Measurement of sigma(e+ e- -> pi+ pi-) from threshold to 0.85 GeV^2 using Initial State Radiation with the KLOE detector

TL;DR

This paper measures the e+e- to pi+pi- cross section using initial state radiation at the KLOE detector, providing precise data relevant for the muon g-2 anomaly and confirming existing discrepancies with the Standard Model.

Contribution

It presents a novel measurement of the pion form factor and the dipion contribution to muon g-2 using ISR at DAPHNE, with improved precision and analysis methods.

Findings

Measured the cross section from threshold to 0.85 GeV^2.

Determined the pion form factor |F_pi|^2 with high accuracy.

Confirmed the discrepancy between Standard Model and experimental muon g-2 values.

Abstract

We have measured the cross section of the radiative process e+e- -> pi+pi-gamma with the KLOE detector at the Frascati phi-factory DAPHNE, from events taken at a CM energy W=1 GeV. Initial state radiation allows us to obtain the cross section for e+e- -> pi+pi-, the pion form factor |F_pi|^2 and the dipion contribution to the muon magnetic moment anomaly, Delta a_mu^{pipi} = (478.5+-2.0_{stat}+-5.0_{syst}+-4.5_{th}) x 10^{-10} in the range 0.1 < M_{pipi}^2 < 0.85 GeV^2, where the theoretical error includes a SU(3) ChPT estimate of the uncertainty on photon radiation from the final pions. The discrepancy between the Standard Model evaluation of a_mu and the value measured by the Muon g-2 collaboration at BNL is confirmed.