Nucleon Form Factors Strike Back

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Nucleon Form FactorsStrike Back

• R. Baldini Ferroli, Centro Fermi/INFN-LNF2
• ee- collisions from f to y
• BINP 27 feb. 2006

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Outline

• Space-like Q2 (lt0) GE/GM measurement at JLAB
• Time-like Q2 (gt0) main features and
  previous data
• BaBar ee-? p p cross section and Form Factors
• The ee-? n n puzzle
• Conclusions

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space-like time-like
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Space-like GE/GM JLab measurement
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• Renewed interest in nucleon FF
• by space-like GE/GM JLab measurement of recoil
  polarization

Crossing zero at Q2 10 GeV2 ?
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T.Massam, A.Zichichi (1966)
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F.Iachello, A.D.Jackson, A.Lande (1973)
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Two photons exchange contribution
Blunden-Melnitchouk-Tjon - PRL 91,142304 (2003)
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• The explanation of this discrepancy should be in
  the
• radiative corrections not properly taken into
  account,
• gg exchange contribution invalidating Born
  approximation
• in the angular distribution, while recoil
  polarizations ratio is less affected.

• Minimal time-like expectations
• GE/GM ? 1 asymptotically
• fEM ? p asymptotically
• GE/GM gtgt1 just above threshold
• Substantial N polarization and asymmetry

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Nucleon Time-like Form Factors
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Cross section ee-? p p
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• C correction to Born approximation for
  Coulomb interaction at threshold.
• Important in understanding threshold behaviour.
• Distorted wave approximation in final state
  interaction (Landau-Lifchitz book)

C 1/b at threshold ? s0 finite
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• GE,GM features due to crossing analyticity
• In principle continuity from ep?ep to
• (space-like)
  (time-like)
• Cuts above inelastic thresh space?time-like
  real?complex
• A(heltot 0) analytical continuation of
  space-like spin flip GM
• A(heltot 1) analytical continuation of
  non spin flip GE
• GM F1F2, GE F1t F2
• F1,F2 analyticity ?
• at threshold S wave only ?
• Relative phase measurement needed to get F1,
  F2
• (from p polarization measurement/polarized
  beams)

GM(2mp) GE(2mp)
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• Previous data
• Poor statistics/angular acceptance ? GMGE

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s µ 2 GM2tGE2 ? GM dominant at high w
Early PQCD scaling F1 GM a2(w)/w4
Gtime-like(w2) Gspace-like(-w2) (but
within a factor of 2)
Space-like
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BaBar ee- ? p p new results by means of
Initial State Radiation
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• Analyzed 232 fb-1
• I.S.R. simulation
• 20 lt qg lt 160 (H.Czyz et al, Eur. Phys. J. C
  35(2004)527)
• Soft photons (M.Caffo et al, N. C.
  110A(1997)515)
• Event selection
• Tracks within DCH and DIRC acceptance
• Very tight proton selector ( 30 good events
  loss)
• pp g kinematical fit
• Eg resolution not reproduced ? 3C fit
• e 18 1
• 4025 selected events

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GE(Q2)/GM(Q2)in theTime-like region
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• BaBar GE/GM measurements vs previous ones
• and dispersion relation prediction (yellow)
  based on JLab space-like GE/GM and analyticity
  only

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Cross section and Form Factors
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• Dressed cross section

• Negative step at w 2.2 GeV (!?)
• Speculations
• Strong resonances interference ?
• A new (inelastic) amplitude
• opening of baryonic excitations?

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• Negative step at w 2.9 GeV (!?)
• More difficult to interpretate !

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Near threshold dressed cross section
Reasonable agreement with previous measurements
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• Very near threshold Form Factor

• Steep behaviour at threshold confirmed
• Steep behaviour at threshold also seen in other
  processes

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• Steep behaviour at threshold
• also seen in other processes,
• with different quantum numbers

BES BaBar BaBar
Corrected for phase space
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• Possible interpretations
• Tail of a narrow resonance below threshold
  (Baryonium ?)
• Dominance of p exchange in pp final state int.
• Underestimation of the Coulomb correction factor
• Possible tests for baryonium
• Narrow vector meson, with vanishing ee-
  coupling,
• on top of a large r/w recurrence dip in some
  cross section

- ee- ? nn (no Coulomb correction)
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• Dips have indeed been seen in diffractive
  photoproduction of 6p by E687

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(No Transcript)
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ee- ? Hadrons


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The ee- ? n n puzzle
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Present Neutron Time-like Data

• One measurement only FENICE at ADONE
• 100 events at 1.9 lt Q lt 2.55 GeV
• Rn s(ee- ? n n) / s(ee- ? p p) gt 1
• Hints for GEltlt GM (sharp thresh. behavior ?)

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• Some expectations for s(ee- ? n n) / s(ee- ? p
  p)
• pQCD (qd/qu)2 0.25
• Soliton models ³ 1
• Future perspectives
• Very difficult, if not impossible, by means of
  ISR
• p p n n , n D ee- (p) at GSI ?
• Diffractive Photoproduction g A (n n)/(p p) A
  ?
• VEPP2000, BEPC2, DAFNE2 ?

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Cross sections nn versus pp
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Slight evidence for GEn ltlt GMn
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Neutron magnetic time-like FF
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Conclusions and perspectives

• Space-like FF unexpected GE behavior
• Looking for a zero _at_ Q2 -10 GeV2
• Compare e-p ? e-p vs ep ? ep
• Time-like p FF
• GE/GM gt1 just above threshold
• steep behavior at threshold
• steps _at_ Q2 5 and 8 GeV2
• Looking for p polarization and asymmetry
• New measurement of Neutron FF
• VEPP2000, DAFNE2, t/Charm, SuperB

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Early successfull predictions

• T.Massam, A.Zichichi, NC 43(66)1137
• (J.Ellis, M.Karliner NewJP 4(02)4)
• VDM SU3 extended source R 1/MN
• first time-like experimental constraint
• FiN 1/(1-Q2/MN2) SV gV fV /(1- Q2/mv2)
  Skyrme model
• F.Iachello, A.D.Jackson,A.Lande, PL 43B(73)191,
• (M.Gari, W.Krumpelmann, PL 173B(86)10)
• VDM extended source R 1/MN Hard core
  (pQCD)
• Both models predict s(ee-?n n) ³ s(ee-?p p)

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I.S.R. Main Features
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I.S.R. versus c.m.

• Advantages
• All energies at the same time ? less systematics
• c.m. boost ? e ? 0 sw 1 MeV at threshold
• Drawbacks
• L µ invariant mass bin Dw
• More background

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cos qp distributions from threshold up to 3 GeV
Histograms show contribution from GM (dashed)
GE (dash-dotted) Transition from
sin2q (GE dominant) to 1cos2q (GM
)
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• 1.92 GeV 1.4 0.5
• APPLE
• (LEAR) 2.05 0.7 0.3
• FENICE DM2 2.1 5.4 4.1
• (Adone) (DCI)
• E835 3.5
  0.7 0.6
• (FNAL)

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B ? K p p
Bo ? Do p p

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FENICE n n typical event
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• SpaceTime-like data it might be possible to
  solve the dispersion relation equations to get
  the FF in between
• (unphysical region)
• A model-independent attempt to solve the integral
  equation by means of a regularization procedure
• Testing the method by means of Fp, assumed like
  pp
• astonishing agreement
• Application to GMN (previous data, BaBar
  ignored)
• r(770), r (1600) and threshold
  OK
• Simplest VDM GMP(r ? pp) mP F(r?pp) OK
• But Gr 350 MeV
  (?)