Meson-baryon scattering

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 $1/2$.