Subthreshold Antiproton Production Research Articles

Antiproton production in heavy-ion collisions at subthreshold energies * * Supported by the National Natural Science Foundation of China under Grant (11722546 and 11675226) and the Talent Program of South China University of Technology

Within the framework of the Lanzhou quantum molecular dynamics model, the deep subthreshold antiproton production in heavy-ion collisions has been investigated thoroughly. The elastic scattering, annihilation and charge exchange reactions associated with antiproton channels are implemented in the model. The attractive antiproton potential extracted from the G-parity transformation of nucleon selfenergies reduces the threshold energies in meson-baryon and baryon-baryon collisions, and consequently enhances the antiproton yields to some extent. The calculated invariant spectra are consistent with the available experimental data. The primordial antiproton yields increase with the mass number of the colliding system. However, annihilation reactions reduce the antiproton production which becomes independent of the colliding partners. Anti-flow phenomena of antiprotons correlated with the mean field potential and annihilation mechanism is found by comparing them with the proton flows. Possible experiments at the high-intensity heavy-ion accelerator facility (HIAF) in China are discussed.

6a-J-1 pA, dA反応におけるSubthreshold p生成実験III

6a-J-1 pA, dA反応におけるSubthreshold p生成実験III

Target mass number dependence of subthreshold antiproton production in proton-, deuteron- and alpha-particle-induced reactions

Data from KEK on subthreshold as well as on π± and K± production in proton-, deuteron- and α-induced reactions at energies between 2.0 and 12.0 A GeV for C, Cu and Pb targets are described within a unified approach. We use a model which considers a nuclear reaction as an incoherent sum over collisions of varying numbers of projectile and target nucleons. It samples complete events and thus allows for the simultaneous consideration of all final particles including the decay products of the nuclear residues. The enormous enhancement of the cross section, as well as the moderate increase of meson production in deuteron and α induced compared to proton-induced reactions, is well reproduced for all target nuclei. In our approach, the observed enhancement near the production threshold is mainly due to the contributions from the interactions of few-nucleon clusters by simultaneously considering fragmentation processes of the nuclear residues. The ability of the model to reproduce the target mass dependence may be considered as a further proof of the validity of the cluster concept.

Subthreshold antiproton production in pC, dC and αC reactions

Data from KEK on subthreshold as well as on π± and K± production in pC, dC and αC reactions at energies between 3.5 and 12.0 A GeV are described for the first time within a unified approach. We use a model which considers a nuclear reaction as an incoherent sum over collisions of varying numbers of projectile and target nucleons. It samples complete events and thus allows for the simultaneous consideration of all final particles including the decay products of the nuclear residues. The enormous enhancement of the cross section, as well as the moderate increase of meson production in dC and αC compared to pC reactions, is well reproduced. In our approach, the observed enhancement near the production threshold is mainly due to the contributions from the interactions of few-nucleon groups.

Subthreshold antiproton production in proton–carbon reactions

Data from KEK on subthreshold as well as on π± and K± production in proton–nucleus reactions are described at projectile energies between 3.5 and 12.0 GeV. We use a model which considers a hadron–nucleus reaction as an incoherent sum over collisions of the projectile with a varying number of target nucleons. It samples complete events and thus allows for the simultaneous consideration of all particle species measured. The overall reproduction of the data is quite satisfactory. It is shown that the contributions from the interaction of the projectile with groups of several target nucleons are decisive for the description of subthreshold production. Since the collective features of subthreshold production become especially significant far below the threshold, the results are extrapolated down to COSY energies. It is concluded that an measurement at ANKE-COSY should be feasible, if the high background of other particles can be efficiently suppressed.

Subthreshold antiproton production in pA, dA and αA reactions

An enormous enhancement of antiproton production in deuteron- and α-induced reactions has been observed in the subthreshold energy region between 2 and 5 GeV/nucleon. Antiprotons produced at 5.1° with a momentum range of between 1.0 and 2.5 GeV/ c were measured by a beam-line spectrometer and identified by the time-of-flight method. The production cross sections in the deuteron- and α-induced reactions at an incident energy of 3.5 GeV/nucleon were 2 and 3 orders of magnitude larger than those in proton-induced reaction at the same energy. The enhancement in light-ion reactions could not be explained by the internal motion in the projectile and target nuclei. The target-mass dependence (C, Al, Cu and Pb) of the cross sections has also been studied. Further, the cross sections of π and K productions were measured.

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

The production of antiprotons in pp collisions is investigated close to threshold where experimental data about the total cross section are not available. We analyze the latter reaction within the LUND string model for inclusive p production and within the framework of a one-boson exchange model for the exclusive reaction pp → ppp p . The application of our new results to the analysis of subthreshold antiproton production in d + p and d + d collisions shows cross sections that are much lower than expected before. Nevertheless, the comparison of experimental p differential cross sections from d + p and d + d is expected to provide valuable information about a nonnucleonic component in the deuteron wavefunction.

Probing the deuteron structure at small NN distances by subthreshold antiproton production

It is shown that subthreshold antiproton production in d - p and d - d collisions can provide new information about the deuteron structure at small internuclear distances. Detailed predictions for future experiments at KEK are made which indicate, that a signal of the non-nucleon component in the deuteron can be obtained from the differential p yield and in an almost model independent way from the ratio of the p spectra from d - d and d - p reactions.

Multi-nucleon mechanism for subthreshold antiproton production in proton-nucleus reactions

We study a many-body (multipion) mechanism for ¯p subthreshold production in proton-nucleus collisions. The incoming proton and the interacting nucleons in the target act as sources of pions that merge to produce a nucleon-antiproton pair. The calculation takes fully into account the initial state interaction and antiproton absorption in the final state. The cooperation of several nucleons in the initial state reduces the threshold for the reaction allowing for production rates that may account for 15–50% of the total ¯p production. We point out that this contribution should be added to the primary ¯p production mechanism due toΔ formation and rescattering, as found by other groups, and should be taken into account when trying to extract information about ¯p properties in the nuclear medium.

Analysis of subthreshold antiproton production in p-nucleus and nucleus-nucleus collisions in the relativistic Boltzmann-Uehling-Uhlenbeck approach.

We calculate the subthreshold production of antiprotons in the Lorentz-covariant relativistic Boltzmann-Uehling-Uhlenbeck (RBUU) approach employing a weighted testparticle method to treat the antiproton propagation and absorption nonperturbatively. We find that the antiproton differential cross sections are highly sensitive to the baryon and antiproton self-energies in the dense baryonic environment. Adopting the baryon scalar and vector self-energies from the empirical optical potential for proton-nucleus elastic scattering and from Dirac-Brueckner calculations at higher density \ensuremath{\rho}>${\mathrm{\ensuremath{\rho}}}_{0}$ we examine the differential antiproton spectra as a function of the antiproton self-energy. A detailed comparison with the available experimental data for p-nucleus and nucleus-nucleus reactions shows that the antiproton feels a moderately attractive mean field at normal nuclear matter density ${\mathrm{\ensuremath{\rho}}}_{0}$ which is in line with a dispersive potential extracted from the free annihilation cross section.

Subthreshold antiproton production in nucleus-nucleus collisions.

Antiproton production in nucleus-nucleus collisions at energies below the threshold for its production from the nucleon-nucleon interaction in free space is studied in the relativistic Vlasov-Uehling-Uhlenbeck model. The antiproton self-energies in the medium are obtained in the mean-field approximation from the G-parity transformation of the nucleon self-energies. Due to the decrease of antiproton mass in the medium as a result of the attractive scalar mean field, there is an enhanced production of primordinal antiprotons which are, however, mostly annihilated by nucelons as they propagate through the matter. The calculated final antiproton momentum spectrum is seen to agree reasonably with the experimental data.

Subthreshold antiproton production in nucleon-nucleus and nucleus-nucleus collisions

New experimental data on antiproton production by carbon and deutron ions on copper and carbon nuclei at 3.65 GeV/nucleon and by proton at 3.65–8.1 GeV incident energy were obtained. Production angle is 24° and antiproton momentum is 0.8 GeV/c in the laboratory system.

Multiparticle interactions in backward proton production, subthreshold antiproton production, and inclusive electron scattering from nuclei.

The phenomena of backward proton emission and subthreshold antiproton production in the proton-nucleus interactions, and findings from the inclusive scattering of electrons from nuclei, are interpreted in terms of multiparticle interactions. Rates for multiparticle collisions are calculated using the results from many-body theory, with the products of the squared interaction amplitudes and Wigner functions. The rates for the processes with different number of particles are proportional to the different powers of spatial density. The many-body amplitudes are expanded in terms of propagators and elementary amplitudes with only two particles in the initial state. The leading processes are isolated on a tree level. They correspond to the expansion of a ground-state nuclear wave function. Reasonable agreement with data is obtained.

Internal nuclear momentum and subthreshold anti-proton production in p-nucleus and nucleus-nucleus collisions

We investigate subthreshold anti-proton production in p-nucleus and nucleus-nucleus collisions in terms of the internal nuclear momentum of the nucleons in the colliding nuclei. We employ a first collision model which contains the kinematics of individual NN collisions, with the inclusion of internal nuclear momentum distributions extracted from data on quasi-elastic electron scattering and backward proton production. Calculations for subthreshold particle production in p-nucleus collisions show excellent agreement with experimental data on subthreshold anti-proton and K + production in p + Cu collisions. An extension of the model to nucleus-nucleus collisions includes also the internal momentum for the incoming projectile nucleons, and contains the parameters obtained from a best fit to the p + Cu → p + X data. Despite the excellent agreement with the p + Cu → p + X data, this model underestimates recent experimental data on subthreshold p production in nucleus-nucleus collisions by large factors. Implications for collective phenomena at large nuclear densities and temperatures are discussed.

Subthreshold antiproton, K-, K+, and energetic-pion production in relativistic nucleus-nucleus collisions.

We present a measurement of subthreshold antiproton, kaon, and energetic pion production at 0\ifmmode^\circ\else\textdegree\fi{} for the reactions $^{28}$Si${+}^{28}$Si at 2.0 GeV/nucleon (${E}_{\mathrm{available}/\mathrm{A}=0.82}$ GeV) and 1.65 GeV/nucleon (${E}_{\mathrm{available}/\mathrm{A}=0.70}$ GeV), and for $^{20}$Ne+NaF at 2.0 GeV/nucleon. For a given reaction, the particle yields, plotted as a function of the required excitation energy, exhibit a scaling behavior independent of particle species. Implications for collectivity and a common production mechanism are discussed.

Subthreshold antiproton production in nucleus-nucleus collisions.

Antiproton production in nucleus-nucleus collisions at energies below the threshold for its production from the nucleon-nucleon interaction in free space is studied in the relativistic Vlasov-Uehling-Uhlenbeck model. The antiproton self-energies in the medium are obtained in the mean-field approximation from the G-parity transformation of the nucleon self-energies. Due to the decrease of antiproton mass in the medium as a result of the attractive scalar mean field, there is an enhanced production of primordinal antiprotons which are, however, mostly annihilated by nucelons as they propagate through the matter. The calculated final antiproton momentum spectrum is seen to agree reasonably with the experimental data.

Subthreshold antiproton production in 28Si+28Si collisions at 2.1 GeV/nucleon.

We report on the first observation of subthreshold antiproton production in nucleus-nucleus collisions. This measurement was made for the system /sup 28/Si+/sup 28/Si at a bombarding energy of 2.1 GeV/nucleon (kinetic energy per NN pair in the c.m. frame /similar to/850 MeV). A differential cross section d/sup 2/sigma/dP d ..cap omega.. of 80 +- 40 nb/sr (GeV/c) was measured for p-bar production at 1.9 GeV/c and 0/sup 0/. This result is 3 orders of magnitude larger than that predicted by a calculation incorporating internal motion of the nucleons in the colliding nuclei.