Strong interaction

The strong interaction is the $SU(3{)}_c$ gauge interaction described by Quantum chromodynamics. It couples quarks and gluons through color charge.

Scattering with one meson and one baryon in the initial and final states. The strong interaction conserves electric charge, baryon number, net strangeness, four-momentum, total angular momentum, and parity. Approximate isospin symmetry imposes additional relations among charge-channel amplitudes.

The ${\pi }^{+}p$ channel has baryon number one, strangeness zero and $I_3=3/2$, matching the ordinary $uuu$ Delta baryon. The $K^{+}p$ channel has strangeness plus one, which no three-light-quark baryon can have because each strange quark contributes minus one. A resonance in that channel would require exotic content such as four quarks and an antiquark; the ordinary low-energy qqq multiplets therefore explain the contrast.

For an initial pion-nucleon state and outgoing meson octet-baryon octet pair, conservation of baryon number and strangeness allows $\pi N$, ${\eta }_{8}N$, $K\Lambda$, and $K\Sigma$, subject to electric charge and production thresholds. Total isospin is $1/2$ or $3/2$; the ${\eta }_{8}N$ and $K\Lambda$ channels contain only $1/2$.