Egle TOMASI-GUSTAFSSON
CEA DSM IRFU
24-VI-2008
1
Electromagnetic Form Factors and
Polarization Phenomena
in Space-like and Time-like Regions
Egle Tomasi-Gustafsson
Saclay, France
Bad-Honnef, 24 –VI – 2008
Egle TOMASI-GUSTAFSSON
CEA DSM IRFU
24-VI-2008
2
Hadron Electromagnetic Form factors
– Characterize the internal structure of a particle (≠ point-like)
– In a P- and T-invariant theory, the EM structure of a particle
of spin S is defined by 2S+1 form factors.
– Elastic form factors contain information on the
hadron ground state.
– Playground for theory and experiment.
– New interest due :
•
to new precise data (polarization method)
• new kinematical range accessible (TL)
Egle TOMASI-GUSTAFSSON
CEA DSM IRFU
24-VI-2008
3
Towards a unified description of Hadron
Form factors to clarify:
- zero of GEp
- asymptotic properties
(Pragmen Lindeloff theorem)
- reaction mechanism
PLAN
Underline the essential information which
becomes accessible through polarization
observables in TL region
Egle TOMASI-GUSTAFSSON
CEA DSM IRFU
24-VI-2008
4
Space-like and Time-like regions
FFs are analytical functions: : real in SL region,
complex TL region
In framework of one photon exchange, FFs are functions of the momentum
transfer squared of the virtual photon, t = q2 = -Q2.
Scatterin
g
e- + h => e- + h
e+ + e- => h + h
_
Annihilation
_
t<0
t>0
Relative phase accessible ONLY through POLARIZATION
Egle TOMASI-GUSTAFSSON
CEA DSM IRFU
24-VI-2008
5
Crossing Symmetry
Scattering and annihilation channels:
- Described by the same amplitude :
- function of two kinematical variables, s and t
p2 → – p2
k2 → – k2
- which scan different kinematical regions
Egle TOMASI-GUSTAFSSON
CEA DSM IRFU
24-VI-2008
6
STATUS on EM Form factors
Space-like region
4) "standard" dipole function for the nucleon
magnetic FFs GMp and GMn
2) linear deviation from the dipole function for the
electric proton FF GEp
3) contradiction between polarized and
unpolarized measurements
4)