A measurement of the vector to pseudoscalar conversion decay $\phi \to \pi^0 e^+e^-$ with the KLOE experiment is presented. A sample of $\sim 9500$ signal events was selected from a data set of 1.7 fb$^{ -1}$ of $e^+e^-$ collisions at $\sqrt{ s} \sim m_{ \phi}$ collected at the DA$\Phi$NE $e^+e^-$ collider. These events were used to obtain the first measurement of the transition form factor $| F_{ \phi \pi^0}(q^2) |$ and a new measurement of the branching ratio of the decay: $\rm{ BR}\,(\phi \to \pi^0 e^+e^-) = (\,1.35 \pm 0.05^{ \,\,+0.05}_{ \,\,-0.10}\,) \times 10 ^{ -5}$. The result improves significantly on previous measurements and is in agreement with theoretical predictions.
Categories: hep-ex hep-ph