Analysis of de,epn in BLAST

1
Analysis of d(e,ep)n in BLAST

• Aaron Maschinot
• Massachusetts Institute of Technology
• Spin 2004 Conference
• Trieste, Italy

2
Deuteron Electro-disintegration

• Loosely-bound deuterium readily breaks up
  electromagnetically into two nucleons
• e d ? e p n
• Most generally, the d(e,eN)N cross section can
  be written as
• In the Born approximation,
• Additionally, vanishes in the L 0 model
  for the deuteron (i.e. no L 2 admixture)
• is a good measure of L 2 component
• Also, is also a good measure of L 2 as
  well as subnuclear degrees of freedom (e.g. MEC,
  IC, RC)

3
The BLAST Program

• Bates Large Acceptance Spectrometer Toroid
• Located at the MIT-Bates Linear Accelerator
  Facility in Massachusetts, USA
• Utilizes polarized beam and polarized targets
• 0.850GeV longitudinally polarized electron beam
• polarized internal atomic beam source (ABS)
  target
• Large acceptance, left-right symmetric
  spectrometer detector
• simultaneous parallel/perpendicular,
  in-plane/out-of-plane asymmetry measurements
• Toroidal magnetic field
• Ideally suited for a comprehensive analysis of
  the spin-dependent electromagnetic response of
  few-body nuclei at momentum transfers up to 1GeV2

4
Polarized Beam at Bates

• 1GeV longitudinally polarized electron beam
• 0.5GeV linear accelerator with recirculator
• Polarized beam fills South Hall storage ring
• location of BLAST experiment
• Longitudinal polarization maintained by Siberian
  snakes
• 25 minute lifetime _at_ 175mA ring current

5
Beam Polarization Measurements

• Beam polarization measured via a Compton
  polarimeter
• polarization amount of back-scattered photons
• nondestructive measurement of polarization
• Long-term beam polarization stability
• average beam polarization 65 4

PRELIMINARY
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The BLAST Targets

• Internal Atomic Beam Source (ABS) target
• Hydrogen and Deuterium gas targets
• Can quickly switch between polarization states
• Hydrogen polarization in two-state mode
• Vector Pz ? -Pz
• Deuterium polarization in tri-state mode
• (Vector, Tensor)
  
• (-Pz, Pzz) ( Pz, Pzz)
• (0, -2Pzz)
• Flow 2.2 ? 1016 atoms/s,
  Density 6.0 ? 1013 atoms/cm2,
  Luminosity 4.0 ? 1031
  /cm2/s _at_ 140mA
• Actual polarization magnitudes from data analysis
• 3He target ready for future running

7
The BLAST Spectrometer

• Left-right symmetric detector
• simultaneous parallel and perpendicular asymmetry
  determination
• Large acceptance
• covers 0.1GeV2 Q2 1GeV2
• out-of-plane measurements
• DRIFT CHAMBERS
• momentum determination, particle identification
• CERENKOV COUNTERS
• electron/pion discrimination
• SCINTILLATORS
• TOF, particle identification
• NEUTRON COUNTERS
• neutron determination
• MAGNETIC COILS
• 4.5kG toroidal field

BEAM
DRIFT CHAMBERS
TARGET
CERENKOV COUNTERS
BEAM
NEUTRON COUNTERS
SCINTILLATORS
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Drift Chambers

• Three wire chambers on either side
• Two superlayers of cells per chamber
• Three sense wires per cell
• 3 ? 2 ? 3 18 hit wires for ionizing particle
• 954 total sense wires, 9888 total wires

• Large acceptance
• 20 ? 80 ,
• -17 ? 17
• 1sr total solid angle
• Each wire 98 efficient

9
Event Reconstruction

• C OOP reconstruction library using ROOT
• Resolutions are a work in progress
• much progress has been made in the last six
  months

.
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Missing Mass and Momentum

• Only the e- and p are measured
• actually measure d(e,ep)X
• need cuts to ensure that X n
• Define missing energy, momentum, and mass
• Demanding that mM mn helps ensure that X n

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Monte Carlo d(e,ep)n Asymmetries

• Using theoretical model from H. Arenhövel
• Data take into account detector acceptance
• Target polarization vector, ,set at 32º on
  the left side
• can access different asymmetry components

12
Background Contributions

• Empty target runs provide a measure of background
• Negligible contribution at small pM
• Larger contribution at high pM due to scattering
  off of Aluminum target

Perpendicular
Parallel
13
Beam-Vector Asymmetry Results

• 200kC of data analyzed so far
• 450kC projected total data
• Vector polarization determined from fitting
  asymmetry below pM 0.15GeV
• Visible correlation with full subnuclear-effects
  model

14
Tensor Asymmetry Results

• Tensor polarization from independent T20 fit
• L2 dips reproducible in the data
• Still working on systematic checks results are
  preliminary

15
Determining the Vector Polarization

• In the quasi-elastic (QE) limit, d(e,ep)n is
  well understood
• reduces to p(e,ep) with spectator n
• lt1 model uncertainty in
• Large asymmetry, high detector efficiency ? small
  statistical uncertainty
• QE d(e,ep)n ? pM ? pN 0
• small uncertainty up to pM 0.15GeV

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Conclusions

• Both the d(e,ep)n beam-vector and tensor
  asymmetries are good measures of the L 2
  deuterium component.
• The d(e,ep)n beam-vector asymmetry is a good
  measure of subnuclear effects (and relativistic
  corrections).
• Both asymmetries are being measured in BLAST
• Final asymmetry results with 450kC expected
  within six months
• Results will offer much discerning power between
  models