First measurement of the π+π− atom lifetime

The goal of the DIRAC experiment at CERN (PS212) is to measure the π+π− atom lifetime with 10% precision. Such a measurement would yield a precision of 5% on the value of the S-wave ππ scattering lengths combination |a0−a2|. Based on part of the collected data we present a first result on the lifetime, τ=[2.91−0.62+0.49]×10−15 s, and discuss the major systematic errors. This lifetime corresponds to |a0−a2|=0.264−0.020+0.033mπ−1.