Antiproton production inp+A andd+A reactions at subthreshold energies

Abstract

The production of antiprotons in proton-nucleus and deuteron-nucleus collisions is analyzed with respect to onestep nucleon-nucleon (NN ---+ NNp/5) and secondary Anucleon (AN ~ NNN/5) production channels within a phase-space model incorporating the selfenergies of the baryons. It is found that contrary to nucleus-nucleus collisions the A induced reaction channels are of minor importance except at very low bombarding energies. The data from KEK are approximately reproduced when accounting for the internal momentum distribution of the deuteron which provides a natural explanation for the large enhancement of the/5 cross section in d+A as compared to p+A. 25.40.-h Antiproton production at energies of a few GeV/u is the most extreme subthreshold production process and has been observed in proton-nucleus collisions some time ago [1][2][3]. In the last years the problem was taken up again at KEK [4] and GSI [5] with new detector setups. Various descriptions for these data have been proposed (cf. PACS:[6] and refs. therein), most of them based on the assumption of kinetic and chemical equilibrium.First fully relativistic transport calculations for antiproton production including/~ annihilation as well as the change of the quasi-particle properties in the medium have been reported in [7]. There it was found that according to the reduced nucleon mass in the medium the threshold for /5-production is shifted to lower energy and the antiproton cross section prior to annihilation becomes dramatically enhanced as compared to a relativistic cascade calculation where no in-medium effects are incorporated. In fact, Teis et al. [6] have shown recently (within the relativistic BUU approach) that the t5 yields for p + A and A + A can be well reproduced simultaneously when employing proper selfenergies for the baryons in the dense medium.It is the aim of the present note to investigate the question, whether the KEK data on/5 production [4] -which show a dramatic enhancement for the deuteron projectile at the same bombarding energy per nucleon -can be understood within the same scheme as well. Since the RBUU transport approach is not well suited for modelling the proSupported by BMFT, GSI Darmstadt and Forschungszentrum J/ilich perties of a weakly bound deuteron, we use a phase-space approach [8,9] which allows for a proper incorporation of the deuteron momentum distribution.The phase-space model adopted is described in more detail in [9,10]. Its ingredients are as follows: apart from the elementary differential cross section E,jda@/dak~ the inclusive t5 cross section for p+ A reactions is determined by the momentum distribution p(p) describing the available momentum components in a nucleus as well as by pd(k) in case of d + A reactions. We use a Ganssian fit to the experimental momentum distribution for 12C from [11] and use pd(k) for the deuteron from the Paris NN-potential [12]. Whereas the latter two distributions are known sufficiently well, the t5 production cross section in the medium is still a matter of debate. Here we ado...

Antiproton production in pp, dp and dd collisions close to threshold

“…It is thus mandatory to analyse the elementary production in situations where the kinematical conditions are more clearly defined and where in-medium potentials as well as intermediate pion induced production channels as well as antiproton annihilation can approximately be neglected. This is clearly the case for pp collisions and also quite well fulfilled for d + p and d + d reactions according to the analysis in [12,13].…”

“…Although the proper magnitude of thep potential and its annihilation cross section in the medium is still a matter of debate, it is clear that the dominant production mechanism in nucleusnucleus collisions proceeds via intermediate baryon resonances since the latter act as short-time energy reservoirs for thep production [8,10,11]. On the other hand the N∆ or even ∆∆ channels play a minor role in proton-nucleus and deuteron-nucleus reactions since the resonances on average decay before colliding with another nucleon due to the much lower densities involved [12]. However, the poor knowledge of the elementary production cross section pp →p+X especially close to threshold leads to large ambiguities in the interpretation of the experimental data about subthresholdp production in p + A and A + A collisions and thus to sizeable uncertainties for the antiproton potential or selfenergy in the nucleus.…”

“…Although the proper magnitude of the p potential and its annihilation cross section in the medium is still a matter of debate, it is clear that the dominant production mechanism in nucleusnucleus collisions proceeds via intermediate baryon resonances since the latter act as short-time energy reservoirs for the p production [8,10,11]. On the other hand the N∆ or even ∆∆ channels play a minor role in proton-nucleus and deuteron-nucleus reactions since the resonances on average decay before colliding with another nucleon due to the much lower densities involved [12].…”

Antiproton Production in p + d Reaction at Subthreshold Energies

“…To explore this suggestion the BUU approach group performed calculations employing the deuteron wave function from 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 [18]. In this note we take up the most simple reaction p+d → p and clarify the enhancement of antiproton productions in comparison with the elementary process when we do not consider non-nucleonic components in the deuteron wave function.…”

“…To explore this suggestion the BUU approach group performed calculations employing the deuteron wave function from 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 [18]. Figure 1: The diagram for antiproton producing mechanism in proton-proton collision.…”