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Event-by-event fluctuations of particle ratios in central Pb + Pb collisions at 20 to 158 AGeV
(2004)
In the vicinity of the QCD phase transition, critical fluctuations have been predicted to lead to non-statistical fluctuations of particle ratios, depending on the nature of the phase transition. Recent results of the NA49 energy scan program show a sharp maximum of the ratio of K+ to Pi+ yields in central Pb+Pb collisions at beam energies of 20-30 AGeV. This observation has been interpreted as an indication of a phase transition at low SPS energies. We present first results on event-by-event fluctuations of the kaon to pion and proton to pion ratios at beam energies close to this maximum.
Evidence for an exotic S=-2, Q=-2 baryon resonance in proton-proton collisions at the CERN SPS
(2004)
Results of resonance searches in the Xi - pi -, Xi - pi +, Xi -bar+ pi -, and Xi -bar+ pi + invariant mass spectra in proton-proton collisions at sqrt[s]=17.2 GeV are presented. Evidence is shown for the existence of a narrow Xi - pi - baryon resonance with mass of 1.862±0.002 GeV/c2 and width below the detector resolution of about 0.018 GeV/c2. The significance is estimated to be above 4.2 sigma . This state is a candidate for the hypothetical exotic Xi --3/2 baryon with S=-2, I=3 / 2, and a quark content of (dsdsu-bar). At the same mass, a peak is observed in the Xi - pi + spectrum which is a candidate for the Xi 03/2 member of this isospin quartet with a quark content of (dsusd-bar). The corresponding antibaryon spectra also show enhancements at the same invariant mass.
We investigate transverse hadron spectra from relativistic nucleus-nucleus collisions which reflect important aspects of the dynamics - such as the generation of pressure - in the hot and dense zone formed in the early phase of the reaction. Our analysis is performed within two independent transport approaches (HSD and UrQMD) that are based on quark, diquark, string and hadronic degrees of freedom. Both transport models show their reliability for elementary pp as well as light-ion (C+C, Si+Si) reactions. However, for central Au+Au (Pb+Pb) collisions at bombarding energies above ~ 5 A.GeV the measured K+- transverse mass spectra have a larger inverse slope parameter than expected from the calculation. Thus the pressure generated by hadronic interactions in the transport models above ~ 5 A.GeV is lower than observed in the experimental data. This finding shows that the additional pressure - as expected from lattice QCD calculations at finite quark chemical potential and temperature - is generated by strong partonic interactions in the early phase of central Au+Au (Pb+Pb) collisions.
We propose a method to experimentally study the equation of state of strongly interacting matter created at the early stage of nucleus--nucleus collisions. The method exploits the relation between relative entropy and energy fluctuations and equation of state. As a measurable quantity, the ratio of properly filtered multiplicity to energy fluctuations is proposed. Within a statistical approach to the early stage of nucleus-nucleus collisions, the fluctuation ratio manifests a non--monotonic collision energy dependence with a maximum in the domain where the onset of deconfinement occurs.
We propose a method to experimentally study the equation of state of strongly interacting matter created at the early stage of nucleus–nucleus collisions. The method exploits the relation between relative entropy and energy fluctuations and equation of state. As a measurable quantity, the ratio of properly filtered multiplicity to energy fluctuations is proposed. Within a statistical approach to the early stage of nucleus–nucleus collisions, the fluctuation ratio manifests a non-monotonic collision energy dependence with a maximum in the domain where the onset of deconfinement occurs.
We suggest that the fluctuations of strange hadron multiplicity could be sensitive to the equation of state and microscopic structure of strongly interacting matter created at the early stage of high energy nucleus-nucleus collisions. They may serve as an important tool in the study of the deconfinement phase transition. We predict, within the statistical model of the early stage, that the ratio of properly filtered fluctuations of strange to non-strange hadron multiplicities should have a non-monotonic energy dependence with a minimum in the mixed phase region.
We suggest that the fluctuations of strange hadron multiplicity could be sensitive to the equation of state and microscopic structure of strongly interacting matter created at the early stage of high energy nucleus–nucleus collisions. They may serve as an important tool in the study of the deconfinement phase transition. We predict, within the statistical model of the early stage, that the ratio of properly filtered fluctuations of strange to non-strange hadron multiplicities should have a non-monotonic energy dependence with a minimum in the mixed phase region.
We point out that during the supernova II type explosion the thermodynamical conditions of stellar matter between the protoneutron star and the shock front correspond to the nuclear liquid–gas coexistence region, which can be investigated in nuclear multifragmentation reactions. We have demonstrated, that neutron-rich hot heavy nuclei can be produced in this region. The production of these nuclei may influence dynamics of the explosion and contribute to the synthesis of heavy elements.
We discuss gapless colour superconductivity for neutral quark matter in β equilibrium at zero as well as at nonzero temperature. Basic properties of gapless superconductors are reviewed. The current progress and the remaining problems in the understanding of the phase diagram of strange quark matter are discussed.
New results on the production of Xi and Omega hyperons in Pb+Pb interactions at 40 A GeV and Lambda at 30 A GeV are presented. Transverse mass spectra as well as rapidity spectra of these hyperons are shown and compared to previously measured data at different beam energies. The energy dependence of hyperon production (4Pi yields) is discussed. Additionally, the centrality dependence of Xi- production at 40 A GeV is presented.
Transverse mass and rapidity distributions for charged pions, charged kaons, protons, and antiprotons are reported for sqrt[sNN]=200 GeV pp and Au+Au collisions at Relativistic Heary Ion Collider (RHIC). Chemical and kinetic equilibrium model fits to our data reveal strong radial flow and long duration from chemical to kinetic freeze-out in central Au+Au collisions. The chemical freeze-out temperature appears to be independent of initial conditions at RHIC energies.
We study the phase diagram of a generalized chiral SU(3)-flavor model in mean-field approxi- mation. In particular, the influence of the baryon resonances, and their couplings to the scalar and vector fields, on the characteristics of the chiral phase transition as a function of temperature and baryon-chemical potential is investigated. Present and future finite-density lattice calculations might constrain the couplings of the fields to the baryons. The results are compared to recent lattice QCD calculations and it is shown that it is non-trivial to obtain, simultaneously, stable cold nuclear matter.
The hadronic final state of central Pb+Pb collisions at 20, 30, 40, 80, and 158 AGeV has been measured by the CERN NA49 collaboration. The mean transverse mass of pions and kaons at midrapidity stays nearly constant in this energy range, whereas at lower energies, at the AGS, a steep increase with beam energy was measured. Compared to p+p collisions as well as to model calculations, anomalies in the energy dependence of pion and kaon production at lower SPS energies are observed. These findings can be explained, assuming that the energy density reached in central A+A collisions at lower SPS energies is sufficient to force the hot and dense nuclear matter into a deconfined phase.
The hadronic final state of central Pb+Pb collisions at 20, 30, 40, 80, and 158 AGeV has been measured by the CERN NA49 collaboration. The mean transverse mass of pions and kaons at midrapidity stays nearly constant in this energy range, whereas at lower energies, at the AGS, a steep increase with beam energy was measured. Compared to p+p collisions as well as to model calculations, anomalies in the energy dependence of pion and kaon production at lower SPS energies are observed. These findings can be explained, assuming that the energy density reached in central A+A collisions at lower SPS energies is sufficient to transform the hot and dense nuclear matter into a deconfined phase.
The enzyme quinol:fumarate reductase (QFR) from the anaerobic e-proteobacterium Wolinella succinogenes is part of the anaerobic respiratory system of this organism. It couples the reduction of fumarate to succinate to the oxidation of menaquinol to menaquinone. W. succinogenes uses fumarate as terminal electron acceptor and can use various substrates (e.g., formate or molecular hydrogen) as electron donors. The concerted catalytic substrate turnover of either a hydrogenase or a formate dehydrogenase in conjunction with QFR contributes to the generation of an electrochemical potential gradient across the bacterial plasma membrane, which is used for the phosphorylation of ADP with inorganic phosphate, Pi, to ATP. In addition to an FAD (in subunit A) and three iron-sulfur clusters (in subunit B), QFR binds a low- and a high-potential heme b group in its transmembrane subunit C, as was ultimately shown in the crystal structure at 2.2 Å resolution (Lancaster et al., 1999, Nature 402, 377– 385). Both hemes are part of the electron transport chain between the two catalytic sites of this redox enzyme. The midpoint potentials of the hemes are well established but their assignment to the distal and proximal positions in the structure had not yet been determined. Furthermore, QFR from W. succinogenes has been proposed to exhibit a novel coupling mechanism of transmembrane electron and proton transfer, which has been described in the so-called “E-pathway” hypothesis (Lancaster, 2002, Biochim. Biophys. Acta 1565, 215–231). The aim of this project was to characterize the relationship between structure and function of QFR and to investigate the details of the proposed coupling mechanism (“Epathway”) with the help of computer-based electrostatic calculations on the QFR wild-type (WT) coordinates, and electrochemically induced FTIR and VIS difference spectroscopy on the QFR WT and available variant enzymes (in particular enzyme variant E180Q, in which the glutamic acid at position C180 has been replaced by a glutamine). 1.) It was demonstrated in this study that the diheme-containing QFR exhibits stable and reproducible electrochemically induced FTIR difference bands in the midinfrared range from 1800 cm-1 to 1000 cm-1 that reflect transitions from the reduced to the oxidized state of the enzyme. The spectral features that were observed in the FTIR difference spectra are fully reversible when changing from a reductive to an oxidative reference potential at the working electrode and vice versa. This indicates that the underlying redox reactions of the enzyme at the gold grid working electrode are also fully reversible under the applied experimental conditions. The same reversible spectral redox behavior in the visible range could also be ascertained for the Soret- and a-band of the two heme b groups of QFR. This behavior allowed to reliably determine the heme b midpoint potentials of QFR at various pH values. Analysis of the FTIR difference spectra in the amide I range yields evidence for structural reorganizations of the polypeptide backbone upon the electrochemically induced redox reaction. 2.) The redox titrations of the high- and low-potential heme b of QFR as simulated by multiconformation continuum electrostatics (MCCE) calculations showed a very high level of agreement with respect to the experimentally observed midpoint potentials of the heme b groups at pH 7. As determined with the help of the theoretical calculations, prominent features governing the differences in redox potential between the two hemes are the higher loss of reaction field energy for the proximal heme and the stronger destabilization of the oxidized form of the proximal heme due to several buried and ionized Arg and Lys residues. The explicit incorporation of crystallographically identified water molecules in the calculations had a noticeable effect on the absolute values of the determined midpoint potentials, although the relative difference of the two obtained midpoints did not change significantly. The results of the electrostatic calculations clearly showed that the lowpotential heme corresponds to the distal position bD in the structure, and that the high-potential heme is identical to the proximal heme bP. This assignment could previously not be achieved unequivocally with experimental methods. 3.) In addition, the currently discussed mechanism of coupled electron and proton transfer in the QFR of W. succinogenes (i.e., the “E-pathway” hypothesis) is further supported by the results of this study. The simulations of intermediate states of electron transfer via the heme b groups show that the protonation state of the key amino acid residue Glu C180 depends on the redox states of the heme groups as suggested in the “E-pathway” hypothesis. This result yields a possible mechanism for the coupling of transient transmembrane proton transfer via Glu C180 to the electron transfer via the heme b groups, since Glu C180 could be part of a “proton wire” and its redox-dependent protonation state could serve as the regulatory element of the “E-pathway”. Furthermore, the results of simulated heme reduction indicate that the side chain of Glu C180 also changes its conformation with respect to the redox state of the hemes. Both major results concerning the role of Glu C180, the change of protonation as well as the reorientation of the side chain upon reduction of the heme groups, are consistent with the results from electrochemically induced FTIR difference spectroscopy: Of particular interest was the spectral range above 1710 cm-1, where C=O stretching vibrations of protonated COOH carboxyl groups absorb, because those groups can act as proton donors, respectively acceptors, and can be involved in intra-protein proton transfer reactions. It was possible to observe signals of such protonated carboxyl groups originating from QFR enzyme, which either change their protonation state and/or experience an environmental change in the course of the induced redox reaction. This finding was supported by the fact that the relevant FTIR difference signals are sensitive to an isotopic hydrogen/deuterium (1H/2H) exchange via the buffer solution, since they were shifted towards lower wavenumbers in D2O. Furthermore, it could be shown with the help of site-directed mutagenesis that the acidic residue Glu C180, which is located in the membranespanning, diheme-containing subunit C of QFR, is contributing to the redox dependent signal of protonated carboxyl groups. The observed residual signal in the FTIR double-difference spectrum of QFR wild-type and enzyme variant E180Q (Glu C180 has been replaced with a Gln residue) could be interpreted as a protonation/deprotonation event that is superimposed by an environmental effect on the specific C=O vibration. This result strongly supports the proposed “E-pathway” of coupled transmembrane electron and proton transfer in the QFR enzyme, which states that residue Glu C180 is an essential constituent of a transient redox-controlled transmembrane proton transfer pathway. 4.) As a second possible constituent of the suggested “E-pathway”, the ring C propionate of the distal heme was found to be unusually fully protonated in all simulated redox states, indicating a possible role as a transient proton donor/acceptor in the “E-pathway”. Similarly to Glu C180, experimental evidence from FTIR difference spectroscopy on a modified QFR with 13C-labeled heme propionates was obtained, which indicates an involvement of at least one of the two propionates of heme bD in proton transfer. The observed signals can tentatively be interpreted as a redox-coupled (de)protonation of the ring C propionate of bD, which is possibly xiii superimposed by a conformational or environmental change of the specific propionate. 5.) Also the observation of a strong redox Bohr effect for both heme b groups in QFR is in line with the proposed “E-pathway” hypothesis, as this effect yields a possible and well-established mechanism for the coupling of proton transfer and redox changes of the heme groups. The comparison of the observed effect in QFR WT and E180Q together with the results from FTIR spectroscopy and MCCE calculation indicate that the ring C propionate of the distal heme is dominating the pHdependence of the midpoint potential of bD, and that the corresponding group for bP is Glu C180. The origin of the redox Bohr effect for bP in the enzyme variant E180Q (which is dramatically changed with respect to the WT) could not be identified unequivocally, but the observation of this redox Bohr effect in the variant implies the presence of other protolytic groups, which interact with heme bP and which may be necessary for a functional “E-pathway”.
Abstract: The medium modification of kaon and antikaon masses, compatible with low energy KN scattering data, are studied in a chiral SU(3) model. The mutual interactions with baryons in hot hadronic matter and the e ects from the baryonic Dirac sea on the K( ¯K ) masses are examined. The in-medium masses from the chiral SU(3) e ective model are compared to those from chiral perturbation theory. Furthermore, the influence of these in-medium e ects on kaon rapidity distributions and transverse energy spectra as well as the K, ¯K flow pattern in heavy-ion collision experiments at 1.5 to 2 A·GeV are investigated within the HSD transport approach. Detailed predictions on the transverse momentum and rapidity dependence of directed flow v1 and the elliptic flow v2 are provided for Ni+Ni at 1.93 A·GeV within the various models, that can be used to determine the in-medium K± properties from the experimental side in the near future.
Production of Lambda and Antilambda hyperons was measured in central Pb-Pb collisions at 40, 80, and 158 A GeV beam energy on a fixed target. Transverse mass spectra and rapidity distributions are given for all three energies. The Lambda/pi ratio at mid-rapidity and in full phase space shows a pronounced maximum between the highest AGS and 40 A GeV SPS energies, whereas the anti-Lambda}/pi ratio exhibits a monotonic increase. PACS numbers: 25.75.-q
Transverse energy ( ET ) distributions have been measured for Au+Au collisions at sqrt[sNN ]=200 GeV by the STAR Collaboration at RHIC. ET is constructed from its hadronic and electromagnetic components, which have been measured separately. ET production for the most central collisions is well described by several theoretical models whose common feature is large energy density achieved early in the fireball evolution. The magnitude and centrality dependence of ET per charged particle agrees well with measurements at lower collision energy, indicating that the growth in ET for larger collision energy results from the growth in particle production. The electromagnetic fraction of the total ET is consistent with a final state dominated by mesons and independent of centrality.
System size dependence of multiplicity fluctuations of charged particles produced in nuclear collisions at 158 A GeV was studied in the NA49 CERN experiment. Results indicate a non-monotonic dependence of the scaled variance of the multiplicity distribution with a maximum for semi-peripheral Pb+Pb interactions with number of projectile participants of about 35. This effect is not observed in a string-hadronic model of nuclear collision HIJING.
The transverse mass spectra and midrapidity yields for Xi s and Omega s are presented. For the 10% most central collisions, the Xi -bar+/h- ratio increases from the Super Proton Synchrotron to the Relativistic Heavy Ion Collider energies while the Xi -/h- stays approximately constant. A hydrodynamically inspired model fit to the Xi spectra, which assumes a thermalized source, seems to indicate that these multistrange particles experience a significant transverse flow effect, but are emitted when the system is hotter and the flow is smaller than values obtained from a combined fit to pi , K, p, and Lambda s.