Shape of Hadrons Athens April 2006 Real Photon Experiments  M. Kotulla MAMI and LEGS magnetic dipole moment of the   

Nucleon Structure

first excited State precise electro- and photoproduction data:  magnetic dipole transition M1  electric quadrupole transition E2  Coulomb quadrupole transition C2 incoming photon: M1, E2, C2 outgoing pion: N   transition  intrinsic

methods:  isospin decomposition:  most sensitive: shape of angular distribution beam asymmetry at 90°  E 1 + M 1 +  multipol analysis, dispersion relation  R EM Extraction

MAMI

MAMI 6  62, 1  138 detectors with individual veto detector, solid angle: ~0.40  4  sr MAMI cm beam

LEGS setup

Photon Asymmetry

Differential Cross Sections

Comparison LEGS - MAMI

P 33 Partial Waves (MAMI)

MAMI Results:  DAPHNE measurement: C || /(12A || ) dispersion relation analysis multiplol ana (S,P waves + Born) LEGS Results:  multipol analysis (up to F waves + Born) R. Beck et al., PRC (2000)G. Blanpied et al., PRC (2001) R EM  for comparison: deuteron

The Beamline LH 2 CB (inside 2MWPCs, PID) TAPS  beam production runs CB:   672 NaI(Tl)  covers 20° - 160°  TAPS:   510 BaF 2  covers 1° - 20° 

Photon Asymmery 15% of total statistics Photon Asymmetry  :   CBT =0.619    Leukel =0.592    CBT =0.632    Leukel =0.591  E. Downie preliminary

  Magnetic Moment p p   ´´  measurement:  excitation of the  + and identification of the    ´p state  reaction description necessary M1, (E2), M3  ´  transition p   p ´´ p p  ´´  basic principle  resonance:  isolated excited state of the nucleon  lifetime  ~ s    M.Kotulla et al., PRL 89, (2002) PDG2004    = (2.7  2.2  3)  N

Reaction Description Pascalutsa, Vanderhaeghen PRL (2005) M. Kotulla et al., PRL 89, (2002) Recent progress:  chiral effective field theory ansatz V. Pascalutsa, M. Vanderhaeghen PRL (2005)  systematic ansatz (NLO) error estimation  rescattering: MDM acquires real and imaginary part

  (1232) MDM cross sections:  angular distribution of  ´  comparison: pioneering experiment (TAPS1999)   = (2.7  2.2  3)   M.K. et al., PRL 89, (2002)  preliminary Crystal Ball / TAPS data much higher precision new Crystal Ball / TAPS Data   (1232) MDM experiment:   p     ´p  exclusive measurement  kinematical overdetermination  identification checks preliminary B. Boillat

new Crystal Ball / TAPS Data checks:  photon beam asymmetry p  meson p ´´ E. Downie  =0.619  S. Schumann preliminary

Summary    magnetic moment  pioneering experiment (proof of principle)  new (CB/TAPS) measurement is currently being analyzed much higher precision  reaction calculation (systematic ansatz): chiral effective field theory  Outlook: experiment feasible S 11 + (1535):  p   ´  p S 11 0 (1535):  n   ´  n(LD 2 target) N  R EM :  precise data from MAMI and LEGS very good agreement in photon asymmetry slight deviation in (differential) cross sections