Measurement of the Pion Formfactor with KLOE and Study of the Reaction f0(980) to pi+pi-

TL;DR

This paper reports on measuring the pion form factor using radiative return at DAPHNE, with a new analysis tagging photons at large angles, enabling study of the f0(980) decay into pi+pi- and testing different theoretical models.

Contribution

It introduces a complementary analysis method for the pion form factor measurement and provides the first fit of the two-pion spectrum with various models for the f0(980) decay.

Findings

Successful measurement of the pion form factor at low M_pipi^2

First fit of the f0(980) decay spectrum with multiple models

Enhanced access to the threshold region for two-pion invariant mass

Abstract

At the Frascati phi-factory DAPHNE the pion formfactor is measured by means of the 'radiative return', i.e. by using events in which one of the collider electrons (positrons) has radiated an initial state radiation photon, lowering in such a way the invariant mass M_pipi of the two-pion-system. In a recent publication of the KLOE collaboration the initial state radiation photon had been required to be at small polar angles with respect to the beam axis. We are presenting results from a new and complementary analysis in which the photon is tagged at large polar angles. Only like this the threshold region M_pipi^2<0.35 GeV^2 becomes accessible. Moreover, the final state pi+pi-gamma allows to study the phi radiative decay into the scalar particle f_0(980) with f_0(980) \to pi+pi-. For the first time the two-pion mass spectrum could be fitted with different theoretical models for the description of this phi radiative decay.