Antikaon production and medium effects in proton-nucleus reactions at subthreshold beam energies

The inclusive K ^--meson production in proton-nucleus collisions in the subthreshold energy regime is analyzed in the framework of an appropriate folding model for incoherent primary proton-nucleon and secondary pion-nucleon production processes, which takes properly into account the struck target nucleon momentum and removal energy distribution (nucleon spectral function), novel elementary cross-sections for proton-nucleon reaction channel close to threshold as well as nuclear mean-field potential effects on the one-step and two-step antikaon creation processes. A detailed comparison of the model calculations of the K ^- differential cross-sections for the reactions p + ⁹Be and p + ⁶³Cu at subthreshold energies with the first experimental data obtained at the ITEP proton synchrotron is given, that displays both the relative role of the primary and secondary production channels at considered incident energies and the contributions to the K ^- production coming from the use of the single-particle part as well as high-momentum-energy part of the nucleon spectral function. It is found that the pion-nucleon production channel does not dominate in the subthreshold “hard” antikaon production in p ⁹Be-, p ⁶³Cu-collisions and the main contributions to the antikaon yields here come from the direct K ^- production mechanism. The influence of the nucleon, kaon and antikaon mean-field potentials on the K ^- yield is explored. It is shown that the effect of the nucleon mean-field is of importance in explaining the considered experimental data on “hard” antikaon production, whereas the K ⁺ and K ^- optical potentials play a minor role. The sensitivity of the subthreshold “soft” antikaon production in p ⁹Be-, p ¹²C-reactions to the nucleon, kaon and antikaon mean fields is studied. It is demonstrated that, contrary to the case of “hard” antikaon production, the K ^- potential has a very strong effect on the K ^- yield, which is greater than that from nucleon effective potential. Received: 20 July 2000 / Accepted: 4 December 2000     

Subthreshold <Emphasis Type="Italic">K</Emphasis><Superscript>−</Superscript>-meson production in proton-nucleus reactions revisited

The inclusive K^?-meson production in proton-nucleus collisions in the subthreshold energy regime is studied in the framework of an appropriate spectral function approach for incoherent primary proton-nucleon and secondary pion-nucleon production processes, which takes properly into account the nuclear mean-field potential effects on these processes as well as the final state interaction (FSI) among the outgoing nucleons participating in the one-step antikaon creation process. A detailed comparison of the model calculations of the K^? differential cross sections is given for the reactions p + ⁹Be, p + ⁶³Cu, and p + ¹⁹⁷Au at subthreshold energies with the currently available experimental data obtained recently at the ITEP proton synchrotron and at SIS/GSI. It is found that the calculations with inclusion of the influence of both the nuclear density-dependent mean-field potentials and the elementary NN-FSI effects on the K^? production from direct mechanism are able to reproduce, contrary to previous estimates based on the use only of the density-dependent mean fields in calculating the K^? yield from this mechanism, the energy dependences of the invariant differential cross sections for “hard” antikaon creation in p⁹Be and p⁶³Cu collisions. It is further shown that the NN-FSI effects play a minor role in describing the data on the spectrum of relatively soft K^? mesons from p¹⁹⁷Au interactions at incident energy of 2.5 GeV. It is also shown that the relative strength of the proton-and pion-induced reaction channels in the subthreshold energy regime is governed by the kinematics of the experiment under consideration. The influence of the nucleon, kaon, and antikaon mean-field potentials on the K^? yield is explored. It is demonstrated that, in line with previous findings, the K^? optical potential has a strong effect on this yield at low antikaon momenta, which is greater than those from nucleon and kaon effective potentials. At high antikaon momenta, the K^? yield is found to be mainly determined, along with the elementary NN-FSI effects, by the nucleon mean field and the scenario with zero K⁺ potential is favorable.     

Subthreshold K<Superscript>-</Superscript>-meson production in proton-nucleus reactions revisited

We study the inclusive K^--meson production in proton-nucleus collisions in the subthreshold energy regime in the framework of a spectral function approach for incoherent primary proton-nucleon and secondary pion-nucleon production processes. Our approach takes properly into account the effect of the nuclear mean-field potentials on these processes, as well as the final-state interaction (FSI) among the outgoing nucleons participating in the one-step antikaon creation process. A detailed comparison of the model calculations of the K^- differential cross-sections for the reactions p + ⁹Be, p + ⁶³Cu, and p + ¹⁹⁷Au at subthreshold energies with the currently available experimental data obtained recently at the ITEP proton synchrotron and at SIS/GSI is given. We find that our calculation, which includes both the nuclear density-dependent mean-field potentials and the elementary NN-FSI effects on the K^- production from the direct mechanism, is able to reproduce the energy dependences of the invariant differential cross-sections for the “hard” antikaon creation in p- ⁹Be and p- ⁶³Cu collisions. This result contradicts previous estimates which used only density-dependent mean fields in calculating the K^- yield from this mechanism. We further show that the NN-FSI effects play a minor role in describing the data on the spectrum of the relatively soft K^--mesons from p- ¹⁹⁷Au interactions at incident energy of 2.5GeV. We find that the relative strength of the proton- and pion-induced reaction channels in the subthreshold energy regime is governed by the kinematics of the experiment under consideration. We also explore the influence of the antikaon mean-field potential on the K^- yield at low antikaon momenta. Received: 26 July 2001 / Accepted: 23 December 2002 / Published online: 8 April 2003     

Subthreshold and near-threshold K +-meson production on light nuclei by protons

Inclusive K ⁺-meson production in proton-nucleus collisions in the near-threshold and subthreshold energy regimes is analyzed with respect to the one-step (pN → K ⁺ YN, Y=Λ, Σ) and two-step (pN → NNπ, NN2π; πN → K ⁺ Y) incoherent production processes. An appropriate folding model is used that properly takes into account the struck-target-nucleon-removal energy and the momentum distribution (nucleon spectral function), novel elementary cross sections for proton-nucleon reaction channel close to threshold, as well as nuclear-mean-field-potential effects on the one-step and two-step kaon-creation processes. A detailed comparison of the model calculations of the K ⁺ total and differential cross sections for the p+⁹Be and p+¹²C interactions with the existing experimental data is given, which displays both the relative role of the primary and secondary production channels at considered incident energies and those features of the cross sections that are sensitive to the high-momentum and high-removal-energy parts of the nucleon spectral function. It is found that, contrary to previous studies known in the literature, the pion-nucleon production channels do not necessarily dominate in pA collisions at subthreshold energies and that the relative strength of the proton-and pion-induced reaction channels for light target nuclei in the subthreshold energy regime is governed by the kinematics of the experiment under consideration.     

SubthresholdK + production in proton-nucleus collisions as a probe for short range nucleon correlations

Double differential cross sections for nearthreshold and subthreshold production ofK ⁺ with momentum 1.28 GeV/c at 10.5 degree on Be, Al and Cu targets had been measured. The experiment was performed with FH Spectrometer at ITEP PS in projectile proton kinetic energy range 2.9 – 1.65 GeV. The performed analysis of the excitation functions and target atomic mass dependencies evidences for high order multinucleon correlations are involved in deep subthreshold kaon production.     

Medium effects in ΛK~+ pair production by 2.83 GeV protons on nuclei

We study ΛK~+ pair production in the interaction of protons of 2.83 GeV kinetic energy with C, Cu,Ag, and Au target nuclei in the framework of the nuclear spectral function approach for incoherent primary proton–nucleon and secondary pion–nucleon production processes, and processes associated with the creation of intermediate Σ~0K~+ pairs. The approach accounts for the initial proton and final Λ hyperon absorption, final K~+ meson distortion in nuclei, target nucleon binding, and Fermi motion, as well as nuclear mean-field potential effects on these processes.We calculate the Λ momentum dependence of the absolute ΛK~+ yield from the target nuclei considered, in the kinematical conditions of the ANKE experiment, performed at COSY, within the different scenarios for the Λ-nucleus effective scalar potential. We show that the above observable is appreciably sensitive to this potential in the low-momentum region. Therefore, direct comparison of the results of our calculations with the data from the ANKE-at-COSY experiment can help to determine the above potential at finite momenta. We also demonstrate that the two-step pion–nucleon production channels dominate in the low-momentum ΛK~+ production in the chosen kinematics and, therefore, they have to be taken into account in the analysis of these data.     

Antikaon production in nucleon- nucleon reactions near threshold

The antikaon production cross section from nucleon-nucleon reactions near threshold is studied in a meson exchange model. We include both pion and kaon exchange, but neglect the interference between the amplitudes. In case of pion exchange the antikaon production cross section can be expressed in terms of the antikaon production cross section from a pion-nucleon interaction, which we take from the experimental data if available. Otherwise, a K* -resonance exchange model is introduced to relate the different reaction cross sections. In case of kaon exchange the antikaon production cross section is related to the elastic KN and K?N cross sections, which are again taken from experimental measurements. We find that the one-meson exchange model gives a satisfactory fit to the available data for the NN → NN KK? cross section at high energies. We compare our predictions for the cross section near threshold with an earlier empirical parameterization and that from phase space models.     

Inclusive neutral kaon production in 70 GeV/c K+p interactions

The inclusive production of neutral kaons in 70 GeV/c K⁺p interactions is studied with the CERN BEBC bubble chamber. The (semi-)inclusive cross sections are interpreted in terms of the various strangeness channels leading to neutral kaon production. The invariant inclusive cross section for kaon production is studied as a function of p_t² and the Feynman variable x. The latter distributions are considered both “raw” and corrected for the presence of K⁰'s resulting from K¹ decay. They are compared with the predictions expected from the Regge-Mueller formalism, the recombination model and fragmentation models.     

SubthresholdK + production on nuclei byπ + mesons

The inclusiveK ⁺ mesons production inπ ⁺-nucleus reactions in the subthreshold energy regime is analyzed with respect to the one-step (π ⁺ n→K ⁺ Λ) and the two-step (π ⁺ n→ηp ₁,ηp ₂→K ⁺ Λ) incoherent production processes on the basis of an appropriate folding model, which allows one to take into account the various forms of an internal nucleon momentum distribution as well as on- and off-shell propagation of the struck target nucleon. Contrary to proton-nucleus reactions primary reaction channel is found to be significant practically at all considered energies. Detailed predictions for theK ⁺ total and invariant differential cross sections fromπ ⁺C¹²- andπ ⁺Pb²⁰⁸-collisions at subthreshold energies are provided.     

Antikaon production in proton-nucleus reactions and the K− properties in nuclear matter

We calculate the momentum-dependent potentials for K⁺ and K⁻ mesons in a dispersion approach at nuclear density ϱ₀ using the information from the vacuum K⁺N and K⁻N scattering amplitudes, however, leaving out the resonance contributions for the in-medium analysis. Whereas the K⁺ potential is found to be repulsive (≈ + 25 MeV) and to show only a moderate momentum dependence, the K⁻ self-energy at normal nuclear matter density turns out to be ≈ − 140 ± 25 MeV at zero momentum roughly in line with K⁻ atomic data, however, decreases rapidly in magnitude for higher momenta. The antikaon production in p + A reactions is calculated within a coupled channel transport approach and compared to the data at KEK including different assumptions for the antikaon potentials. Furthermore, detailed predictions are made for p+¹²C and p+²⁰⁷Pb reactions at 2.5 GeV in order to determine the momentum-dependent antikaon potential experimentally.     

K + production in proton and deuteron induced reactions at subthreshold energies

The production ofK ⁺ mesons in proton-nucleus and deuteron-nucleus collisions is analyzed with respect to one-step nucleon-nucleon (NN → N ΛK ⁺),Δ- nucleon (ΔN → N Λ K⁺) and two-step pion-nucleon (πN → K ⁺ Λ) production channels on the basis of experimental ground state momentum distributions and free on-shell production processes. Whereas forK ⁺ production in proton-nucleus reactions the secondary channelπN clearly dominates at subthreshold energies, meson and nucleon induced channels are of similar magnitude in deuteron-nucleus reactions. Contrary to nucleus-nucleus collisions theΔ induced reaction channels are found to be of minor importance. The experimental differentiation of the underlying microscopic reaction channels appears possible via differential proton —K ⁺ coincidence measurements as shown in detail by the microscopic simulations including proton rescattering.     

Investigation of the subthreshold production of <Emphasis Type="Italic">K</Emphasis><Superscript>−</Superscript> mesons in proton-nucleus collisions

The differential cross sections for the proton-induced production of K ^? mesons on Be, Al, and Cu nuclei in the subthreshold energy region have been measured. Folding-model calculation including three channels of their production satisfactorily reproduces the experimental data. It has been shown that the main contribution to the production of hard K ^? mesons comes from the nonresonant direct process. Evidence for the existence of an attractive optical potential with a magnitude of about 140 MeV for K ^? at normal nuclear density has been obtained.     

Fragmentation of 7- to 9-GeV/c deuterons to cumulative kaons

Experimental results concerning the kaon yield in the reaction d + Be → K ^±(0°) + X for values of the cumulative variable x _c in the range between 0.88 and 1.37 are presented. Values in the region of x _c ≥ 1 correspond to short nucleon-nucleon distances (deuteron-core region) at which nucleon wave functions begin overlapping each other, forming a cluster (flucton) where the hadron-matter density is higher than the average matter density in the nucleus. The behavior of the K ⁺/K ^? yield ratio in this region may be interpreted within the hypothesis that the quark sea in nuclei becomes harder because of the presence of a flucton component in nuclear matter.     

Subthreshold and near-threshold K +-meson photoproduction on nuclei

The inclusive K ⁺-meson production in photon-induced reactions in the near-threshold and subthreshold energy regimes is analyzed for the one-step (γN → K ⁺ Y, Y=Λ, Σ) incoherent production processes on the basis of an appropriate new folding model that takes properly into account the struck-target nucleonremoval energy and the internal momentum distribution (nucleon spectral function), extracted from recent quasielastic-electron-scattering experiments and from many-body calculations based on realistic models of NN interaction. Simple parametrizations of the total and differential cross sections for K ⁺ production in photon-nucleon collisions are presented. A comparison of the model calculations of the K ⁺ differential cross sections for γ¹²C interactions in the threshold region with existing experimental data is given, which displays the contributions to K ⁺ production at considered incident energies from the use of the single-particle part, as well as high momentum and high removal energy part, of the nucleon spectral function. Detailed predictions for the K ⁺ total and differential cross sections for γ²H, γ¹²C, and γ²⁰⁸Pb interactions at subthreshold and near-threshold energies are provided. The effect of the uncertainties in the elementary K ⁺-production cross sections on the K ⁺ yield is explored.     

Kaon production in S+nucleus and Pb+Pb collisions at CERN SPS

Charged and neutral kaon production in S-nucleus and Pb-Pb collisions has been measured at CERN in the NA35 and NA49 experiments, close to mid-rapidity. The production ofK ^± in S+S, Ag, Au at 200 GeV/nucleon was measured via their one-prong (kink) topology inside a Time Projection Chamber (TPC). The resulting charged kaon yields are in agreement with other measurements done in the same experiment. TheK ⁺/K ^? ratio was found to be 1.30±0.15 in central S+Au interactions. The production ofK _S ⁰ in Pb+Pb collisions was measured using the NA49’s large (Main) TPCs outside the magnetic field. The resulting yields are compatible with other measurements performed in the same experiment, within the quoted systematic errors. The preliminary rapidity distribution in Pb+Pb scales with the number of participants if compared to S+S collisions. From this we conclude that no further strangeness enhancement (relative to nucleon-nucleon collisions) is observed in going from S+S to Pb+Pb.     

Production of charged pions, kaons and antikaons in relativistic C + C and C + Au collisions

Production cross-sections of charged pions, kaons and antikaons have been measured in C+C and C+Au collisions at beam energies of 1.0 and 1.8 AGeV for different polar emission angles. The kaon and antikaon energy spectra can be described by Boltzmann distributions whereas the pion spectra exhibit an additional enhancement at low energies. The pion multiplicity per participating nucleon M(π⁺)/<A _part> is a factor of about 3 smaller in C+Au than in C+C collisions at 1.0 AGeV whereas it differs only little for the C and the Au target at a beam energy of 1.8 AGeV. The K⁺ multiplicities per participating nucleon M(K⁺)/ <A _part> are independent of the target size at 1 AGeV and at 1.8 AGeV. The K^- multiplicity per participating nucleon M(K^-)/ <A _part> is reduced by a factor of about 2 in C+Au as compared to C+C collisions at 1.8 AGeV. This effect might be caused by the absorption of antikaons in the heavy target nucleus. Transport model calculations underestimate the K^-/K⁺ ratio for C+C collisions at 1.8 AGeV by a factor of about 4 if in-medium modifications of K-mesons are neglected. Received: 10 December 1999 / Accepted: 14 November 2000     

Unified description of transverse momentum spectrums contributed by soft and hard processes in high-energy nuclear collisions

The transverse momentum spectrums of π ^?, π ⁺, K ^?, K ⁺, \(\bar p\) , and p produced in p-Pb collisions at √s _NN = 502 TeV measured by the CMS Collaboration and in Pb-Pb collisions at √s _NN = 2.76 TeV measured by the ALICE Collaboration are described by a two-component Erlang distribution. The first component corresponds to “soft” excitation process and contributes in the low transverse momentum region, which is contributed by 2–5 partons (sea quarks and gluons) with strong interactions. The second component corresponds to “hard” scattering process and contributes in the high transverse momentum region, which is contributed by 2 partons (valent quarks) with violent head-on collision. Each parton source contributes an exponential transverse momentum spectrum. Both the soft and hard processes result in an Erlang distribution. The transverse momentum spectrums of final-state charged particles are then described by the two-component Erlang distribution. The contribution ratio (30–40%) of the hard process extracted from nuclear collisions at the Large Hadron Collider is consistent with that (17–46%) obtained from other methods.     

On large transverse momentum production in high-energy proton-nucleus collisions

Experimental data from inclusive production of large p_T hadronsin high-energy proton-nucleus collisions are analysed with emphasis on the influence of the size of the target nucleus. It is found that data at p_T?3 GeV/c are consistent with the hypothesis that the incoming proton collides “simultaneously” with all the nucleons in its way, resulting in an effective collision energy larger than the one expected in a single proton-nucleon hit. At p_T > 3 GeV/c, data are in conflict with such a simple model, a result we think is a consequence of the dominance of hard parton collisions at very large p_T.     

Inclusive single and double diffractive dissociations inK + p interactions at 32 GeV/c

We study single and double inclusive diffractive production in a 32 GeV/c K ⁺ p experiment in MIRABELLE at the Serpukhov accelerator. From reactionsK ⁺ p→K ⁺+X ⁺ andK ⁺ p→X ⁺+p we determine the total proton and kaon single diffractive dissociation cross sections \(\sigma (p\xrightarrow{{K^ + }}p_D ) = 0.90 \pm 0.12 mb\) and \(\sigma (K^ + \xrightarrow{p}K_D^ + ) = 0\) . 90±0.17 mb, respectively. In either case the only notable contributions come from dissociations into 1 and 3 charged particles. Kaon dissociation exhibits a pronounced slope-mass correlation. The search for double diffractive production in reactionsK ⁺ p→(K ⁺π^?π⁺)+X ⁺ andK ⁺ p→(pπ ⁺ π ^?)+X ⁺ leads in either case to an estimated total double diffractive cross section σ(K ⁺p→K _D ⁺ p_D) of ?220 μb. The double dissociation differential cross section exhibits a large slope of ?10GeV^?2 in the nearthreshold mass region, rapidly decreasing to ?4 GeV^?2 with increasing excitation mass. At our c.m. energy \((\sqrt s \simeq 8 GeV)\) the ratio σ_{inel dif}/σ_el is 0.85±0.10, the total diffractive cross section σ_dif≡σ_el+σ_{inel dif}=4.41±0.24 mb and the fraction σ_dif(K ⁺ p)/σ_tot(K ⁺ p) is 25±2%. TheK ⁺ andK ^? diffractive excitation mass spectra, differential cross sections and total diffractive cross sections are very similar for both single and double dissociations.     

Experimental cross section for dimuon production and the Drell-Yan model

We have measured the absolute cross section for dimuon production in hadron collisions at 200 GeV/c in the continuum region M = 4–8.5 GeV. In all the channels studied (pN, $\text{p}$N, π^±N and π^?H₂) the experimental cross section is significantly larger by a factor of 2.3 ± 0.5 than expected from the Drell-Yan model. Furthermore, our proton-nucleon data allow a determination of the nucleon valence structure function which agrees with the deep inelastic lepton scattering data.