Electromagnetic Form Factors John Arrington Argonne National Lab

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Electromagnetic Form Factors John
ArringtonArgonne National Lab

• Long Range Plan QCD Town Meeting
• Piscataway, NJ, 12 Jan 2007

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Nucleon Form Factors

• Fundamental properties of the nucleon
• Connected to charge, magnetization distribution
• Crucial testing ground for models of the nucleon
  internal structure
• Necessary input for experiments probing nuclear
  structure, or trying to understand modification
  of nucleon structure in nuclear medium
• Recent revolution in experiments last 5-10 yrs
• Dramatically improved precision, Q2 coverage
• New program of parity-violating measurements
• Revelation of importance of two-photon exchange
• Driving renewed activity on theory side
• Models trying to explain all four electromagnetic
  form factors
• Trying to explain data at both low and high Q2
• Progress in QCD based calculations

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Status Ten Years Ago (end of 1997)
Proton
Neutron
Range allowed by e-d elastic
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Unpolarized Elastic e-N Scattering

• Nearly all of these measurements used Rosenbluth
  separation
• sR ds/dW e(1t)/sMott tGM2 eGE2
  t Q2/4M2

• Reduced sensitivity to
• GM if Q2 ltlt 1
• GE if Q2 gtgt 1
• GE if GE2ltltGM2 (e.g. neutron)
• Form factor extraction is very sensitive to
  angle-dependent corrections in these cases
• Lack of a free neutron target correct for
  nuclear effects (FSI, MEC) and proton
  contributions

GE2
tGM2
q180o
q0o
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New techniques Polarization and A(e,eN)

• Mid 90s brought measurements using improved
  techniques
• Polarized beams with polarized target or recoil
  polarimeter
• Large, efficient neutron detectors for 2H(e,en)
• Improved models for nuclear corrections

L/T tGM2 eGE2
Pol GE/GM
BLAST at MIT-Bates
Polarized 3He target
Focal plane polarimeter Jefferson Lab
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Example GE /GM from Recoil Polarization
Similar expressions for cross section asymmetry
from polarized target
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Progress in the last decade (since 1997)
Magenta underway or approved
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Results from BLAST (unpublished)
GEn
2H(e,en)
GEp / GMp
1H(e,ep)
PRELIMINARY
GMn
2H(e,e)
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Insight from New Measurements

• New information on proton structure
• GE, GM differ for the proton different charge,
  magnetization distributions
• Connection to GPDs spin-space-momentum
  correlations

Model-dependent extraction of charge,
magnetization distribution of proton J. Kelly,
Phys. Rev. C 66, 065203 (2002)
A.Belitsky, X.Ji, F.Yuan, PRD69074014
(2004) G.Miller, PRC 68022201 (2003)
x0.1
x0.7
x0.4
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Insight from New Measurements

• Can test models with data on both proton and
  neutron form factors
• Previously, precise data and large Q2 range only
  for GMp, lower precision and limited Q2 range for
  GEp, GMn, little data for GEn

• Data for all FFs at low Q2
• GEp, GMn, GEn known to greater precision
  discrepancies resolved
• Soon, FFs known to 4-5 GeV2
• GEp changed dramatically, GMp also modified
• Complete data set in quark core and pion
  cloud region

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Small Sample of Recent Calculations
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Pion Form Factor Fp

• The pion form factor is of fundamental importance
  to our understanding of hadronic structure
• The pion is the lightest QCD system and one of
  the simplest
• The positronium atom of QCD
• Excellent test case for non-perturbative models
  of hadronic structure
• Test case for study of transition between
  non-perturbative and perturbative regions of QCD

• Fp is experimentally challenging to determine
• Above Q2gt0.3 GeV2, one must employ the
  1H(e,ep)n reaction
• At small t lt 0.2 GeV2, the t-channel diagram
• dominates sL In the t-pole approximation

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Projected JLab 12 GeV Data

• A program that can only be performed at Jefferson
  Lab
• Experiments performed in 1997 and 2003
  established the validity of the experimental
  technique and extended measurements to Q22.45
  GeV2

Higher Q2 data will challenge QCD-based models in
the most rigorous manner and provide a real
advance in our understanding of light quark
systems 12 GeV JLab upgrade and proposed
forward-angle SHMS spectrometer are essential to
the measurement
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Parity Violating Elastic e-p Scattering

• Nucleon charge, mag. distributions determined by
  quark distributions

Experiment Q2 APV ppm Notes SAMPLE 0.1 6ppm 19
97 0.1 7 deuterium 0.04 2 deuterium HAPPEX
0.5 15 0.1 2 0.1 6 4He 0.5 - G0 0.1-1 1-
10 0.4 - 0.7 - PVA4 0.1 1 0.2 5 0.2 -
backward angle Magneta for planned or ongoing
measurements
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Present Status

• Recent and near-future measurements 1997-2007
• Most of the worlds high-Q2 data, most of the
  worlds high-precision data
• Demonstrated problems with previous GEp AND GMp
  data
• New program of parity violating elastic
  scattering
• For isovector (protonneutron) form factors or
  flavor decomposition, need precise data covering
  similar Q2 range, careful understanding of
  systematics, including correlations between
  measurements
• TPE contributions
• Large effect on GEp (up to 100), smaller effect
  on GMp
• Corrections can propagate from proton to neutron
  (as extracted from 2H)
• While direct TPE corrections to parity violation
  are small, the effect of TPE corrections to the
  EM FFs changes the expected asymmetry

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Two-Photon Exchange

• Proton form factor measurements
• Comparison of precise Rosenbluth and Polarization
  measurements of GEp/GMp show clear discrepancy at
  high Q2
• Two-photon exchange corrections believed to
  explain the discrepancy

P.A.M.Guichon and M.Vanderhaeghen, PRL 91, 142303
(2003)

• Compatible with e/e- ?
• Yes previous data limited to low Q2 or small
  scattering angle
• Still lack direct evidence of effect on cross
  section
• Beam normal spin asymmetry the only observable in
  elastic e-p where TPE observed

M.K.Jones, et al., PRL 84, 1398 (2000) O.Gayou,
et al., PRL 88, 092301 (2003) I.A.Qattan, et al.,
PRL 94, 142301 (2005)
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Two-Photon Exchange Measurements

• Comparisons of e-p and e--p scattering
  VEPP-III, JLab-Hall B
• e dependence of polarization transfer and
  unpolarized se-p JLab-Hall C
• More quantitative measure of the discrepancy
• Test against models of TPE at both low and high
  Q2
• TPE effects in Born-forbidden observables
  JLab-Hall A, Hall C, Mainz
• Target single spin asymmetry, Ay in e-n
  scattering
• Induced polarization, py, in e-p scattering
• Vector analyzing power, AN, in e-p scattering

Evidence (3s level) for TPE in existing data J.
Arrington, PRC 69, 032201(R) (2004)
Worlds data Novosibirsk JLab Hall B
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Two-Photon Exchange Calculations

• Significant progress in theoretical understanding
• Hadronic calculations appear sufficient up to 2-3
  GeV2
• GPD-based calculations used at higher Q2
• Experimental program will quantify TPE for
  several e-p observables

• Precise test of calculations
• Tests against different observables
• Want calculations well tested for elastic e-p,
  reliable enough to be used for other reactions

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TPE Beyond the Elastic Cross Section

• Two-photon exchange (TPE) corrections
• Direct impact on extraction of form factors
• Important direct and indirect consequences on
  other experiments
• Neutron form factor measurements
• Strangeness from parity violation
• High-precision quasi-elastic experiments
• ? - N scattering measurements
• Proton charge radius, hyperfine splitting

P.Blunden, et al, PRC72, 034612
(2005) A.Afanasev, et al., PRD 72, 013008
(2005) A.Afanasev and C.Carlson, PRL 94, 212301
(2005) J.Arrington and I.Sick, nucl-th/0612079 D.D
utta, et al., PRC 68, 064603 (2003) J.Arrington,
PRC 69, 022201(R) (2004) H.Budd, A.Bodek, and
J.Arrington, hep-ex/0308005 P.Blunden and I.Sick,
PRC 72, 057601 (2005) S.Brodsky, et al., PRL 94,
022001 (2005)
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Summary Next few years

• Data being analyzed
• BLAST
• JLab GEn at high Q2
• Upcoming experiments
• GEp/GMp at high Q2 (zero crossing?)
• TPE corrections
• Cross section, polarization,
• Born-forbidden observables
• Parity measurements (HAPPEX,G0,A4)
• New experiments being planned
• Extend GMn to higher Q2
• Improve GEp/GMp precision at low Q2
• Global analysis of form factor, TPE measurements
• Extract corrected proton, neutron, and
  strangeness form factors
• Precise, complete data set for nucleon form
  factors to moderate Q2
• Constraints for GPDs, proton and neutron,
  extending to high Q2

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Extensions with JLab 12 GeV Upgrade
8 GeV2

• BLUE CDR or PAC30 approved, GREEN new ideas
  under development

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Electromagnetic Form Factors

• Part of the mission of Hadronic physics
• 2002 Long Range Plan, Hadronic physics milestone
  (2010)
• Electromagentic form factors up to 3.5 GeV2
• Parity measurements up to 1 GeV2
• These measurements completed or currently in
  progress
• Driving rapid progress in theory
• Pion form factor measurements to challenge
  QCD-based calculations
• Delivered, and still delivering, new insight and
  surprises
• Decrease of GE/GM at high Q2
• Reexamination and modification of pQCD
  predictions
• Emphasized effects of relativity, quark angular
  momentum
• Two-photon exchange
• Complicated task of making precise extractions
• Will be thoroughly tested in next few years
• High Q2 extensions probe quark structure, provide
  input to GPDs, sensitive to relativity and quark
  angular momentum
• High precision data at lower Q2, probing pion
  cloud contributions

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Fin