Proton Form Factor Measurements with Polarization Method

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Proton Form Factor Measurements with Polarization
Method
L.Pentchev The College of William and Mary For
the GEp-2g and GEp-III collaborations
JLab , June 8-10, 2009
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Outline

• GEp-III (E04-108) and GEp-2g (E04-019)
  experiments
• Polarization transfer method, experimental
  set-up, kinematics
• Elastic/background separation
• Spin transport in HMS
• GEp-2g experiment precise (1) measurement of
  two polarization quantities test of the limits
  of Born approximation in polarization method
• 2g exchange theoretical calculations
• Longitudinal transferred polarization
  (preliminary results), beam polarization
  measurements
• e-dependence of the form factor ratio
  (preliminary results)
• Reconstruction of the real part of the ep elastic
  amplitudes
• GEp-III measurement of the proton form factor at
  high Q2
• Preliminary results
• Comparison with theoretical calculation,
  asymptotic behavior
• Summary

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Polarization Method
In Born (one-photon exchange) approximation

• Form Factor ratio can be obtained without knowing
  analyzing power, Ay, and beam helicity, h, (both
  cancel out in the ratio), and without measuring
  cross-section.
• Systematic uncertainty dominated by the spin
  transport from the polarimeter to the target.

A.I.Akhiezer and M.P.Rekalo, Sov.J.Part.Nucl. 3,
277 (1974) R.Arnold, C.Carlson, and F.Gross,
Phys. Rev. C 23, 363 (1981)
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GEP-3 and GEP-2gamma experimental set-up in Hall
C
e
e
Big E.M. Calorimeter
p
High Momentum Spectrometer Double Focal Plane
Polarimeter
1.87- 5.71 GeV beam 80-100 mA beam current 80-85
pol. 20cm LH target
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Detectors

• Changes in standard HMS detector package
• Focal Plane Polarimeter with Double Analyzer
• -gt 70 increased efficiency (30 for FOM)
• Scintillator plane S0 in front of drift chambers
• -gt deteriorates angular resolution but
  needed for triggering

• 1744 channel E.M. Calorimeter (BigCal)
• from (due to radiation
  damage) needed for triggering
• beter than 10 mm position resolution most
  important parameter for elastic separation

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Goal of The Experiments

• GEp-2gamma e dependence of R at 2.5 GeV2

• KEY IDEA OF THE METHOD FIXED Q2
• same spin transport
• same analyzing power

Two polarization observables are measured Pt/Pl
and Pl separately
precision limited only by statistics ( 1),
very small p.t.p systematics Ay , h cancel out
in the Pt/Pl ratio Q2 fixed, Pp fixed, spin
precession fixed

• GEp-3 high Q2 measurements

• 5.2 GeV2 point overlapping with GEp-II (4.0 and
  5.6 GeV2)
• two higher Q2 points

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Data analyses elastic separation
All triggers Elastics after BigCal-HMS
correlations Estimated background Range used in
analyses
2.5 GeV2 e0.15
8.5 GeV2 e0.24
s0.10
s0.11
Background contribution 13 Absolute
correction to mR 0.10
Background contribution 0.5 Correction to mR
0.35

• (PCAL-PHMS)/P0 gives better resolution then
  (Pqp-PHMS)/P0, because of worse HMS angular
  resolution
• Background estimated by interpolation, dominated
  by g p -gt p0 p
• Polarization of the background measured below
  the elastic peak looking at events with hits at
  the calorimeter outside expected position of the
  elastic electron (p0 -gt gg)

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Spin transport in HMS
QQQD type spectrometer rotations are additive in
the quads and total precession is sum of
dispersive (main) and non-dispersive precession
Dispersive precession
Non-dispersive precession
Allows to use simple geometrical model, giving
results very similar to COSY calculations used
for the results presented here

• Non-dispersive precession the dominant source
  of systematics, because it mixes the two
  polarization components in the reaction plane
• Requires very good knowledge of non-dispersive
  bend angle Df
• uncertainty of Df used for the preliminary
  analyses of 1mrad
• using dedicated optical studies, we expect to
  reduce the uncertainty by factor of 3

2.5 GeV2 e0.15
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GEp/GMp Crisis discrepancy in the data

• The discrepancy is a serious problem as it
  generates confusion and doubt about the whole
  methodology of lepton scattering experiments
• P.A.M. Guichon and
• M.Vanderhaeghen

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GEp-2g Beyond Born Approximation
Mo and Tsai, and others

• prescriptions for radiative corrections commonly
  used
• two-photon exchange (e), (f) only with one
  soft photon, neglecting proton structure

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Generalized Form Factors (ep elastic amplitudes)
this experiment
e/e- x-section ratio Rosenbluth non-linearity
Born Approximation
Beyond Born Approximation
P.A.M. Guichon and M.Vanderhaeghen,
Phys.Rev.Lett. 91, 142303 (2003) M.P. Rekalo and
E. Tomasi-Gustafsson, E.P.J. A 22, 331 (2004)
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Two-Photon Exchange theoretical predictions

• Hadronic calculations
• P.Blunden et al., Phys.Rev.C72 034612 (2005)
  elastic (at the figure)
• S.Kondratyuk et al., Phys.Rev.Lett. 95 172503
  (2005) including Delta reduces the effect
• S.Kondratyuk et al., nucl-th/0701003 (2007)
  including 1/2 and 3/2 resonances no effect

GPD A.Afanasev et al., Phys.Rev.D72013008
(2005) GPD models Gauss (figure), smaller
effect with Regge, or non-zero quark mass Valid
at high e region (vertical line at figure)

LO pQCD N. Kivel and M. Vanderhaeghen
arXiv0905.0282 hep-ph LO pQCD using two
different distribution amplitude models BLW
(good agreement with lattice QCD!) and COZ Valid
in high e region (vertical line at figure)
.
Both theories describe Rosenbluth data but have
opposite predictions for mGE/GM
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Longitudinal transferred polarization stability
of the measurements
Beam polarization dominant source of systematic
error for PL measurements
PRELIMINARY

• open circles this experiment
• (hAyPl)meas/(Plborn Ay(q))
• filled circles Moller measurements of beam
  polarization (h)
• open boxes (connected with line) beam
  polarization predicted from quantum efficiency
  measurements (Dave Gaskell, private comm.)
• 1.873 GeV beam energy, e0.15
• 2.846 GeV e0.64
  
• 3.549 GeV e0.78
• 3.680 GeV e0.79

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Longitudinal transferred polarization stability
of the measurements

• open circles this experiment
• (hAyPl)meas/(Plborn Ay(q))
• filled circles Moller measurements of beam
  polarization (h)
• open boxes (connected with line) beam
  polazrization predicted from quantum efficiency
  measurements (Dave Gaskell, private comm.)
• 1.873 GeV beam energy, e0.15
• 2.846 GeV e0.64
  
• 3.549 GeV e0.78
• 3.680 GeV e0.79

PRELIMINARY
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Preliminary results longitudinal polarization
PRELIMINARY
PRELIMINARY
NO RADIATIVE CORRECTIONS APPLIED, Less than 1
(Afanasev et.al, Phys.Rev. D64 (2001) 113009)
Uncertainties in the overall normalization of the
data due to uncertainties in Ay
Beam polarization p.t.p. systematics 0.5
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Preliminary results form factor ratio
PRELIMINARY
Narrow acc. matching all kinematics
Wide acc. matching e0.64 and e0.79
Theoretical predictions are with respect to the
Born approximation
NO RADIATIVE CORRECTIONS APPLIED, Less than 1
(Afanasev et.al, Phys.Rev. D64 (2001) 113009)
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GEP3 preliminary results FF ratio

• Results at 2.5 and 5.2 GeV2 agree (within one
  sigma) with previous Hall A results
• No zero crossing slower decrease with Q2

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GEP3 results

• No evidence for the Q2 F2/F1 scaling
• Modified (logarithmic) scaling still holds

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CONCLUSIONS

• GEp-2g POLARIZATION METHOD PASSED THE TEST no
  evidence for effects beyond Born approximation at
  2 level in the polarization data at Q2 of 2.5
  GeV2
• Slight deviations from Born approximation at
  two sigma level both of longitudinal polarization
  and of form factor ratio require further
  investigations possible standard radiative
  corrections, not applied yet
• The preliminary results do not exclude with high
  confidence any of existing 2g-exchange
  theoretical models yet high-e data favor GPD and
  pQCD models. Expected reduction of systematic
  error and especially, knowledge of Born FF ratio
  (from e/e- experiments) will greatly help in
  constraining theoretical predictions.
• Measuring two polarization observables for a
  fixed Q2 in a wide kinematical range with 1
  precision allows to constrain the real parts of
  both, ratio of the generalized electric to
  magnetic form factors, and the third non-Born
  amplitude contribution Y2g, without model
  assumptions.
• GEp-III First high Q2 proton FF ratio
  measurements outside Hall A confirm previous
  results at one sigma level, though Hall C data
  possibly slightly higher
• New FF ratio data up to 8.5 GeV2 exhibit slower
  decrease with Q2 (favoring existing VMD, GPD
  models) still consistent with modified
  (logarithmic) scaling of F2/F1 no zero crossing
  yet
• Measurements above 8.5 GeV2 with 12 GeV machine
  are certainly very important

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BACK-UP SLIDES STARTING HERE
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Elastic amplitude reconstruction

• Three observables measured at 2.5 GeV2
• Pt/Pl
• AyPl
• ds

PRELIMINARY
Important note Elastic amplitude reconstruction
is different from full Born / non-Born
separation need e/e- data and triple
polarization observables (M.P.Rekalo and E.
Tomasi-Gustafsson Nucl.Phys.A740271-286,2004)
Still here one can constrain the contribution
from the third non-Born amplitude Y2g.
Three amplitudes (Re parts) RmRe(GE)/Re(GM),
Y2g, Re(GM) and Ay unknown Plotted Re(GM)
(ds, Pt/Pl,R), Y2g(Pt/Pl,R),
Ay(AyPl,R)
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Background corrections
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Two-Photon Exchange theoretical predictions

• Hadronic calculations
• P.Blunden et al., Phys.Rev.C72 034612 (2005)
  elastic (Figure)
• S.Kondratyuk et al., Phys.Rev.Lett. 95 172503
  (2005) including Delta reduces the effect
• S.Kondratyuk et al., nucl-th/0701003 (2007)
  including 1/2 and 3/2 resonances no effect

• Yu. Bystricky, E.A.Kuraev, E. Tomasi-Gustafsson
  Phys. Rev. C75, 015207 (2007) structure function
  method 2g effects small, higher orders change
  Rosenbluth slope (Figure)
• D.Borisuyk, A.Kobushkin arXiv0804.4128 proton
  off-shell form factors are not needed to
  calculate TPE amplitudes

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Two-Photon Exchange theoretical predictions
GPD calculations

• Absolute correction to FF ratio mGe/Gm
• slow Q2 variation, strong effects at low e
• valid for high Q2 or high e

• A.Afanasev et al., Phys.Rev.D72013008 (2005)
  GPD models Gauss on Fig., smaller effect with
  Regge, or non-zero quark mass

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Analyzing Power
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Polarization Method Spin Transport
Non-dispersive precession
Dispersive precession
Target
Target
to Reaction Plane
Reaction Plane
Longitudinal and transverse polarizations Pt and
Pl are helicity dependent (transferred) Normal
polarization Pn is helicity independent zero in
Born approximation
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GEp/GMp Crisis asymptotic behavior
Dirac and Pauli form factors
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Polarization Method Systematics
Relate the evolution of the velocity (trajectory)
to the evolution of the spin
COSY Geom. Approx.
Geometrical Approx.
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High Q2 Measurements