Search Results (26 total)

We compute the mass shifts and mixing of the ω and φ mesons at finite temperature due to scattering from thermal pions. The ρ and b₁ mesons are important intermediate states. Up to a temperature of 140 MeV the ω mass increases by 12 MeV and the φ mass decreases by 0.6 MeV. The change in mixing angles is negligible.

We use a boost-invariant one-dimensional (cylindrically symmetric) fluid dynamics code to calculate e⁺e^- production from ρ⁰ and ω decay in the central rapidity region of a central S+Au collision at sqrt[s]=20 GeV/nucleon. We use equations of state with a first-order phase transition between a massless pion gas and quark gluon plasma, with transition temperatures in the range 150-200 MeV. The production cross section at the ρ mass loosely constrains the transition and freeze-out temperatures, and we find that the m_T spectrum is a good thermometer for sufficiently high T_c.

We estimate the numbers and mass spectra of observed lepton and kaon pairs produced from φ meson decays in the central rapidity region of an Au+Au collision at lab energy 11.6 GeV/nucleon. The following effects are considered: possible mass shifts, thermal broadening due to collisions with hadronic resonances, and superheating of the resonance gas. Changes in the dilepton mass spectrum may be seen, but changes in the dikaon spectrum are too small to be detectable.

We use a boost-invariant one-dimensional (cylindrically symmetric) fluid dynamics code to calculate thermal photon production in the central rapidity region of S+Au and Pb+Pb collisions at SPS energy (√s =20 GeV/nucleon). We assume that the hot matter is in thermal equilibrium throughout the expansion, but consider deviations from chemical equilibrium in the high temperature (deconfined) phase. We use equations of state with a first-order phase transition between a massless pion gas and quark gluon plasma, with transition temperatures in the range 150≤T_c≤200 MeV.

We estimate freezeout conditions for s, c, and b quarks in high energy nuclear collisions. Freezeout is due either to loss of thermal contact, or to particles ‘‘wandering’’ out of the region of hot matter. We then develop a thermal recombination model in which both single-particle (quark and antiquark) and two-particle (quark-antiquark) densities are conserved. Conservation of two-particle densities is necessary because quarks and antiquarks are always produced in coincidence, so that the local two-particle density can be much larger than the product of the single-particle densities. We use the freezeout conditions and recombination model to discuss heavy resonance production at zero baryon density in high energy nuclear collisions.

We derive classically an expression for a hadron width in a two-phase region of hadron gas and quark-gluon plasma (QGP). The presence of QGP gives hadrons larger widths than they would have in a pure hadron gas. We find that the φ width observed in a central Au+Au collision at √s=200 GeV/nucleon is a few MeV greater than the width in a pure hadron gas. The part of observed hadron widths due to QGP is approximately proportional to (dN/dy)^-1/3.

Regression with χ² constructed from covariance matrices should not be used for some combinations of covariance matrices and fitting functions. Using the technique for unsuitable combinations can amplify systematic errors. This amplification is uncontrolled, and can produce arbitrarily inaccurate results that might not be ruled out by a χ² test. In addition, this technique can give incorrect (artificially small) errors for fit parameters. I give a test for this instability and a more robust (but computationally more intensive) method for fitting correlated data.

The decay width of a phi meson is reduced from its vacuum value as its mass decreases in hot hadronic matter as a result of the partial restoration of chiral symmetry. This reduction is, however, cancelled by collisional broadening through the reactions φπ→KK^*, φK→φK, φρ→KK, and φφ→KK. The resulting phi meson width in hot hadronic matter is found to be less than about 10 MeV for temperatures below 200 MeV. If hadronic matter has a strong first-order phase transtion, this narrow phi meson with reduced mass will appear as a second peak in the dilepton spectrum in ultrarelativistic heavy-ion collisions. We discuss use of this second phi peak to determine the transition temperature and the lifetime of the two-phase coexistence region in the case of a strong first-order phase transition. We also discuss using the peak to determine the range of temperatures over which the transition occurs in the case of a smooth but fast change in the entropy density.

We calculate thermal production of u, d, s, c, and b quarks in ultrarelativistic heavy ion collisions. The following processes are taken into account: thermal gluon decay (g→i i¯), gluon fusion (gg→i i¯) and quark-antiquark annihilation ( j¯→i ī), where i and j represent quark species. We use the thermal quark masses, m_i²(T)≃m_i²+(2g²/9)T², in all the rates. At small mass [m_i(T)<2T], the production is largely dominated by the thermal gluon decay channel. We obtain numerical and analytic solutions of one-dimensional hydrodynamic expansion of an initially pure glue plasma. Our results show that even in a quite optimistic scenario, all quarks are far from chemical equilibrium throughout the expansion. Thermal production of light quarks (u, d, and s) is nearly independent of species. Heavy quark (c and b) production is quite independent of the transition temperature and could serve as a very good probe of the initial temperature. Thermal quark production measurements could also be used to determine the gluon damping rate, or equivalently the magnetic mass.

The dilepton mass distribution from preequilibrium matter in ultrarelativistic nuclear collisions is indistinguishable from a thermally produced distribution.

We show how to determine the ratio of the transverse velocity of a source to the velocity of emitted particles, using split-bin correlation functions. The technique is to measure S₂ and S₂^φ, subtract the contributions from the single-particle distribution, and take the ratio as the bin size goes to zero. We demonstrate the technique for two cases: each source decays into two particles, and each source emits a large number of particles.

Requirements for correlation measurements in high-multiplicity events are discussed. Attention is focused on detection of so-called hot spots, two-particle rapidity correlations, two-particle momentum correlations (for quantum interferometry), and higher-order correlations. The signal-to-noise ratio may become large in the high-multiplicity limit, allowing meaningful single-event measurements, only if the correlations are due to collective behavior.

We calculate the transverse-mass distribution of lepton pairs in the ρ⁰-ω peak. We use two different ultrarelativistic nuclear collision scenarios: one with a strong first-order deconfinement phase transition, and one with a first-order chiral phase transition. In both cases, we assume thermal and chemical equilibrium throughout the collision, and consider transition temperatures in the range 150–200 MeV. We then follow the experimental procedure and fit the transverse-mass distribution with a thermal distribution. The fitting temperature is near the transition temperature for both scenarios.

I propose to use the transverse momentum distribution of lepton pairs in the ρ^0-ω peak to measure the QCD transition temperature in ultrarelativistic nuclear collisions. The signal-to-background ratio is approximately unity, and the transition temperature can be determined with roughly 15% accuracy.

Photons in the energy range of about one-half to several GeV have been proposed as a signal of the formation of a quark-gluon plasma in high-energy collisions. To lowest order the thermal emission rate is infrared divergent for massless quarks, but we regulate this divergence using the resummation technique of Braaten and Pisarski. Photons can also be produced in the hadron phase. We find that the dominant contribution comes from the reactions ππ→ργ and πρ→πγ; the decays ω→πγ and ρ→ππγ are also significant. Comparing the thermal emission rates at a temperature T=200 MeV we conclude that the hadron gas shines just as brightly as the quark-gluon plasma.

We consider a two-dimensional Chern-Simons gauge theory where the gauge field couples to an internal degree of freedom of the fermion. For a fixed gauge coupling g there exists a finite window of Chern-Simons magnetic moment coupling g’ for which a nonzero Chern-Simons magnetic field is generated at T=0. The system then is superconducting without Cooper pairing. There is a second-order thermodynamic phase transition at T_c>0 where the Chern-Simons magnetic field goes to zero and superconductivity terminates.

I present the results of a split-bin correlation-function analysis of O+Ag(Br) and S+Ag(Br) collisions at 200 GeV/nucleon. The data are corrected for the shape of the single-particle pseudorapidity distribution, and are also corrected event by event for multiplicity. The observed pseudorapidity correlations are an order of magnitude larger than correlations from p¯p and e⁺e^- collisions.

Entropy generation during an ultrarelativistic nuclear collision is discussed. It is shown that the maximum entropy densities attained are limited by the time during which the collision takes place. This provides a strong upper limit on the maximum entropy density attained in a collision of given multiplicity density.

We discuss the study of correlations in multiparticle processes, using split-bin correlation functions (SBCF's). We show how SBCF's can be used to study different production mechanisms, such as production via jetlike or resonancelike sources. We illustrate some possibilities with calculations of various SBCF's in simple models. One of the main advantages of SBCF's is the possibility of using transverse-energy correlations as well as multiplicity correlations in order to differentiate the various mechanisms of particle production. We show that in general the transverse-energy SBCF's and the multiplicity SBCF's are very similar, but also discuss some models for which these calsses of SBCF's will be different. Finally, we provide useful formulas for the analysis of SBCF data using simple analytic models, including effects due to curvature of the single-particle distribution.

We discuss the two standard constructions used in the search for intermittency, the exclusive and inclusive scaled factorial moments. We propose the use of a new scaled factorial moment that reduces to the exclusive moment in the appropriate limit and is free of undesirable multiplicity correlations that are contained in the inclusive moment. We show that there are some similarities among most of the models that have been proposed to explain factorial-moment data, and that these similarities can be used to increase the efficiency of testing these models. We begin by calculating factorial moments from a simple independent-cluster model that assumes only approximate boost invariance of the cluster rapidity distribution and an approximate relation among the moments of the cluster multiplicity distribution. We find two scaling laws that are essentially model independent. The first scaling law relates the moments to each other with a simple formula, indicating that the different factorial moments are not independent. The second scaling law relates samples with different rapidity densities. We find evidence for much larger clusters in heavy-ion data than in light-ion data, indicating possible spatial intermittency in the heavy-ion events.

We explain ‘‘intermittency’’ in recent data, using a simple model in which pions are emitted from quark-gluon-plasma droplets. We find that droplets with a temperature near the QCD phase transition temperature that emit 10–15 pions, with a droplet volume of 5–10 fm³, reproduce the observations very well.

We extend our previous analysis of hydrodynamic stability of hadronization solutions to include solutions which produce hadrons via a region of mixed phase. We find that the transition from quark matter to mixed-phase matter must occur via a shock with supersonic velocities for both incoming and outgoing matter. The transition from mixed-phase matter to hadronic matter can occur either via a detonation solution or across a supersonic shock as in the quark–mixed-phase matter transition. Nucleation of both mixed-phase matter and hadronic matter is discussed using the criteria developed in our previous paper. We find that mixed-phase matter can nucleate in quark matter only when quark matter is at the transition temperature and that the region of nucleating solutions is a single point. Hadronic matter can nucleate in mixed-phase matter when the energy density of the mixed-phase matter reaches the hadronic transition energy density, and there is a region of possible nucleation solutions for any value of the mixed-phase energy density. We calculate entropy production and velocity boosts for the transitions from quark matter to mixed-phase matter and from mixed-phase matter to hadronic matter. We find that the expected entropy increase during hadronization is about 6.1% in the central rapidity region of a heavy-ion collision and about 4.2% in the early Universe in the absence of supercooling. If supercooling occurs, the entropy production will be increased by an amount which depends on the degree of supercooling. The expected velocity boost during hadronization is about 0.50c in heavy-ion collisions and about 0.33c in the early Universe, again assuming no supercooling. If supercooling occurs, the velocity boost will depend strongly on the degree of supercooling.

We discuss some of the properties of deflagration and detonation solutions for the hadronization process which are mentioned in the article ‘‘Burning of baryon-rich quark-gluon plasmas’’ which was recently published in Physical Review D. These properties include the hadronization velocities and thermodynamic-stability conditions of the two types of solutions and the signals for hadronization through detonation.