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

TL;DR

This paper reports the first measurement of the running of the effective QED coupling constant below 1 GeV using the KLOE detector, providing strong evidence of hadronic contributions in this energy region.

Contribution

It presents the first measurement of the running of $ ext{alpha}(s)$ in the 0.6 to 0.975 GeV range, with significant evidence of hadronic effects, and extracts the $ ext{BR}( ext{omega} o ext{mu}^+ ext{mu}^-)$.

Findings

Over 5σ significance of hadronic contribution to $ ext{alpha}(s)$

First direct evidence of running in this energy region

Determined $ ext{BR}( ext{omega} o ext{mu}^+ ext{mu}^-)$

Abstract

We have measured the running of the effective QED coupling constant $\alpha(s)$ in the time-like region $0.6<\sqrt s< 0.975$ GeV with the KLOE detector at DA$\Phi$NE using the Initial State Radiation process $e^+e^-\to\mu^+ \mu^-\gamma$. It represents the first measurement of the running of $\alpha(s)$ in this energy region. Our results show a more than 5$\sigma$ significance of the hadronic contribution to the running of $\alpha(s)$, which is the strongest direct evidence both in time- and space-like regions achieved in a single measurement. By using the $e^+e^-\to\pi^+\pi^-$ cross section measured by KLOE, the real and imaginary part of the shift $\Delta\alpha(s)$ has been extracted. By a fit of the real part of $\Delta\alpha(s)$ and assuming the lepton universality the branching ratio $BR(\omega\to\mu^+\mu^-) = (6.6\pm1.4_{stat}\pm1.7_{syst})\cdot 10^{-5} $ has been determined.