Measurement of the Electric Form Factor of the Neutron at low Q2 from a Vector Polarized Deuterium Target at BLAST

1
Measurement of the Electric Form Factor of the
Neutron at low Q2 from a Vector Polarized
Deuterium Target at BLAST
Vitaliy Ziskin PhD Thesis March 29, 2005
2
Measurement of the Electric Form Factor of the
Neutron at low Q2 from a Vector Polarized
Deuterium Target at BLAST

• BLAST Experiment
• Polarized Hydrogen and Deuterium Gas Target
• EM Structure of the Deuteron
• Measurements of GnE from Unpolarized
  Electron-Deuteron Scattering
• Measurements of GnE from Polarized Quasielastic
  Electron Scattering on Deuteron
• BLAST Results
• Phenomenological Fit to the Results
• Conclusion

Vitaliy Ziskin PhD Thesis March 29, 2005
3
Polarized Electron Beam in the South Hall Ring

• Laser driven GaAs crystal is a source of the
  polarized electron
• 500 MeV Linac with a recirculator capable of up
  to 1 GeV beams
• Siberian snake in the east straight section to
  cancel g-2 spin precession
• Compton polarimeter is used to monitor beam
  polarization
• Injection current is as high as 200 mA
• Average current during the experiment Iave 95
  mA
• Average beam polarization Pe 65 4

4
BLAST Detector

• Eight symmetrically spaced coils provide toroidal
  field which was carefully mapped
• Two sectors are instrumented with Drift Chambers,
  TOFs, Cerenkov counters and Neutron.
• Detectors Drift Chambers, TOFs and Cerenkov
  counters are mounted on a subframe. Can be moved
  out of the coil region

5
Drift Chambers

• Three trapezoidal drift chambers share the same
  gas volume
• 2 super layers with 3 layers of sense wires in
  each chamber
• Define the BLAST acceptance
• Used in charged particle veto for enhanced veto
  efficiency
• Resolutions

Design Measured
D pe 2 3
D qe 0.3 0.45
D fe 0.5 0.56
D ze 1 cm 1cm
6
Neutron Detector

• Enhanced detection in the right sector
• Negatively charged track in coincidence with a
  straight track that leaves no wire hits in the WC
  and no hit in TOF
• Ohio Wall and Large Acceptance Detector System
• Average detection efficiency is 30 in the right
  sector and 10 in the left
• Cosmic events are used to calibrate neutron
  timing
• Time-walk correction is applied
• Flasher system is used to monitor timing shifts
  during the experiment

7
Polarized 1H and 2H Gas Target

• Received from NIKHEF in 2000
• Has to fit inside of two BLAST coils
• Operates in high BLAST toroidal field
• Operations must be reliable and stable over a
  long period of time
• Improvement of figure-of-merit
• Rapid switching between various spin states and
  deuterium and hydrogen gases

8
RF Dissociator

• 1H2 or 2H2 gas is injected by PGFS
• Fixed RF frequency of 27.12 MHz
• Coil ( L ), Capacitor ( C ) and Plasma ( R ) make
  an LRC circuit with Q-value of 150
• DI water is used to cool the discharge plasma
• Tuning network consisting of two capacitors is
  used to tune the dissociator RF circuit
• Nozzle is cooled to avoid recombination and to
  cool the atomic beam for more efficient focusing
• 0.5 of O2 is injected

9
Dissociator Performance
hydrogen

• Fraction of dissociation was studied with
  Quadrupole Mass Analyzer (QMA)
• kdet is a function of QMA acceptance and gas
  type only and kv is a function of relative
  velocities
• aH/D gt 90 is achieved for all flows into
  dissociator

aH
deuterium
aD
10
Focusing in the Sextupole System

• The force on atom in sextupole in absence of
  external magnetic field
• In presence of external magnetic field
• Parameter bBext/B0 is a function of external
  magnetic field, pole-tip field strength
• Force magnitude remains the same, while the
  direction changes
• In a weak b regime the force is the weaker but in
  the right direction
• In a strong b regime Fx remains the same, while
  Fy completely reverses direction

11
Focusing Simulation

• The effect of the external magnetic field was put
  into the ray tracing program
• The atomic beam has a Maxwellian velocity
  distribution

• The external field reduces the intensity almost a
  factor of two

12
Hyper-Fine Structure of a Single Electron Atom

• Interaction Hamiltonian
• At B0 the multiplet is doubly degenerate
• At Bgt0 splits into 2F1 levels

hydrogen
deuterium
13
Polarization States
14
RF Transitions

• s-trasition
• Only states that are a function of the external
  magnetic field exchange populations
• Selection rule D mF 0

• p-trasition
• Selection rule D mF 1

• Since atoms in the beam have different velocities
  the transition efficiency is limited
• In the presence of the gradient field the
  efficiency of an RF transition is close to 100 if

15
Medium, Weak and Strong Field Transitions
p - transition
s - transition
SFT
MFT 3-4
MFT 1-4
16
Polarization Scheme at BLAST
17
Polarized Gas in the Storage Cell

• Triangular distribution
• Drifilm coating
• Beam Collimator to protect the cell
• Cooled to 90 K

• Polarization loss is due to

1. Recombination on the walls
2. Wall depolarization
3. Spin-exchange collisions

18
Performance (Atomic Beam Intensity)

• The intensity is limited by the rest gas
  scattering
• where Q is the flow into the dissociator.
• I0 is a function of focusing in sextupoles,
  fraction of dissociation, etc.
• Q0 is a function of vacuum in the ABS
• Iave ¼2.5 1016 atoms/sec
• rave ¼4.5 1013 cm-2
• Lave ¼2.7 1031 cm-2s-1

19
Performance (Polarization)

• Polarization is measured in nuclear reaction,
  2H(e,ep)n for Pz and 2H(e,ed) for Pzz
• Polarization remained stable over the course of
  the experiments
• Average polarizations in deuteriumPz 80 4
  Pzz 68 6
• Average polarization in hydrogenPz 80 4

20
Why Measure GnE

• GnE is a very sensitive test of the QCD models in
  the non-perturbative limit (low Q2)
• Relativistic quark models predict charge
  distribution in the neutron due to the quark
  interactions (SU6 breaking, hyper-fine, etc.)
• Most successful effective field models include
  both relativistic quarks and a pion cloud (cloudy
  bag models)
• GnE is the least known of all four EM form
  factors
• Precise value of GnE, particularly at low
  momentum transfer, is of great interest to parity
  violating experiments

21
Neutron Form Factor Measurements

• No free neutron targets. Instead, nuclear targets
  with a loosely bound neutron are used, typically
  2H2 and more recently 3He2
• Inclusive and exclusive quasielastic cross
  section measurements are sensitive to GnM only
• Large systematic uncertainty in neutron detection
  efficiency
• Polarization techniques are used in stead for
  both GnM and especially GnE

22
Deuteron Nuclear Structure

• Measurement in this work are done on Deuterium
• The only bound N-N system
• Jp 1, Eb 2.225 MeV
• 96 S-wave and 4 D-wave
• Electric quadrupole moment
• Magnetic Moment

23
Elastic Electron Scattering on Deuterium

• Unpolarized cross section
• Rosenbluth separation can be used
• EM current of the deuterium is characterized by
  the elastic form factors, GC, GM and GQ with
  boundary conditions determined by static
  properties

• T20 observable from tensor polarized target is
  used to determine all three elastic form factors

24
Extraction of GnE from the Elastic Scattering on
Deuterium

• At low Q2 , A(Q2) is largely determined by GC
• In non-relativistic approximation with no
  exchange currents
• GC(Q2) GsE(Q2)DC(Q2)
• GQ(Q2) GsE(Q2)DQ(Q2)
• Body Form Factors
• Isoscalar Form Factors

25
Galster Analysis

• Platchkov et al. used existing and new data on
  A(Q2) and best potentials available in 1990 to
  fit Galster form
• Best fit was obtained with Paris potential
• a 1.25 0.13, b 18.3 3.4
• However, the neutron charge radius prediction is
  violated
• ltr2ngtchexp -0.115 0.003 0.004

26
Measurements of GnE with 2H(e,en)p

• In PWBA vector polarization direction of a
  nucleon in the S-wave is parallel to the
  deuterium targets polarization
• Target polarization tensor is
• Polarized differential cross section is

• AVed is measured in spin-perpendicular kinematics

27
Electro-Disintegration of Deuterium

• Target polarization angle is set in plane at 32
  into beam left sector
• Perpendicular kinematics is
• Corresponds to right sector at BLAST
• q varies with Q2

28
Sensitivity to GnE and Reaction Mechanism

• In PWBA vector polarization observable in
  perpendicular kinematics is
• where t Q2/4M2n

• Sensitivity to FSI, MEC, IC and RC on the neutron
  side
• No sensitivity on proton side
• Allows to measure hPz

(e,en)
(e,ep)
29
Experimental details

• Start date 05/29/2004
• Finish date 10/15/2004
• 3-State injection vector plus, vector minus and
  tensor minus states
• Total charge 420 kC
• Total luminocity 1.32 mb-1
• Average quasielastic rate 0.1 Hz
• Total number of quasielastic neutrons 0.27 M in
  three target polarization states
• Five Q2 points in the rage of 0.1 lt Q2 lt 0.60
  (GeV/c)2
• Higher Q2 data are available

30
Identification of the 2H(e,en)p events
Time of flight ? g/neutron separation
31
Experimental Asymmetry

• Experimental vector beam-target asymmetry with
  unpolarized background
• The true asymmetry is determined as
• is a ratio between target full and empty
  target rate

32
BLAST Monte Carlo

• Calculations by H. Arenhovel using Bonn potential
• Full GEANT model of the BLAST detector
• Calculations on a grid are extrapolated

• Reconstructed variables are convoluted with
  realistic resolutions
• No radiative effects are taken into account

33
Extraction of GnE from the Experimental Asymmetry

• Experimental asymmetry is compared with BLASTMC
  predictions with various a0, 0.5, 1.0, 1.5 and
  2.0
• Minimize c2, where c2 is defined as
• Parabolic around c2min

34
Results
ltQ2gt (GeV/c)2 GnE/GnM D GnE/GnM (stat) D GnE/GnM (syst) C2min/ndf
0.14 0.0438 0.0070 0.0031 1.05
0.20 0.0463 0.0062 0.0036 1.27
0.29 0.0624 0.0076 0.0039 0.40
0.38 0.0537 0.0099 0.0040 1.08
0.50 0.0519 0.0155 0.0039 0.55
35
Systematic Uncertainties (Target Angle)

• Significant sensitivity to the target
  polarization angle due to a contribution from the
  parallel asymmetry proportional to (GnM)2. 12
  per degree
• BLASTMC uses holding field map
• Good agreement between target polarization angle
  from the holding field map and T20 calculations
• Error has been determined by the precision of T20
  analysis
• Contributes 5 to the total systematic uncertainty

qd
z
36
Systematic Uncertainties(Product of Beam and
Target Polarizations)

• Product of beam and target polarizations (hPz) is
  determined from 2H(e,ep)n reaction
• Verified with 2H(e,en)p in parallel kinematics
• Variation as a function of Q2, limited in the low
  Q2 region
• Contributes 2.5 of Galster to total systematic
  uncertainty

37
Systematic Uncertainties (Radiative Corrections)

• Radiative corrections are not implemented into
  BLASTMC
• Though the effect on the cross section is
  sizable, most of the radiative corrections are
  not beam and target helicity dependent
• Contribution is lt 1.0

38
Systematic Uncertainties (Value of GnM)

• Only GnM from more recent experiment is used
• Friedrich and Walcher parameterization is used
• Contribution to the systematic uncertainty is
  1.5

39
Phenomenological Fit to the Results
40
Phenomenological Fit to the Results

• Platchkov fit with the slope at Q2 0, with
  a0.903(fixed) and b3.47
• Does not fit well to high Q2 data

41
Phenomenological Fit to the Results

• Platchkov fit with the slope at Q2 0, with
  a0.903(fixed) and b3.47
• Does not fit well to high Q2 data

• A1 collaboration fit Friedrich and Walcher
  parameterization

42
Phenomenological Fit to the Results

• Platchkov fit with the slope at Q2 0, with
  a0.903(fixed) and b3.47
• Does not fit well to high Q2 data

• A1 collaboration fit Friedrich and Walcher
  parameterization

• A1 fit under predicts slope at Q2. Slope is
  defined as
• BLAST fit explicitly constrains the slope

43
BLAST Fit

• At Q2 0 slope is dominated by the bump term in
  the parameterization

• At Q2 0 the uncertainty is due to the precision
  of the neutron charge radius

44
Density from the BLAST Fit

• Non-relativistic Fourier transform of the neutron
  form factor
• Smooth dipole corresponds to the constituent
  quark core
• Bump corresponds to a diffuse pion cloud

45
QCD Models
46
Conclusion
Thank You !!!

• Measurements of the neutron form factor at five
  Q2 points have been accomplished
• A new BLAST phenomenological fit achieves good
  agreement with data at low and high Q2 regimes
• BLAST fit of GnE has a precision of DGnE / GnE ¼
  6.5
• More data is available for analysis
• Systematic uncertainties are expected to be
  significantly reduced

47
(No Transcript)
48
Focusing in the Sextupole System
Force in the Absence of
Force in the Presence of
49
Breit-Rabi Polarimeter
Magnet coils
Magnet pole tips
Vacuum Chamber
Ion Gauge
Compression Tubes

• Quadrupole Mass Analyzer can not be used in a
  magnetic field