Aula Seminari (Laboratori Nazionali di Frascati, Edificio Alte Energie)
Aula Seminari
Laboratori Nazionali di Frascati, Edificio Alte Energie
Via E. Fermi, 40
00044 Frascati (RM)
Description
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, omega 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, 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 (V P gamma
and P gamma gamma couplings) and leptonic (Ve+e- couplings) decays and also the isospin breaking
(omega/ phi --> 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 V P gamma, Pgamma gamma as well as the isospin
breaking decay modes. It leads us to propose new reference values for the rho^0 --> e+ e- and
omega --> pi+ pi- partial widths which are part of our description of the pion form factor.
Other topics (phi --> K anti K, the rho meson mass and width parameters) are briefly discussed.
Therefore, we confirm the 3.3 sigma discrepancy between the theoretical estimate of a_mu based
on e+e- and its direct BNL measurement.