= Inclusive electron-positron annihilation into hadrons
{title2=$\sigma_h(s)$}
At leading electromagnetic order and with negligible electron mass, the photon-channel <relativistic cross-section> is $\sigma_h=4\pi^2\alpha\rho_h=16\pi^3\alpha^2\rho_J/s$, in the stated <hadronic photon spectral density> and <hadronic electromagnetic-current spectral density> conventions. Initial electron and positron spins are averaged; final hadronic states are summed inclusively. Electroweak channels beyond photon exchange are separate contributions.
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