Forward scattering amplitude
= Forward scattering amplitude
{title2=$f(0)$}
The <scattering amplitude> at zero deflection angle. Its real part changes the phase of a coherent beam in matter; its imaginary part is related by the <optical theorem> to removal from that beam. Different forward amplitudes for a <kaon> and a <neutral antikaon> allow <neutral kaon regeneration> without fundamental <CP violation>.