Working group e e- @ Frascati

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Working group ee- _at_ Frascati
Milano 4 Nov. 2005
Time - like form factors
Marco Radici INFN - Pavia
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Outline

• Why should we measure time-like (TL)
• form factors (FF) ?

• What should we learn from TL FF
• with respect to SL FF ?

3. Which measurements are needed ?
4. Available data and calculations ?
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1. Questions in SL region
2. Surprise in SL TL
3. Few and poor TL data
4. Better constrain models

1. Why TL FF ?
2. What do we learn ?
3. Which measurements ?
4. Available data and models ?

elastic scattering cross section
Rosenbluth separation
fixed Q2, vary GE slope , GM intercept
large errors in GE
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1. Questions in SL region
2. Surprise in SL TL
3. Few and poor TL data
4. Better constrain models

1. Why TL FF ?
2. What do we learn ?
3. Which measurements ?
4. Available data and models ?

polarization transfer
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1. Questions in SL region
2. Surprise in SL TL
3. Few and poor TL data
4. Better constrain models

1. Why TL FF ?
2. What do we learn ?
3. Which measurements ?
4. Available data and models ?

JLab Hall A Qattan et al.
P.R.L. 94 (05) 142301
Blunden et al. nucl-th/0506039
2? ?
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1. Questions in SL region
2. Surprise in SL TL
3. Few and poor TL data
4. Better constrain models

1. Why TL FF ?
2. What do we learn ?
3. Which measurements ?
4. Available data and models ?

pQCD
JLab polar. transfer data
where is onset of pQCD scaling ?
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GEp
GEn
GMp
GMn
Miller, P.R. C66 (02) 032201(R) LFCBM
Ma, Qing, Schmidt, P.R. C65 (02) 035205 LF
diquark
Lomon, P.R. C66 (02) 045501 VMD
from H. Gao Int. J. Mod. Phys. E12 (03) 1 erratum
E12 (03) 567
Holzwarth, hep-ph/0201138
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1. Questions in SL region
2. Surprise in SL TL
3. Few and poor TL data
4. Better constrain models

1. Why TL FF ?
2. What do we learn ?
3. Which measurements ?
4. Available data and models ?

SpaceLike
TimeLike
analytic continuation by Dispersion Relations
(DR)
But
fit to pQCD TimeLike
fit to pQCD SpaceLike
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1. Questions in SL region
2. Surprise in SL TL
3. Few and poor TL data
4. Better constrain models

1. Why TL FF ?
2. What do we learn ?
3. Which measurements ?
4. Available data and models ?

pQCD analyticity
But
fit to GMp
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1. Questions in SL region
2. Surprise in SL TL
3. Few and poor TL data
4. Better constrain models

1. Why TL FF ?
2. What do we learn ?
3. Which measurements ?
4. Available data and models ?

poor statistic
integrate d? over wide angular range
all data assume
its true only at
where steep rise is observed
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1. Questions in SL region
2. Surprise in SL TL
3. Few and poor TL data
4. Better constrain models

1. Why TL FF ?
2. What do we learn ?
3. Which measurements ?
4. Available data and models ?

only 1 measurement for neutron (ADONE-1998) again
with
scarce angular distributions and with low
statistics
no polarization of protons and/or electrons
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1. Questions in SL region
2. Surprise in SL TL
3. Few and poor TL data
4. Better constrain models

1. Why TL FF ?
2. What do we learn ?
3. Which measurements ?
4. Available data and models ?

IJL
CQM
pQCD improved
Brodsky et al. P.R. D69 (04) 054022
Dispersive approach on BaBar and Lear Pacetti
talk at Nucleon 05 - Frascati
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1. Form factors are complex
2. FF from unpol. cross section
3. Phases from polarization
4. Unphysical region

1. Why TL FF ?
2. What do we learn ?
3. Which measurements ?
4. Available data and models ?

F(t) analytic function in t 2 C with cut t0
4m?2, 1)
Dispersion Relation (DR)
R
Im F(t) ? 0 only in t04m?2, 1 )
C
vector-meson poles and multi-hadron continuum
dipole fit to F2 (q2e-i? ) / F1 (q2 e-i? ) ! R
!
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1. Form factors are complex
2. FF from unpol. cross section
3. Phases from polarization
4. Unphysical region

1. Why TL FF ?
2. What do we learn ?
3. Which measurements ?
4. Available data and models ?

unpolarized cross section for
R angular asymmetry
q2lt0 Rosenbluth plot change E, ?e at fixed q2 )
linear plot in ?
q2gt0 measure 2 ? at fixed q2 ) get R cos2?
typical of Born diagram
measure 3 ? ) explore 2? mechanisms
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1. Form factors are complex
2. FF from unpol. cross section
3. Phases from polarization
4. Unphysical region

1. Why TL FF ?
2. What do we learn ?
3. Which measurements ?
4. Available data and models ?

polarized cross section for
Ax, Az require polarization of the electron beam
Pe ? 0
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1. Form factors are complex
2. FF from unpol. cross section
3. Phases from polarization
4. Unphysical region

1. Why TL FF ?
2. What do we learn ?
3. Which measurements ?
4. Available data and models ?

phases of FF from Final-State Interactions (FSI)
of final baryon system interference of
channels with different phases ( Im (GEGM) )
pQCD FSI ! 0 for Q2 ! 1 ) test transition
to scaling and Color Transparency (CT)
FSI ! T-odd mechanisms are allowed
generates
GE F1 ? F2 GE ei ?E GM F1 F2
GM ei ?M
not possible in elastic scattering
Im (GEGM) (? -1) Im F2F1 threshold t14M2 !
Im( )0 consistent with GEGM from GT1(t1)0
ambiguity ? ? - ? solved by
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1. Form factors are complex
2. FF from unpol. cross section
3. Phases from polarization
4. Unphysical region

1. Why TL FF ?
2. What do we learn ?
3. Which measurements ?
4. Available data and models ?

discriminate among models that are close in SL
region
IJL
CQM
pQCD improved
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1. Form factors are complex
2. FF from unpol. cross section
3. Phases from polarization
4. Unphysical region

1. Why TL FF ?
2. What do we learn ?
3. Which measurements ?
4. Available data and models ?

steep rise at threshold
tail of narrow resonance at t 4M2 (baryonium) ?
should show up as a dip in some hadronic cross
section
E687 diffractive photoproduction of 6? Upcoming
results from BaBar also
W 1.9 GeV
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1. Why TL FF ?
2. What do we learn ?
3. Which measurements ?
4. Available data and models ?

measure angular dependence of unpol. cross
section ) angular asymmetry R and test 2?
mechanisms
measure Ay / sin (?E - ?M)
measure Ay / Ax / tan (?E - ?M)
utopia measure Ai j
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1. data proton
2. models

1. Why TL FF ?
2. What do we learn ?
3. Which measurements ?
4. Available data and models ?

ADONE Q2 4.4 GeV (1973)
CERN Q2 3.6 (1977)
Orsay-DM1 Q2 3.75-4.56 (1979)
Orsay-DM2 Q2 4-5 (1983)
LEAR Q2 3.5-4.2 (1994)
E760 Q2 8.9-13 (1993)
FENICE Q2 3.7-6 (1994)
E835 Q2 8.8-18.4 (1999)
11.6-18.2 (2003)
CLEO Q2 11-12 (2005)
BES Q2 4-9 (2005)
BaBar Q2 2-20 (2005)
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1. data proton
2. models

1. Why TL FF ?
2. What do we learn ?
3. Which measurements ?
4. Available data and models ?

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1. data neutron
2. models

1. Why TL FF ?
2. What do we learn ?
3. Which measurements ?
4. Available data and models ?

FENICE Q2 3.7-6 (1994)
but always assumed GEN GMN Np,n
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1. data
2. models

1. Why TL FF ?
2. What do we learn ?
3. Which measurements ?
4. Available data and models ?

• Dispersion Relations (DR)
• analyticityunitarityVector Mesons
  (VM) (Drechsel, Meissner,
• 
  Hammer,
  Hoehler,..)
• 
  input exp. data (Baldini, Pacetti, )

• VM Dominance (VMD) based models (Iachello,
  Bijker, Lomon, )

• Soliton (Holzwarth)

• CQM Light Front Form (Pace, Salmè, Simula,
  )
• \pi
  cloud (Miller, Jennings, .. )
• Point Form (Pavia Graz
  collaboration)
• Diquark (Ma, )

• pQCD inspired (Brodsky, Ji, Belitski, Yuan,
  ..)

• reviews Brodsky et al. P.R. D69 (04) 054022
• Tomasi-Gustafsson et al.
  E.P.J. A24 (05) 419

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Possible logo ?
(from Pacetti talk at Nucleon05)