BLAST _at_ 2.3 GeV
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pOsitron-proton and eLectron-proton elastic
scattering to test the hYpothesis
of Multi- Photon exchange Using DoriS
2008 Full proposal (in preparation) 2009
Transfer and setup of BLAST 2010 Engineering run
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1000 hours each for e and e- Lumi2x1033 cm-2s-1
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Super-ratio
Cycle of four states e ? , BLAST magnetic field
polarity ? Repeat cycle many times

• Change between electrons and positrons regularly
• Change BLAST polarity every day
• Left-right symmetry provides additional
  redundancy two identical experiments
  simultaneously taking data

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• USA
• Arizona State University
• University of Colorado
• Hampton University
• University of Kentucky
• Massachusetts Institute of Technology
• University of New Hampshire
• Germany
• Universität Bonn
• DESY, Hamburg
• Universität Erlangen-Nürnberg
• Universität Mainz
• Italy
• INFN, Ferrara
• INFN, Frascati
• INFN, Rome
• Russia
• St. Petersburg Nuclear Physics Institute
• United Kingdom

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• The current dramatic discrepancy between recoil
  polarization and Rosenbluth measurements of the
  elastic form factor ratio GEp/GMp constitutes a
  serious challenge to our understanding of the
  structure of the proton.
• The widely accepted explanation in terms of
  multiple photon exchange demands a definitive
  confirmation. A precision measurement of the
  ep/e-p cross section ratio will directly test
  the contribution of multiple photon effects.
• As the prediction of the magnitude of these
  effects is model-dependent, the experiment
  described here will provide a strong constraint
  on theoretical calculations.
• The proposed experiment takes advantage of unique
  features of the BLAST detector combined with an
  internal hydrogen gas target and the DORIS
  storage ring operated with both electrons and
  positrons.
• The systematic uncertainties are controllable at
  the percent level, and with the superior
  luminosity that can be provided at DORIS, this
  experiment will not be limited in statistical
  precision.

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• BLAST OLYMPUS has a future
• Stay tuned!

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BACKUP slides
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Relativistic effects involve factors of
Product gigf minimized in the Breit frame where
q/2
q
-q/2
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Final result expected soon!
PRELIMINARY
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Ph.D. work of P. Karpius (UNH)
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Deuteron Electrodisintegration

• Quasielastic breakupe d ? e p n
• D(e,ep), PWIApm q pp -pp,I
• Beam-vector asymmetry(PWIA)
• Nucleon spins parallel ?
  changes sign

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Deuteron Electrodisintegration
D(e,ep) momentum distribution
(Mainz, Bates, Nikhef)

• D-wave dominant at pmgt300 MeV/c
• FSI,MEC,IC subtle effects in cross section lt
  450 MeV/c

PRELIMINARY
D(e,ep) beam-vector asymmetry Observing expected
sign change!
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Ph.D. work of A. Maschinot and A. DeGrush (MIT)
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Quasielastic Tensor Asymmetry
PRELIMINARY
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M0
M1
Ph.D. work of A. Maschinot and A. DeGrush (MIT)
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H(e,e)D inclusive

5k ev. / 299 kC 3.7M elastic
PRELIMINARY
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Ph.D. work of O. Filoti (UNH)
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H(e,ep)n Double Asymmetry

PRELIMINARY
Analyses by A. Shinozaki (MIT) Ph.D. work of Y.
Xiao (MIT)
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H(e,ep)p0 Double Asymmetry

PRELIMINARY
Analyses by A. Shinozaki (MIT) Ph.D. work of Y.
Xiao (MIT)
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Determination of the spin angle
2005 ?d32
2004 ?d47