Resonance model study of strangeness production in pp collisions

Results for the energy dependence of the elementary kaon production cross sections in proton-proton collisions are reported. Calculations are performed within an extended version of the resonance model which was used for the previous studies of elementary kaon production in pion-nucleon and pion-Δ collisions. Although the model treatment is within theempirical tree level (observed widths for the resonances are used), it is fully relativistic, and includes all relevant baryon resonances up to 2 GeV. One of the purposes of this study is to provide the results for the simulation codes of subthreshold kaon production in heavy ion collisions. This is the first, consistent study of the elementary kaon production reactions including both πB and BB (B = N,Δ) collisions on the same footing. Comparisons are made between the calculated results and the existing semi-empirical parametrizations which are widely used for the simulation codes, as well as the experimental data.     

Baryon yields, isospin effects and strangeness production in elementary hadronic interactions

New NA49 data from hadron+proton and hadron+Pb interactions are used to extract detailed information on projectile isospin effects and baryon, baryon pair and strangeness production. Consequences for the interpretation of hadron production and strangeness enhancement in nuclear collisions are discussed.     

Rapid hadronization and strangeness production

A fast hadronization scenario is presented where a central role is played by the chiral symmetry break-down in the expanding Quark-Gluon Plasma. This mechanism can become effective when thermal damping ceases after the thermal freeze-out of the quark system. We estimate time scales and spatial characteristics of chiral-symmetry breaking instabilities on the basis of an effective field-theoretical model. It is argued that this fast process provides a basis to understand the measured large abundances of strange antibaryons in heavy ion collisions.     

Associated strangeness production at threshold

The associated strangeness dissociation at threshold has been studied at the COSY-11 facility measuring the hyperon and the K ⁺ K ^- meson pair production. Measurements of the near-threshold and production via the reaction (S. Sewerinet al., Phys. Rev. Lett. 83, 682 (1999)) at COSY-11 have shown that the cross-section ratio exceeds the value at high excess energies ( MeV (A. Baldiniet al., Total Cross-Sections for Reactions of High-Energy Particles, Landolt-Börnstein, New Series, Vol. I/12 (Springer, Berlin, 1988))) by an order of magnitude. For a better understanding additional data have been taken between 13 MeV and 60 MeV excess energy. The near-threshold production of the charged kaon-antikaon pair is related to the discussion about the nature of the scalar states in the 1 GeV/c ² mass range, i.e. the f ₀(980) and a ₀(980) (O. Krehl, R. Rapp, J. Speth, Phys. Lett. B 390, 23 (1997)). The interpretation as a molecule is strongly dependent on the K- interaction which can be studied via the production channel. A first total cross-section value on the reaction at an excess energy of 17 MeV (C. Quentmeieret al., Phys. Lett. B 515, 276 (2001)), i.e. below the production threshold, was measured.Received: 30 September 2002, Published online: 22 October 2003PACS: 13.75.-n Hadron-induced low- and intermediate-energy reactions and scattering (energy GeV) - 14.20.Jn Hyperons - 14.40.Aq , K, and mesons - 25.40.Ep Inelastic proton scattering     

Photon production in 69 GeV pp interactions

We present results on 728 photon conversions into electron pairs observed in the hydrogen bubble chamber MIRABELLE. The average number of π° per inelastic pp collision is found to be 2.57 ± 0.13 and this number increases with the number n of charged particles up to n=12. The comparison of our invariant cross section F(x, p⊥, s) for the reaction pp → γ X with higher energy results shows that scaling behaviour is probably reached at 70 GeV.     

Charge and strangeness transfer in multiparticle production

A simple explicit model for charge production is found to agree well with available data. This model combined with unitarity gives a realistic estimate for the suppression of two-body charge exchange scattering. Similar considerations are applied to strangeness transfer.     

Double strangeness production in antiproton annihilation on nuclei

The recent data on double strangeness production after antiproton annihilation on nuclei are analyzed within the conventional picture of the reaction, namely annihilation on a single nucleon, followed by rescattering of the products in the nucleus, assumed to proceed in a hadronic phase. General expressions for single and double strangeness are derived within this picture. The relevant parameters are determined, as far as possible, from the single strangeness production yields and are used to predict double strangeness production yields. The later are compared with experiment and found consistent with the conventional picture. However, the data seem to indicate that the strangeness production in the primordial annihilation is different from what it is in nucleon-antinucleon reactions. An estimate of theΞ production rate is also performed. It is shown that the latter is much less sensitive to the rescattering process than in theΛ production case.     

Dilepton and strangeness production probed with HADES

With the High Acceptance Di-Electron Spectrometer (HADES) at GSI we have studied dilepton production in the few-GeV energy regime in various collisions systems, from elementary NN, over pA, up to the medium-heavy Ar + KCl system. We have thus confirmed the puzzling results of the former DLS Collaboration at the Bevalac. While we have traced the origin of the excess pair yield in CC collisions to elementary pp and pn processes, in our Ar + KCl data a contribution from the dense phase of the collision has been identified. Together with the e ⁺ e ^? pairs, we have also obtained in the Ar + KCl system at 1.76 A GeV a high-statistics data set on open and hidden strangeness, i.e. K ^±, K _s ⁰ , ??, ?, and ??^?, allowing for a comprehensive discussion of strangeness production in this system.     

Probing dense matter with strangeness production in nuclear collisions

Experiments on strangeness production in nucleus–nucleus collisions at SIS energies address fundamental questions of modern nuclear physics: the determination of the nuclear equation-of-state at high baryon densities and the properties of hadrons in dense nuclear matter. From the yields of K⁺ mesons measured in heavy-ion collisions a value for the nuclear compressibility of is extracted for nuclear densities around twice saturation density using different microscopic transport models. Both the yield of K⁺ mesons and their anisotropic azimuthal angular distribution (elliptic flow) exhibit strong evidence for a repulsive K⁺N potential. The yields of K⁺ and K⁻ mesons measured in proton–nucleus collisions can be reproduced by transport calculations assuming at saturation density a repulsive K⁺N potential of and an attractive K⁻N potential of .     

Intriguing aspects of strangeness production at CERN energies

Strange particle production in pp, pA and AA collisions at CERN SPS energies is reviewed. First results from Pb beam experiments are briefly presented. The emerging picture (still incomplete) is discussed.     

Soft hadron production in pp interactions up to ISR energies

Soft hadron production is described as a two-step process, where the interaction of the partonic constituents of the colliding hadrons leads to the production of intermediate subsystems (fireballs), which decay subsequently into hadrons. The weights of the various final states are derived from the corresponding phase-space factors modified by empirical transition elements. The results compare well with data at energies between particle production thresholds and ISR energies. Special emphasis is put on correlation data, which offer the opportunity to shed some light on the question whether particle production proceeds via fireballs or strings. Received: 20 July 2000 / Revised version: 26 October 2000 / Published online: 21 December 2000     

Inclusive neutral kaon and lambda production in 69 GeV pp interactions

From a study of 180 neutral vees in the hydrogen bubble chamber Mirabell, we have obtained respectively, for the cross sections of the inclusive reactions pp → K⁰X and pp → λX, 8.4 ± 1.1 and 3.6 ± 0.5 mb. Data presented for the neutral kaons and λ in terms of the invariant cross sections F(x, p_⊥, s) show, when compared with lower and higher energy results, that scaling is approximately valid at our energy for K^o and λ production.     

Search for exotic baryons with hidden strangeness in diffractive production processes

Experimental results of the SPHINX Collaboration on studying proton diffractive production processes are presented. Evidences for new baryon states with masses 2 GeV were obtained in hyperon-kaon effective mass spectra in several reactions. Unusual properties of these massive states (small enough decay widths, large branching ratios for their decays with strange particles in final states) make them serious candidates for cryptoexotic pentaquark baryons with hidden strangeness . New results for these states are presented here, as well as recent data on large violation of the OZI rule in proton diffractive dissociation.     

Multi-reaction-channel fitting calculations in a coupled-channel model: Photoinduced strangeness production

To describe photo- and meson-induced reactions on the nucleon, one is faced with a rather extensive coupled-channel problem. Ignoring the effects of channel coupling, as one would do in describing a certain reaction at the tree level, invariably creates a large inconsistency between the different reactions that are described. In addition, the imaginary parts of the amplitude, which are related through the optical theorem, to total cross-sections, are directly reflected in certain polarization observables. Performing a full coupled-channel calculation thus offers the possibility to implement the maximum number of constraints. The drawback one is faced with is to arrive at a simultaneous fit of a large number of reaction channels. While some of the parameters are common to many reactions, one is still faced with the challenge to optimize a large number of parameters in a highly non-linear calculation. Here we show that such an approach is possible and present some results for photoinduced strangeness production.