Le facteur de forme du pion dans les annihilations e+e- et dans la desintegration du tau -- M. Benayoun -- LPNHE

jeudi 20 mars 2008, 16:30 → 17:30 Europe/Paris

LPNHE

We readdress the problem of finding a simultaneous description of the pion form factor data in $e^+e^-$ annihilations and in $\tau$ decays. For this purpose, we work in the framework of the Hidden Local Symmetry (HLS) Lagrangian and modify the vector meson mass term by including the pion and kaon loop contributions. This leads us to define the physical $\rho$, $\omg$ and $\phi$ fields as linear combinations of their ideal partners, with coefficients being meromorphic functions of $s$, the square of the 4--momentum flowing into the vector meson lines. This allows us to define a dynamical, {\it i.e.} $s$-dependent, vector meson mixing scheme. The model is overconstrained by extending the framework in order to include the description of all meson radiative ($VP\gamma$ and $P\gamma \gamma$ couplings) and leptonic ($Ve^+e^-$ couplings) decays and also the isospin breaking ($\omg/\phi \ra \pi^+ \pi^-)$ decay modes. The model provides a simultaneous, consistent and good description of the $e^+e^-$ and $\tau$ dipion spectra. The expression for pion form factor in the latter case is derived from those in the former case by switching off the isospin breaking effects specific to $e^+e^-$ and switching on those for $\tau$ decays. Besides, the model also provides a good account of all decay modes of the form $VP\gamma$, $P\gamma \gamma$ as well as the isospin breaking decay modes. It leads us to propose new reference values for the $\rho^0 \ra e^+ e^-$ and $\omg \ra \pi^+ \pi^-$ partial widths which are part of our description of the pion form factor.

Slides M_Benayoun.pdf