H(e,e

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H(e,ep)n Analysis in BLAST
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• Aaron Maschinot
• Massachusetts Institute of Technology
• Ph.D. Thesis Defense
• 09/02/05

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Outline of Presentation

• Physics Motivation and Theory
• Overview of BLAST Project
• BLAST Drift Chambers
• Data Analysis
• Results and Monte Carlo Comparison
• Summary

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Deuteron Wave Functions
(Bonn Potential)

• NN interaction conserves only total angular
  momentum
• Spin-1 nucleus lies in L 0, 2 admixture state
• Tensor component must be present to allow ?L 2
• Fourier transform into momentum space
• L 2 component is dominant at p 0.3GeV

(Bonn Potential)
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Deuteron Density Functions

• Calculate density functions
• Straightforward form
• Possess azimuthal degree of symmetry
• Famous donut and dumbbell shapes
• In absence of tensor NN component, plots are
  spherical and identical

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Donuts and Dumbbells
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Deuteron Electrodisintegration

• Loosely-bound deuteron readily breaks up
• electromagnetically into two nucleons
• cross section can be
  written as
• In Born approximation, Ae AVd ATed 0
• ATd vanishes in L 0 model for deuteron (i.e. no
  L 2 admixture)
• Measure of L 2 contribution and thus tensor NN
  component
• Reaction mechanism effects (MEC, IC, RC)
  convoluted with tensor contribution
• AVed provides a measure of reaction mechanisms
• Also measure of L 2 contribution
• Provides measurement of beam-vector polarization
  product (hPZ)

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Tensor Asymmetry in PWIA

• In PWIA, ATd is a function of only the missing
  momentum
• ATd has a straightforward form

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The BLAST Project

• Bates Large Acceptance Spectrometer Toroid
• Utilizes polarized beam and polarized targets
• 0.850 GeV longitudinally polarized electron beam
• Vector/tensor 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
• BLAST is ideally suited for comprehensive
  analysis of spin-dependent electromagnetic
  responses of few-body nuclei at momentum
  transfers up to 1(GeV/c)2
• Nucleon form factors
• Deuteron form factors
• Study few body effects, pion production,

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Polarized Electron Beam at Bates

• 0.850 GeV longitudinally-polarized electron beam
• 0.500 GeV linac with recirculator
• Polarized laser incident on GaAs crystal
• 25 minute lifetime at 200 mA ring current
• Polarization measured via Compton polarimeter
• Polarization amount of back-scattered photons
• Nondestructive measurement of polarization
• Beam helicity flipped with each fill
• Long-term beam polarization stability
• Average beam polarization 65 4

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The BLAST Targets

• Internal Atomic Beam Source (ABS) target
• Hydrogen and deuteron gas targets
• Rapidly switch between polarization states
• Hydrogen polarization in two-state mode
• Vector Pz ? -Pz
• Deuteron polarization in three-state mode
• (Vector, Tensor)
  
• (-Pz, Pzz) ( Pz, Pzz)
• (0, -2Pzz)
• Flow 2.6 ? 1016 atoms/s
• Density 6.0 ? 1013 atoms/cm2
• Luminosity 4.6 ? 1031 /cm2/s _at_ 160mA
• Actual polarization magnitudes from data analysis
• Pz 86 5, Pzz 68 6

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The BLAST Spectrometer
BEAM

• Left-right symmetric detector
• Simultaneous parallel and
• perpendicular asymmetry
• determination
• Large acceptance
• Covers 0.1(GeV/c)2 Q2 0.8(GeV/c)2
• Out-of-plane measurements
• DRIFT CHAMBERS
• momentum determination,
• kinematic variables
• CERENKOV COUNTERS
• electron/pion discrimination
• SCINTILLATORS
• TOF, particle identification
• NEUTRON COUNTERS
• neutron determination
• MAGNETIC COILS
• 3.8kG toroidal field

DRIFT CHAMBERS
TARGET
CERENKOV COUNTERS
BEAM
NEUTRON COUNTERS
SCINTILLATORS
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Drift Chamber Theory

• Charged particles leave stochastic trail of
  ionized electrons
• Apply uniform electric field
• Function of HV wire setup
• Electrons drift to readout wires
• Series of accelerations and decelerations
• Electron amplification near readout wires (105)
• Pulses ? TDCs ? distances

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Drift Chamber Design

• Three drift chambers in either detector sector
• Each chamber consists of two layers of drift
  cells
• Each drift cell consists of three sense wires
• 3 ? 2 ? 3 18 hits per track
• 1000 total sense wires
• 9000 total field wires

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Drift Wire Tensioning

• Wire positions must be known accurately (10 µm)
• Wires strung under tension
• Resist electromagnetism, gravity
• Chambers pre-stressed before wiring
• Tension must be measured
• AC signal on HV DC level
• Induces charge on nearby wires
• Wires vibrate in EM field
• Stop generating signal
• Only harmonic frequency remains after 100 ms
• Readout voltage info
• FFT to get wires tension

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Detector Performance

• All detectors operating at or near designed level
• Drift chambers 98 efficient per wire
• TOF resolution of 300 ps
• Clean event selection
• Cerenkov counters 85 efficient in electron/pion
  discrimination
• Neutron counters 10 (25-30) efficient in left
  (right) sectors
• Reconstruction resolutions good but still being
  improved

current goal
?p 3 2
?? 0.5 0.3
?? 0.6 0.5º
?z 1 cm 1 cm
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Deuteron Data Summary

• Runs consist of multiple fills and all (beam,
  target) spin states
• Beam helicity flipped every fill (25 min)
• Target (vector,tensor) state shuffled
  semi-randomly (5 min)
• All states in each run (60 min)
• Deuteron data set taken during June - October
  2004
• 400 kC (150 pb-1) of data collected
• 5700k 2H(e,ep)n events

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Monte Carlo 2H(e,ep)n Asymmetries

• Based on theoretical model from H. Arenhövel
• Emphasis on Bonn potential but others considered,
  too (e.g. Paris and V18)
• Reaction mechanism effects considered (e.g. FSI,
  MEC, IC, RC)
• Detector acceptance taken into account in Monte
  Carlo results
• Target polarization vector, , set at 32º on
  left side
• Can access different (i.e. parallel and
  perpendicular) asymmetry components

electron side side asymmetry component
left right perpendicular
right left parallel
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Kinematics Monte Carlo Vs. Data

• Compare electron and proton momenta
• Polar angle, ?
• Azimuthal angle, ?
• Magnitude, p
• Good agreement in polar and azimuthal angles
• Momentum magnitudes show nonnegligible
  discrepancies

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Momentum Magnitude Corrections

• Nonnegligible discrepancies with momentum
  magnitudes
• reconstruction errors
• energy loss
• Empirical fits needed to match-up data
• Shift data peak to match MC for different Q2
  bins
• Fit correction factors to polynomial function in
  Q2

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Missing Mass

• Only scattered electron and proton are detected
• Actually measure 2H(e,ep)X
• Need extra 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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Missing Momentum Magnitude, pM
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Background Contributions

• Empty target runs provide a measure of
  background
• Negligible contribution at small pM , 5
  contribution at large pM
• 1 contribution for all cos ?M
• Beam collimator greatly reduces background

f vs pM
f vs cos ?M
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Tensor Asymmetry Vs pM
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Tensor Asymmetry Vs pM
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Tensor Asymmetry Vs cos ?M
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Tensor Asymmetry Vs cos ?M
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Beam-Vector Asymmetry Vs pM
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Beam-Vector Asymmetry Vs pM
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Target Angle Systematic Error

• Polarization set nominally at 32
• Variation with vertex position
• Good agreement between holding field map and T20
  calculations
• Polarization angle known to 1
• Uncertainty introduces asymmetry error
• Studied via Monte Carlo perturbation
• Negligible contribution to beam-vector
  asymmetries
• Dominant contribution to tensor asymetries at
  high pM

?d
z
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Target Polarization Systematic Error

• Polarization uncertainty leads to asymmetry
  error
• Dominant contribution to beam-vector asymmetries
• Dominant contribution to tensor asymmetries at
  low pM
• Contribution comparable to tensor asymmetry spin
  angle error at high pM

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False Asymmetries

• 2H(e,ep)n AVd, Ae, and ATed asymmetries are very
  small
• All three vanish in PWIA
• Inconsistency implies target polarization
  deviations
• Nonequal PZ/PZZ magnitudes in different states
• False asymmetries consistent with zero

AVd
Ae
ATed
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Determining hPZ

• Need to determine beam-vector polarization
  product (hPZ)
• Determination of GnE
• Determination of beam-vector asymmetries
• In QE limit, 2H(e,ep)n is well understood
• reduces to H(e,ep) with spectator n
• lt1 model error for pM lt 0.15 GeV/c
• Compare to Arenhovels deuteron model
• uses dipole form factors
• low-Q2 extraction is most reliable

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Dipole Form Factor Corrections

• Arenhovel uses dipole nucleon form factors
• Use elastic e-p beam-vector asymmetry
• Use more realistic parameterization
• Friedrich and Walcher
• Eur. Phys. J. A17607-623 (2003)
• Compute FW to dipole asymmetry ratio
• r 1.01 (1.02) for perp (para) kinematics

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hPZ Results and Systematic Error

• Dominant error from spin angle determination
  uncertainty
• Overall, hPZ 0.558 0.007
• Target has PZ 0.86 0.05

SOURCE CONTRIBUTION
Target Polarization Angle 0.004
Dipole Approximation 0.003
NN Potential Dependence 0.003
Missing Mass Cut 0.002
TOTAL 0.006
Perp Kine Para Kine
hPZ DIPOLE 0.572 0.564
rDIP?FW 1.01 1.02
hPZ FW 0.567 0.553
hPZ OVERALL 0.558 0.009STAT 0.006SYST 0.558 0.009STAT 0.006SYST
h 0.65 0.04 0.65 0.04
PZ 0.86 0.05 0.86 0.05
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Summary and Conclusions

• ATd reproduces Monte Carlo results well
• Overall consistency with tensor component
  existence in Arenhovels representation of total
  NN potential
• Evidence of D-state onset at slightly lower pM
  (20MeV/c)
• Importance of reaction mechanism effects
• AVed has same basic form as Monte Carlo
  predictions
• Unexplained rise in asymmetry above predictions
• Importance of reaction mechanism effects
• ABS target vector highly polarized at Pz ? 86

Thank You Very Much!