Antiproton Production in p + d Reaction at Subthreshold Energies

Abstract

An enhancement of antiprotons produced in the p + d reaction in comparison over those produced in the p + p elementary reaction is investigated. In the neighborhood of the subthreshold energy, the enhancement is caused by the difference in threshold energies for antiproton production. The cross section of the p + d reaction, on the other hand, becomes just twice that in the elementary p + p reaction at the incident energy far from the threshold energy when non-nucleon components in the deuteron target are not considered.Antiproton production has been one of the most interesting topics in the study of subthreshold production processes since they were first observed in proton-nucleus reactions more than thirty years ago. 1) -3) About a decade ago, the production of antiprotons at subt4reshold in nucleus-nucleus reactions was observed 4) -6) and various models have been proposed to explain experimental data, e.g., models based on the assumption of kinetic and chemical equilibrium, 7) -9) and models in terms of multiple interactions. 10), 11) The first fully relativistic transport calculations have shown that the antiproton yields for p + A and A + A can be reproduced simultaneously quite well when employing proper self-energies for the baryon and antiproton in a dense medium and taking into account the effect of in-medium antiproton absorption (the BUU and the relativistic BUU approach). 12) -15)Five years ago the problem was taken up at KEK 16) with the goal of carrying out measurements of subthreshold antiproton production with light ions such as d and 0:. The most surprising finding was an enormous enhancement of antiproton productions in the d + A reaction at lower incident energies per nucleon. As an explanation of this enhancement, the existence of non-nucleon components in the deuteron wave function was suggested.To explore this suggestion, the BUU approach group performed calculations employing the deuteron wave function which had been fitted to deuteron fragmentation data at high energy and noted that the present data do not yet provide clear evidence for non-nucleon components in the deuteron wave function. 17) Recently