Study of the $e^+e^-\to K^+K^-$ reaction in the energy range from 2.6 to 8.0 GeV

TL;DR

This study measures the $e^+e^- o K^+K^-$ cross section and kaon form factor from 2.6 to 8.0 GeV using initial-state radiation, revealing their energy dependence, resonance contributions, and interference effects with implications for QCD and meson decays.

Contribution

It provides the first detailed measurement of the kaon electromagnetic form factor in this energy range and analyzes interference effects in $J/A$ and $A(2S)$ decays, offering new insights into strong and electromagnetic amplitude phases.

Findings

Kaon form factor decreases faster than $1/E^2$ with energy.

Resonant production via $J/A$ and $A(2S)$ observed.

Interference effects cause shifts in measured branching fractions.

Abstract

The $e^+e^-\to K^+K^-$ cross section and charged-kaon electromagnetic form factor are measured in the $e^+e^-$ center-of-mass energy range ($E$) from 2.6 to 8.0 GeV using the initial-state radiation technique with an undetected photon. The study is performed using 469 fb$^{-1}$ of data collected with the BABAR detector at the PEP-II $e^+e^-$ collider at center-of-mass energies near 10.6 GeV. The form factor is found to decrease with energy faster than $1/E^2$, and approaches the asymptotic QCD prediction. Production of the $K^+K^-$ final state through the $J/\psi$ and $\psi(2S)$ intermediate states is observed. The results for the kaon form factor are used together with data from other experiments to perform a model-independent determination of the relative phases between single-photon and strong amplitudes in $J/\psi$ and $\psi(2S)\to K^+K^-$ decays. The values of the branching fractions measured in the reaction $e^+e^- \to K^+K^-$ are shifted relative to their true values due to interference between resonant and nonresonant amplitudes. The values of these shifts are determined to be about $\pm5\%$ for the $J/\psi$ meson and $\pm15\%$ for the $\psi(2S)$ meson.