Nucleon Form Factors John Arrington Argonne National Lab

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Nucleon Form Factors John ArringtonArgonne
National Lab

• Hall A Collaboration Meeting
• 4 Jan 2007

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Early History of the Proton
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Electron Scattering and Form Factors
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Proton Form Factors
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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
• Early measurements found
• Neutron form factor small
• Others well approximated by dipole form
• Only proton magnetic form factor measured
  precisely over large Q2 range

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Significant Program Over Next 40 Years
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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 tltlt1 (low Q2)
• GE if tgtgt1
• GE if GE2ltltGM2 (e.g. neutron)
• Form factor extraction is very sensitive to
  angle-dependent corrections in these cases
• Lack of a free neutron requires deuteron target
  correct for proton contribution and nuclear
  effects (e.g. FSI, MEC)

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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
• Rosenbluth (L-T) technique has severe limitations
  in some regions
• Recent revolution (last 10 yrs) due to new
  experimental techniques
• Dramatically improved precision, Q2 coverage
• Most previous data now obsolete (or incorrect)
• 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 EM FFs
• Trying to explain data at both low and high Q2
• Model-independent interpretations of, e.g. flavor
  dependence

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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
Bigbite in Hall A at JLab
Polarized 3He target
Focal plane polarimeter
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Example GE /GM from Recoil Polarization

• Note that PL and PT (or Ax, Az) depend only on
  GE/GM
• ? Need additional data to separate GE and GM
  (e.g. se-p)
• ? GMn measurements require nuclear targets
  (polarized 2H or 3He)

Similar expressions for cross section asymmetry
from polarized target
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Nucleon Form Factors Recent Advancements

• Neutron form factor measurements
• 1997 Mainly d(e,e) - limited (e,en),
  (e,en/e,ep), polarization data
• Uncertainties and scatter made it difficult to
  evaluate models

GMn as of 1997 Inclusive, ratio, and
polarization measurements
GMn as of 1997 Inclusive, ratio, and
polarization measurements
GMn as of 1997 Inclusive, ratio, and
polarization measurements Since 1997 new
polarization, ratio measurements (CLAS
preliminary)
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Nucleon Form Factors Recent Advancements

• Neutron form factor measurements as of 1997
• GEn very poorly known
• Mostly from elastic e-d ? very large
  model-dependence

GEn as of 1997 elastic e-d and polarization
measurements
GEn as of 1997 elastic e-d and polarization
measurements Since 1997 2H and 3He polarized
target and recoil polarization data, along with
improved e-d analysis
GEn as of 1997 elastic e-d and polarization
measurements Since 1997 2H and 3He polarized
target and recoil polarization data, along with
improved e-d analysis and projected future
measurements
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Nucleon Form Factors Recent Advancements

• Proton form factor measurements from Rosenbluth
  separations
• GMp well measured to 10 GeV2, data out to 30 GeV2
• GEp well known to 1-2 GeV2, data to 6 GeV2

GMp from inclusive measurements data extend to
30 GeV2
GMp from inclusive measurements data extend to
30 GeV2 With TPE corrections (Blunden, et al.),
GMp shifts by up to 2-3 sigma, maybe more
mpGEp/GMp from inclusive Rosenbluth measurements
mpGEp/GMp from inclusive Rosenbluth measurements
New data Recoil polarization
mpGEp/GMp from inclusive Rosenbluth measurements
New data Recoil polarization and p(e,p)
Super-Rosenbluth
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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)
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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 higher Q2 and greater
  precision
• GEp has changed dramatically, GMp also
  non-trivially modified
• Soon, all four FFs known with high precision to
  4-5 GeV2
• Complete data set in quark core and pion
  cloud region
• Precise non-singlet (p-n) extraction over large
  range

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Small Sample of Recent Calculations
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Pion Cloud

• 
  charge and magnetization density
• Dipole form factor
  within 5 for
  Q2 lt 1.0 (GeV/c)2
• Deviations from dipole at low Q2 ? effects of
  meson cloude.g. Friedrich-Walcher
  parameterization

smooth part
bump part
Slide courtesy of Michael Kohl
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Pion Cloud Data minus smooth fit
Slide courtesy of Michael Kohl
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Pion Cloud Some cautions

• Bump is not simply the pions contribution
• p ? np x of the time implies x of GEp(0)
  should be from the pion
• Fourier transform of the bump does not isolate
  pion
• Deviation from dipole(s) is just that a
  deviation from the dipole form

• Important to GEn, but not the full story
• Global analysis of high-precision data can, in a
  model-dependent way, tell us about the pion cloud
• Precise comparison of low-Q FFs, e.g. GMp vs. GEp
  or GMp vs. GMn can provide additional information
• Model-independent Information on differences.
  Still need model to tell pion cloud vs.
  relativistic effects, etc

Proton
Neutron
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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
measurements
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Nucleon Form Factors Parity Violation

• Parity-violating elastic electron scattering
• APV depends on EM form factors, RC, and
  strangeness content
• Combine with EM FF to perform full flavor
  decomposition of form factors into Gu(Q2),
  Gd(Q2), Gs(Q2)

Separation at Q20.1
Contours from R.D.Young, et al, PRL97102002
(2006) Bands are latest HAPPEX A.Acha, et al.,
nucl-ex/0609002
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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 non-singlet (p-n) form factors or flavor
  decomposition, need precise data covering similar
  Q2 range, careful understanding of systematics,
  including correlations between measurements
• TPE contributions
• Effect on GEp (up to 100) much larger than for
  GMp (several )
• Impact of GMp corrections can be more important
  in global fitting
• Corrections can propagate from proton to neutron
  (as extracted from 2H) to strangeness
  contribution from parity measurements
• 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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Radiative Corrections Beyond the Born
Approximation
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Tests of Two-Photon Exchange (50s and 60s)
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Two-Photon Exchange Strikes Back

• 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- ?
• 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, Hall B
• e dependence of polarization transfer and
  unpolarized se-p 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 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
  (beam normal spin asymmetry)

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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Status of TPE

• Significant progress in theoretical understanding
• Multiple calculations that explain most of the
  effect at high Q2
• Hadronic calculations appear sufficient up to 2-3
  GeV2
• Experimental program will quantify TPE for
  several e-p observables

• Precise test of calculations in explaining
  discrepancy
• 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
• Precise tests of TPE calculations for the proton
• Calculations for several observables for proton
  and neutron
• Important direct and indirect consequences on
  other experiments
• High-precision quasi-elastic expts.
• ? - N scattering measurements
• Proton charge radius, hyperfine splitting
• Strangeness from parity violation
• Neutron form factor measurements

D.Dutta, 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) A.Afanasev and C.Carlson, PRL 94, 212301
(2005) J.Arrington and I.Sick, nucl-th/0612079 P.B
lunden, W.Melnitchouk, and J.Tjon, PRC72, 034612
(2005) A.Afanasev, et al., PRD 72, 013008 (2005)
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Example TPE Effects on Parity Measurements

• Direct TPE effect small (top left)
• Total effect (bottom left) noticably larger
• TPE effect goes thru zero for Q20.3 GeV2

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Summary Next few years

• Results for GEn at high Q2
• New measurements of GEp/GMp at high Q2
• Look for zero crossing of GEp
• Tests of TPE corrections
• Cross section, polarization,
• Born-forbidden observables
• Parity measurements (HAPPEX,G0,A4)
• Strangeness contributions
• 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
• 12 GeV upgrade More to come

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Nucleon Form Factors at Jefferson Lab

• Part of the initial scientific mission of CEBAF
• A promise fulfilled
• Nearly complete set of measurements
• Driving rapid progress in theory
• Delivered more than initially expected
• New Parity program
• Unexpected Decrease of GE/GM at high Q2
• New Two-photon exchange
• Hall A had a major role in nearly all aspects of
  the program
• Polarimeter GEp/GMp at high Q2 ?
  Super-Rosenbluth, TPE
• Polarized 3He GEn at high Q2 ,GMn at low Q2
• Parity program Strangeness contributions
• More to come TPE measurements, Low Q2 GEp/GMp
  proposal,

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For more information
Nucleon form factors C.F.Perdrisat, V.Punjabi,
and M.Vanderhaeghen, arXivhep-ph/0612014
(2006) J.Arrington, C.D.Roberts, and J.M.Zanotti,
arXivnucl-th/0611050 (2006) C.E.Hyde-Write and
K. de Jager, Ann. Rev. Nucl. Part. Sci. 54, 217
(2004) H.Gao, Int. J. Mod. Phys. E12, 1 (2003)
Erratum-ibid 567, (2003)
Parity, GPDs, TPE, etc E.J.Beise, M.L.Pitt, and
D.T.Spayde, Prog. Part. Nucl. Phys. 54, 289
(2005) D.H.Beck and R.D.McKeown, Ann. Rev. Nucl.
Part. Sci. 51, 189 (2001) D.H.Beck and
B.R.Holstein, Int.J.Mod.Phys. E10, 1
(2000) K.Kumar and P.Souder, Prog.Part.Nucl.Phys.
45, S333 (2000) X.Ji, Ann. Rev. Nucl. Part. Sci.
54, 413 (2004) M.Vanderhaeghen and C.E.Carlson
coming in 2007