Antonelli, M (reprint author), Ist Nazl Fis Nucl, Lab Nazl Frascati, Casella Postale 13, I-00044 Frascati, Italy.
We present a measurement of the branching ratio of the CP violating decay K-L -> pi(+)pi(-) performed by the KLOE experiment at the phi factory DA Phi NE. We use 328pb(-1) of data collected in 2001 and 2002, corresponding to similar to 150 million tagged K-L mesons. We find BR(K-L -> pi(+)pi(-)) = (1.963 +/- 0.012(stat). +/- 0.017(syst.)) x 10(-3). This branching ratio measurement is fully inclusive of final-state radiation. Using the above result, we determine the modulus of the amplitude ratio vertical bar eta(+-)| to be (2.219 +/- 0.013) x 10(-3) and vertical bar epsilon vertical bar to be (2.216 +/- 0.013) x 10(-3). (c) 2006 Elsevier B.V. All rights reserved.