Showing papers on "Strangeness published in 1998"

On Chemical Equilibrium in Nuclear Collisions

Abstract: The data on average hadron multiplicities in central A+A collisions measured at CERN SPS are analysed with the ideal hadron gas model. It is shown that the full chemical equilibrium version of the model fails to describe the experimental results. The agreement of the data with the off-equilibrium version allowing for partial strangeness saturation is significantly better. The chemical freeze-out temperature of about 180 MeV seems to be independent of the system size (from S+S to Pb+Pb) and in agreement with that extracted in e+ + e−, p + p and $$p + \bar p$$ collisions. The strangeness suppression is discussed at both hadron and valence quark level. It is found that the hadronic strangeness saturation factor γs increases from about 0.45 for p + p interactions to about 0.7 for central A+A collisions with no significant change from S+S to Pb+Pb collisions indicating that the strangeness enhancement in heavy ion collisions cannot be fully attributed to the increased system size. The quark strangeness suppression factor As is found to be about 0.2 for elementary collisions and about 0.4 for heavy ion collisions independently of collision energy and type of colliding system.

On the Early Stage of Nucleus--Nucleus Collisions

Abstract: A statistical model of the early stage of central nucleus--nucleus (A+A) collisions is developed. We suggest a description of the confined state with several free parameters fitted to a compilation of A+A data at the AGS. For the deconfined state a simple Bag model equation of state is assumed. The model leads to the conclusion that a Quark Gluon Plasma is created in central nucleus--nucleus collisions at the SPS. This result is in quantitative agreement with existing SPS data on pion and strangeness production and gives a natural explanation for their scaling behaviour. The localization and the properties of the transition region are discussed. It is shown that the deconfinement transition can be detected by observation of the characteristic energy dependence of pion and strangeness multiplicities, and by an increase of the event--by--event fluctuations. An attempt to understand the data on J/psi production in Pb+Pb collisions at the SPS within the same approach is presented.

On the early stage of nucleus-nucleus collisions

Abstract: A statistical model of the early stage of central nucleus–nucleus (A+A) collisions is developed. We suggest a description of the confined state with several free parameters fitted to a compilation of A+A data at the AGS. For the deconfined state a simple Bag model equation of state is assumed. The model leads to the conclusion that a Quark Gluon Plasma is created in central nucleus–nucleus collisions at the SPS. This result is in quantitative agreement with existing SPS data on pion and strangeness production and gives a natural explanation for their scaling behaviour. The localization and the properties of the transition region are discussed. It is shown that the deconfinement transition can be detected by observation of the characteristic energy dependence of pion and strangeness multiplicities, and by an increase of the event–by–event fluctuations. An attempt to understand the data on J/ production in Pb+Pb collisions at the SPS within the same approach is presented.

Lattice calculation of the strangeness magnetic moment of the nucleon

Abstract: We report on a lattice QCD calculation of the strangeness magnetic moment of the nucleon. Our result is G{sub M}{sup s}(0)={minus}0.36{plus_minus}0.20. The sea contributions from the u and d quarks are about 80{percent} larger. However, they cancel to a large extent due to their electric charges, resulting in a smaller net sea contribution of {minus}0.097{plus_minus}0.037{mu}{sub N} to the nucleon magnetic moment. As far as the neutron to proton magnetic moment ratio is concerned, this sea contribution tends to cancel out the cloud-quark effect from the Z graphs and results in a ratio of {minus}0.68{plus_minus}0.04 which is close to the SU(6) relation and the experiment. The strangeness Sachs electric mean-square radius {l_angle}r{sub s}{sup 2}{r_angle}{sub E} is found to be small and negative. {copyright} {ital 1998} {ital The American Physical Society}

Effects of hyperons on the dynamical deconfinement transition in cold neutron star matter

Abstract: The influence of the presence of hyperons in dense hadronic matter on the quantum nucleation of quark matter is examined at low temperatures relevant to neutron star cores. We calculate the equation of state and the composition of matter before and after deconfinement by using a relativistic mean-field theory and an MIT bag model, respectively; the case in which hyperons are present in the hadronic system is considered, together with the case of the system without hyperons. We find that strangeness contained in hyperons acts to reduce a density jump at deconfinement as well as a lepton fraction in the hadronic phase. As a result of these reductions, a quark matter droplet being in a virtual or real state has its effective mass lightened and its electric charge diminished into nearly zero. The Coulomb screening of leptons on the droplet charge, which has significance to the droplet growth after nucleation in the absence of hyperons, is thus shown to be of little consequence. If the effective droplet mass is small enough to become comparable to the height of the potential barrier, the effect of relativity brings about an exponential increase in the rate of droplet formation via quantum tunneling, whereas the role played by energy dissipation in decelerating the droplet formation, dominant for matter without hyperons, becomes of less importance. For matter with and without hyperons, we estimate the overpressure needed to form the first droplet in the star during the compression due to stellar spin-down or mass accretion from a companion star. The temperature at which a crossover from the quantum nucleation to the Arrhenius-type thermal nucleation takes place is shown to be large compared with the temperature of matter in the core.

Off-shell effects in the electromagnetic production of strangeness

Abstract: Previous approaches to the photo- and electro-production of strangeness off the proton, based upon effective hadronic Lagrangians, are extended here to incorporate the so called off-shell effects inherent to the fermions with spin >= 3/2. A formalism for intermediate-state, spin 3/2, nucleonic and hyperonic resonances is presented and applied to the processes $\gamma + p ---> K^{+} + \Lambda$, for $E_{\gamma}^{lab}$ e' + K^+ + \Lambda$, as well as the branching ratio for the crossed channel reaction $K^- + p ---> \gamma + \Lambda$, with stopped kaons. The sensitivity, from moderate to significant, of various observables to such effects are discussed.

Thermal model analysis of particle ratios in Ni+Ni experiments using exact strangeness conservation

Abstract: The production of hadrons in Ni-Ni at the GSI laboratory is considered in a hadronic gas model with chemical equilibrium. Special attention is given to the abundance of strange particles which are treated using the exact conservation of strangeness. It is found that all the data can be described using a temperature $T=70\ifmmode\pm\else\textpm\fi{}10$ MeV and a baryon chemical potential ${\ensuremath{\mu}}_{B}=720\ifmmode\pm\else\textpm\fi{}30$ MeV.

Photoproduction of φ mesons from the proton: Polarization observables and strangeness in the nucleon

Abstract: The polarization observables in $\ensuremath{\varphi}$ meson photoproduction are studied to probe the strangeness content of the nucleon. In addition to the dominant diffractive production and the one-pion-exchange process, we take into account the direct knockout mechanism that arises from the possible hidden strangeness content of the nucleon. We find that some double polarization observables are very sensitive to the strangeness content of the proton because of the different spin structures of the amplitudes associated with different mechanisms. This suggests that such measurements could be very useful in probing the strangeness content in the proton. The orbitally excited quark-cluster configurations in the proton are included in the calculation and found to have little effect.

Hadron production in relativistic nuclear collisions: thermal hadron source or hadronizing quark-gluon plasma?

Abstract: Measured hadron yields from relativistic nuclear collisions can be equally well understood in two physically distinct models, namely a static thermal hadronic source vs. a time-dependent, nonequilibrium hadronization off a quark-gluon plasma droplet. Due to the time-dependent particle evaporation off the hadronic surface in the latter approach the hadron ratios change (by factors of \(\lessapprox 5\)) in time. Final particle yields reflect time averages over the actual thermodynamic properties of the system at a certain stage of the evolution. Calculated hadron, strangelet and (anti-)cluster yields as well as freeze-out times are presented for different systems. Due to strangeness distillation the system moves rapidly out of the \(T\), \(\mu_q\) plane into the \(\mu_s\)-sector. Strangeness to baryon ratios \(f_s=1-2\) prevail during a considerable fraction (50%) of the time evolution (i.e. \(\Lambda\)-droplets or even \(\Xi^-\)-droplets form the system at the late stage: The possibility of observing this time evolution via two-particle correlations is discussed). The observed hadron ratios require \(T_c\approx 160\) MeV and \(B^{1/4}\gtrapprox 200\) MeV. If the present model is fit to the extrapolated hadron yields, metastable hypermatter can only be produced with a probability \(p<10^{-8}\) for \(A \ge 4\).

K − / K + ratios in relativistic heavy-ion collisions

Abstract: We study $K^-/K^+$ ratios as a function of centrality (participant nucleon number), transverse mass ($m_t$), and rapidity, in heavy-ion collisions at beam energies between 1A and 2A GeV. We use the relativistic transport model that includes expicitly the strangeness degrees of freedom and consider two scenarios for kaon properties in dense matter, one with and one without medium modifications of their properties. In both scenarios, The $K^-/K^+$ ratio does not change very much with the centrality, while the $K/\pi$ and ${\bar K}/\pi$ ratios increase with increasing centrality. Significant differences are predicted, both in magnitudes and shapes, for the $m_t$ spectra and rapidity distributions of $K^-/K^+$ ratio. Experimental measurement of these ratios, currently under investigation by the FOPI, KaoS, E866, and E895 collaborations, will be useful in revealing the kaon in-medium properties.

Experiments with $\Xi^-$ atoms

Abstract: Experiments with $\Xi^-$ atoms are proposed in order to study the nuclear interaction of $\Xi$ hyperons. The production of $\Xi^-$ in the ($K^{-}$, $K^{+}$) reaction, the $\Xi^-$ stopping in matter, and its atomic cascade are incorporated within a realistic evaluation of the results expected for $\Xi^-$ X-ray spectra across the periodic table, using an assumed $\Xi$-nucleus optical potential $V_{opt}$. Several optimal targets for measuring the strong-interaction shift and width of the X-ray transition to the `last' atomic level observed are singled out: F, Cl, I, Pb. The sensitivity of these observables to the parameters of $V_{opt}$ is considered. The relevance of such experiments is discussed in the context of strangeness -2 nuclear physics and multistrange nuclear matter. Finally, with particular reference to searches for the $H$ dibaryon, the properties of $\Xi^-d$ atoms are also discussed. The role of Stark mixing, its effect on $S$ and $P$ state capture of $\Xi^-$ by the deuteron, together with estimates of the resulting probability for producing the $H$ dibaryon are considered in detail.

Physics of Strange Matter

Abstract: Relativistic heavy ion collisions offer the possibility to produce exotic metastable states of nuclear matter containing (roughly) equal number of strangeness compared to the content in baryon number. The reasoning of both their stability and existence, the possible distillation of strangeness necessary for their formation and the chances for their detection are reviewed. In the later respect emphasize is put on the properties of small lumps of strange quark matter with respect to their stability against strong or weak hadronic decays. In addition, implications in astrophysics like the properties of neutron stars and the issue of baryonic dark matter will be discussed.

Radiative decays of decuplet hyperons

Abstract: We calculate the radiative decay widths of decuplet hyperons in a chiral constituent quark model including electromagnetic exchange currents between quarks. Exchange currents contribute significantly to the E2 transition amplitude, while they largely cancel for the M1 transition amplitude. Strangeness suppression of the radiative hyperon decays is found to be weakened by exchange currents. Differences and similarities between our results and other recent model predictions are discussed.

Probing the boundaries of the hadronic phase through a strangeness- including statistical bootstrap model

Abstract: A recently constructed strangeness-including Statistical Bootstrap Model (S-SBM), which defines the limits of the hadronic phase and provides for a phase beyond, is further extended so as to include a factor that describes strangeness suppression. The model is then used to analyse the multiplicity data from collision experiments in which the colliding entities form isospin symmetric systems, the primary focus being on S+S interactions (NA35 collaboration). An optimal set of thermodynamical variables is extracted through a fit to both the inclusive full phase space and midrapidity data. The assumption that the measured particles originate from a thermally and partial-chemically equilibrated source described by the S-SBM is satisfactorily established. The proximity of the thermodynamical variables extracted from the S+S data to the limits of the hadronic phase is systematically investigated. Finally, experimental data from proton-antiproton collisions (UA5 collaboration) are similarly analysed.

Exchange currents in octet hyperon charge radii

Abstract: Octet hyperon charge radii are calculated in a chiral constituent quark model including electromagnetic exchange currents between quarks. In impulse approximation one observes a decrease of the hyperon charge radii with increasing strangeness. This effect is reduced by exchange currents. Due to exchange currents, the charge radius of the negatively charged hyperons are close to the proton charge radius.

An extension of the statistical bootstrap model to include strangeness. Implications on particle ratios

Abstract: The Statistical Bootstrap Model (SBM) is extended to describe hadronic systems which carry the quantum number of strangeness. The study is conducted in the three-dimensional space of temperature, up-down and strange chemical potentials, wherein the existence of a ``critical'' surface is established, which sets the limits of the hadronic phase of matter. A second surface, defined by the null expectation value of strangeness number is also determined. The approach of the latter surface to the critical one becomes the focal point of the present considerations. Two different versions of the extended SBM are examined, corresponding to the values 2 and 4 for the exponent, which determines the asymptotic fall-off of the mass spectrum. It is found that the version with the value 4 has decisive physical advantages. This model is subsequently adopted to discuss (strange) particle ratios pertaining to multiparticle production processes, for which a thermal equilibrium mode of description applies.

Production of minijets (gluons) and strangeness enhancement in pA and AA collisions at relativistic energies

Abstract: The idea that effective string tension increases as a result of the hard gluon kinks on a string is applied to study the strange particle production in proton-nucleus and nucleus-nucleus collisions. It is found that the effective string tension increases with the increase of centrality and mass of the colliding system as a consequence of the minijet (gluon) production stemming from the collective string-string interaction. This mechanism leads to strangeness enhancement in pA and AA collisions through the enhanced production of the strange quark pairs from the color field of strings. We discuss different roles played by this mechanism and rescattering of the final state hadrons in the production of strange particles and compare our results with experimental data.

Confronting particle emission scenarios with strangeness data

Abstract: An enhancement of strangeness production in relativistic nuclear collisions (compared to, e.g., proton-proton collisions at the same energy) is a possible signature [1] of the much sought-after quark-gluon plasma. It is therefore particularly interesting that current data at AGS (Alternating Gradient Synchroton) and SPS (Super Proton Synchrotron) energies do show an increase in strangeness production (see, e.g., [2]). At SPS energies, this increase seems to imply that something new is happening: In microscopical models, one has to postulate some previously unseen reaction mechanism (color rope formation in the RQMD code [3], multiquark clusters in the VENUS code [4], etc.) while hydrodynamical models have their own problems (be it those with a rapidly hadronizing plasma [5] or those with an equilibrated hadronic phase, preceded or not by a plasma phase). In this paper, we examine the shortcomings of the latter class of hydrodynamical models and suggest that they might be due to a too rough description of particle emission. (The main problem for the former class of hydrodynamical models is the difficulty to yield enough entropy after hadronization.)