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

Abstract

The precision measurement of the $d\sigma (e^{+}{e}^{-}\to {\mu }^{+}{\mu }^{-}\gamma )/d\sqrt{s}$ cross section with the photon emitted in the initial state with the KLOE detector has been used to measure the running of the QED coupling constant $\alpha (s)$ in the energy range 0.6 $<\sqrt{s}<$ 0.975 GeV in the time-like region. We were able to achieve a significance of the hadronic contribution to the running of $\alpha (s)$ of more than 5$\sigma$ with a clear contribution of the $\rho -\omega$ resonances to the photon propagator. The real and imaginary part of the shift $\Delta \alpha$ to the running has been estracted and a fit of the real part allowed us to measure the branching fraction $BR(\omega \to {\mu }^{+}{\mu }^{-})$ = (6.6$\pm$1.4${}_{stat}$$\pm$1.7${}_{syst}$ )$\cdot$10${}^{-5}$.

5 pages, 3 figures, Rencontres de Moriond, QCD and High Energy Interactions 2017