Importance of the NΔ inelastic threshold for understanding low-energy NN amplitudes

The authors relate three interesting aspects of low-energy NN reaction: (1) The large value of Δsigma/subt/ = sigma/subT/(arrow-down)- sigma/subt/(arrow-down) for pp scattering for P/sub LAB/ below 3 GeV/c; (2) the existence at low energies of large unnatural-parity t-channel amplitudes with nonpion quantum numbers; (3) the anomalous variation with energy of the slope and differential cross section at t =0 in np → pn in the range P/sub LAB/ = 0.5--2 GeV/c. Based on the somewhat crude phase-shift analyses which exist above the single-pion inelastic threshold, it is argued that all three of the above aspects of NN reactions are the reflections of the dominant inelastic channel pp → NΔ. The well-known possibility that the large amplitudes so generated are related to resonant behavior (in the 1D2 partial wave of pp → pp) is discussed. That some low-energy amplitudes are strongly affected by s-channel threshold effects demonstrates one cannot, in general, equate features of high-energy amplitudes with those of low-energy amplitudes whenever inelastic thresholds are important