Nonpole Backgrounds in the Extraction of Fp

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Non-pole Backgrounds in the Extraction of Fp
H. Avakian, P. Bosted, H. Fenker, R. Feuerbach,
D. Gaskell, D. Higinbotham, T. Horn, M. Jones,
D. Mack, C. Butuceanu, G. Huber, A. Sarty, W.
Boeglin, P. Markowitz, J. Reinhold, D. Dutta, V.
Koubarovski, P. Stoler, A. Asaturyan, A.
Mkrtchyan, H. Mkrtchyan, V. Tadevosyan, E. Brash,
K. Aniol, J. Calarco, P. King, J. Roche JLab,
Regina, Saint Marys, Florida International,
Mississippi State, RPI, Yerevan, CNU, California
State, New Hampshire, Ohio University

• Motivation
• Experimental Details
• Summary

Hall A Collaboration Meeting
January 2006
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Extracting Fp from sL data in p production

• In t-pole approximation
• Want smallest possible -t to ensure t-channel
  dominance

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Results from Fp-2

• The VGL Regge model describes sL for p well
• Note that at tmin (maximal pole contribution)
  still only have sL/sT 1 at Q22.45 GeV2
• Constraint on non-pole backgrounds requires
  experimental data

Horn et al., Phys. Rev. Lett. 97, 192001 (2006)
Vanderhaeghen, Guidal and Laget, Phys. Rev. C57,
1454 (1998).
4
Context

• Understanding of hadronic structure via
  measurement of Fp is one of the high priorities
  at 12 GeV
• Extraction of Fp relies on pion pole dominance
  what about other processes?
• Limited knowledge of non-pole contributions
  limits kinematic range of Fp measurement
• Interpretation on experimental data widely
  considered reliable only below t0.2 GeV2
• This kinematic contraint is the primary reason
  why we are limited to Q22.5 GeV2 at JLab at 6 GeV

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Size of non-pole contributions

• CarlsonMilana indicated a significant
  contribution of non-leading processes
  complicating the extraction of Fp
• Background ratio rises dramatically once tmingt0.2
• Other theoretical predictions can be obtained
  from
• VGL/Regge model
• GPD formalism

Interpretation of Fp data considered reliable for
-tlt0.2 GeV2

• But constructing an upper bound on -t difficult
  due to poor quality of existing data.

Carlson Milana, Phys. Rev. Lett. 65, 1717
(1990)
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Motivation -

• Non-pole contributions can be constrained using
  the po longitudinal cross section
• Can be related to the one from p using e.g. GPD
  formalism
• Many studies of po unseparated cross sections in
  the resonance region, but contribution of sL
  effectively unknown above the resonance region
• JLab preliminary data from Hall A (DVCS,
  Q21.5-2.5 GeV2, W1.9-2.3 GeV) and Hall B (e16,
  Q21-5 GeV2) available both unseparated

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Theoretical Predictions for p and po cross
sections
Separated sL

• Theoretical models based on Regge and GPD
  formalism describe sL for p quite well
• But po prediction for sL differs by order of
  magnitude
• Theoretical uncertainty quite large
• Preliminary unseparated po data from Hall A/B in
  this kinematic region
• No information on relative sL contribution

Vanderhaeghen, Guidal and Laget, Phys. Rev. C57,
1454 (1998).
Vanderhaeghen, Guichon and Guidal, Phys. Rev.
D60 (1999).
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Non-pole contributions in the GPD Framework
VGG/GPD prediction

• Amplitudes for p and po composed of the same
  GPDs, but different linear combinations

po
p

• Obtain non-pole contributions by comparing po and
  p production amplitudes, MLApNBpN
• In the limit t ?(mp)2 the p amplitude contains a
  strong singularity (pion pole)

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Motivation Summary

• Constraining the non-pole contributions in the
  extraction of Fp requires experimental data
• Systematic measurement of po cross section could
  constrain the size
• If the non-pole contributions are smaller than
  anticipated this would significantly increase
  the kinematic range accessible for the Fp
  measurement at 12 GeV
• Constraining the contribution of sL in po
  production will allow for easier planning of
  Rosenbluth separations

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Cross Section Separation via Rosenbluth Technique

• Cross Section Extraction
• For uniform f-acceptance, sTT, sLT 0 when
  integrated over f
• Determine sT e sL for high and low e in each
  t-bin for each Q2
• Isolate sL, by varying photon polarization, e
• Small sL makes traditional Rosenbluth separation
  difficult due to unfavorable error propagation
  with two different acceptances

VGG/GPD
VGL/Regge
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Cross Sections via Recoil Polarization

• In parallel kinematics can relate sL/sT to recoil
  polarization observables
• Avoids some of the adverse systematic effects due
  to small R in Rosenbluth technique.

• Using the properties of the independent helicity
  amplitudes in parallel kinematics sL/sT is
  related to Pz

• From the combination of R and s0 one can obtain sL

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Experiment Overview

• 100uA, 5.75 GeV beam, 80 polarized, 10-cm LH2
  target
• Standard Hall A setup
• Coincidence measurement with recoil proton into
  HRS with FPP and electrons in the electron arm,
  H(e,ep)po
• FPP analyzing power relatively large in this
  region
• Kinematics chosen to overlap with p data from
  Fp-2 and pCT allowing for direct comparison

Fp-2
pCT
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Parallel Kinematics

• For recoil polarization analysis all data taken
  in parallel kinematics
• Cuts in ?/f select events
• Taking data to left and right of virtual photon
  could allow for t-dependent studies of
  unseparated cross section

• Radial coordinate ?
• Azimuthal coordinate f

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FPP analyzing power

• Low momentum protons lt760 MeV Los Alamos fit
  applicable, McNaughton et al., Nucl. Instrum.
  Meth. A241, 435 (1985)
• But in 2006 LEDEX took data for similar proton
  momenta in Hall A, so use this
• FoM relatively large for proposed kinematics

Preliminary LEDEX data courtesy of R. Gilman et
al.
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Hard Photon Backgrounds

• Reconstructed photon smeared out under po peak
  Mx cuts not useful
• Use simulation for fitting both peaks
• Subtract bin-by-bin from azimuthal dependence of
  the asymmetry in FPP
• Relative contribution of po and hard photon
  background in good agreement with Hall B
  preliminary data
• Relative scaling based on VGL po cross sections
  and VGG DVCSBH cross sections

Q23.8, W2.0, x0.55
po
?
Vanderhaeghen, Guichon and Guidal, Phys. Rev.
D60 (1999).
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DVCS and Bethe-Heitler Contributions

• Contribution of hard photons requires full
  background subtraction.
• Bethe-Heitler process dominates photon cross
  section
• Contribution of DVCS to total cross section
  10-20
• Bethe-Heitler propagators are no problem for
  these kinematics

Vanderhaeghen, Guichon and Guidal, Phys. Rev.
D60 (1999).
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Other Backgrounds

• End caps subtracted using dummy target data
• Online singles rates are low, removed with
  offline cuts
• Electron momentum is too low for elastics to be
  in acceptance
• Missing mass cuts separate po/?

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What is needed?

• Standard Hall A HRS configuration with FPP
• Installation of standard 10-cm LH2 cryotarget
• 5.75 GeV beam, 80 polarization
• Some FPP checkout and calibration

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Projected Uncertainties

• Measure po cross section at Q22.45, 4.0 GeV2,
  where p data are available
• Statistical uncertainty in recoil polarization
  measurement dominates uncertainty
• Measure sL/s0 contribution to 10

po
p
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Projected Uncertainties

• Fixed W scan at three values of Q2 up to Q24.0
  GeV2
• Measure relative sL/s0 contribution of to 10

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Beam Time Estimate

• Elastic data for proton absorption studies and
  calibration
• Spectrometer angle and momentum changes
• Some FPP checkout
• Polarization measurement

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Summary

• Non-pole contributions in the extraction of Fp
  are an unresolved issue constraint on non-pole
  backgrounds requires experimental data.
• Open door for larger kinematic reach in Fp
  measurement at 12 GeV
• Increase knowledge of largely unknown po sL at
  large Q2 above resonance region
• Easier planning for future Rosenbluth separation
  exp.
• Requires 17 days of data at 5.75 GeV
• An relatively easy experiment making use of
  existing Hall A standard equipment.