Antiproton Production in p+d Reaction at Subthreshold Energies

An enhancement of antiprotons produced in p+ d reaction in comparison with ones in p+p elementary reaction is investigated. In the neighborhood of subthreshold energy the enhancement is caused by the difference of available energies for antiproton production. The cross section in p+ d reaction, on the other hand, becomes just twice of the one in elementary p+ p reaction at the incident energy far from the threshold energy when non-nucleonic components in deuteron target are not considered. Antiproton productions have been one of the most interesting subthreshold production process since they have been observed in proton-nucleus reactions more than thirty years ago [1-3]. About a decade ago, the production of antiprotons at subthreshold in nucleus-nucleus was observed [4-6] and various models have been proposed to explain experimental data, models based on the assumption of kinetic and chemical equilibrium [7-9], and models in terms of multiple interactions [10,11]. First fully relativistic transport calculations for antiproton production including antiproton annihilation as well as the change of the quasi-particle properties in the medium have shown that the antiproton yields for p + A and A + A can be well reproduced simultaneously when employing proper self energies for the baryons in the dense medium (the relativistic BUU approach)[12-16]. Five years ago the problem was taken up at KEK[17] aiming to carry out measurements of subthreshold antiproton productions with light ions such as d and α . In particular, the cross section ratio of antiproton productions between proton induced and deuteron induced reactions is insensitive to uncertainties of antiproton reabsorption in a target nucleus and of the antiproton production cross section in elementary NN interaction near the subthrshold, which give large ambiguity in any theoretical model calculation. The most surprising finding was an enormous enhancement of antiproton productions in d + A reaction at lower incident energies per nucleon and the existence of non-nucleon components in the deuteron wave function was suggested to cure the situation.