Deuteron normalization and channel-dependent formation dynamics in pion and kaon color transparency

Abstract

A combined view of the Jefferson Lab data on nuclear transparency in $A(e,e'\pi^+)$ and $A(e,e'K^+)$ reveals two simple but nontrivial features of the onset of color transparency. First, normalization to deuterium does not play the same role in the two reactions. In pion electroproduction, the missing-mass selection suppresses the neutron-induced $\Delta$ channel so strongly that the deuteron normalization becomes effectively proton dominated. In kaon electroproduction, the nearby hyperon channels cannot be removed in the same way, and the deuteron retains a genuine proton--neutron average. Second, the $Q^2$ dependence indicates different in-medium formation dynamics. The pion transparency is well reproduced by the standard quantum diffusion model with $\Delta M_\pi^2 \simeq 0.7~\mathrm{GeV}^2$, whereas the kaon data favor a faster geometric expansion characterized by the scale $R_K \sim \sqrt{\sigma_{KN}/\pi}$ and are strongly underestimated by the same pion-like diffusion scale. These results suggest that the pion and kaon data already contain evidence that the onset of color transparency is reaction dependent both in normalization and in propagation through nuclear matter.