Deuteron-stripping reactions at high energy: George W. Barry. The Enrico Fermi Institute and the Department of Physics, The University of Chicago, Chicago, Illinois

Abstract

Abstract The bootstrap principle suggests that nuclei (B ⩾ 2) should be granted equal status in the nuclear democracy, along with the ordinary (B ⩽ 1) hadrons, simply because they interact strongly. Naively, one might expect nuclei to lie on Regge trajectories, and nuclear reactions to be dual, with resonances in the direct channel building up Regge poles in the crossed channel. With these concepts in mind, we examine in detail the simplest deuteron-stripping reactions, viz., p + p → d + π+, p + p → d + ϱ+, and p + d → d + p. Overall, the most satisfactory model is the One-Pion-Exchange (OPE) model of Yao. We present a revised version of the Yao model, and demonstrate that it gives a good description of the experimental pp → dπ forward differential cross section from threshold up to the highest energies measured. Furthermore, the model quantitatively accounts for the high-energy behaviors of the p + p → d + π and p + p → d + ϱ forward differential cross sections, which were recently found to have very puzzling high-energy behaviors. Canonical Regge behavior should set in only at much higher energies. The bumps in the resonance region of the pp → dπ cross section do not represent genuine dibaryon resonances, but are merely manifestations of Δ-isobar excitation of one of the nucleons in the deuteron. These results suggest that nuclei do not lie on Regge trajectories, although they do couple to nonexotic trajectories. The OPE mechanism can account for the pp → dπ cross section even at threshold, where the one-nucleon-exchange contribution is greatly suppressed by the p-d-n vertex form factor. To test the general validity of the OPE model, we apply it to p + d → d + p at high energy. The model gives an excellent (one-parameter) fit to the experimental differential cross section over a wide range of energies and backward angles. The excited-deuteron model of Kerman and Kisslinger is in principle equivalent to the OPE model. The OPE approximation corresponds to writing for the amplitude an unsubtracted dispersion relation in u, and then assuming that the π-N anomalous cut saturates the dispersion integral. In this approach, the absolute scale of the amplitude is calculable, whereas before it was left as a free parameter. The calculated normalization turns out to be somewhat larger than the experimental value, and to improve on this would require a much deeper understanding of the deuteron structure. Since nuclei apparently lie outside of the hadron bootstrap, we offer the speculation that nuclei may not be hadrons (strongly interacting particles) after all. Their exotic quantum numbers may render them “inert” to the SU(3)-symmetric strong force, leaving only the SU(3)-breaking medium-strong force through which they can bind together.