Measurement of the running of the fine structure constant below 1 GeV with the KLOE detector

We have measured the running of the effective QED coupling constant α(s)in the time-like region 0.6 <√s<0.975GeVwith the KLOE detector at DAFNE using the Initial-State Radiation process e+e−→μ+μ−γ. It represents the first measurement of the running of α(s)in this energy region. Our results show a more than 5σ significance of the hadronic contribution to the running of α(s), which is the strongest direct evidence both in time-and space-like regions achieved in a single measurement. By using the e+e−→π+π− cross section measured by KLOE, the real and imaginary parts of the shift Dα(s)have been extracted. From a fit of the real part of Dα(s)and assuming the lepton universality the branching ratio BR(ω→μ+μ−) =(6.6 ±1.4stat±1.7syst) ·10−5 has been determined.