arXiv subjects
Publications and source records attributed to David Seibert.
We compute the mass shifts and mixing of the Omega and Phi mesons at finite temperature due to scattering from thermal pions. The Rho and b_1 mesons are important intermediate states. Up to a temperature of 140 MeV the Omega mass increases by 12 MeV and the Phi 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 $ρ^0$ 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 $T_c$.
Are heavy quarks produced thermally or only by hard parton collisions? What is the probability that a produced heavy quark or antiquark is observed in a given resonance?
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 ($\sqrt{s}=20$ 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 $150 \leq T_c \leq 200$ MeV.
The observation of scaling relations in ultra-relativistic nuclear collisions would not by itself signal that the hot matter produced in these collisions has passed through a phase transition.
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.
This comment is withdrawn, as the figure on which it was based is inconsistent with other figures in the text and thus probably incorrect. Using the other figures, I find that the parton cascade model does approach chemical equilibrium correctly.
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 $\sqrt{s}=200$ 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 $(dN/dy)^{-1/3}$.
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 $ϕπ\to KK^*$, $ϕK\toϕK$, $ϕρ\to KK$, and $ϕϕ\to KK$. 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 transition, 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 ultra-relativistic heavy ion collisions. The following processes are taken into account: thermal gluon decay (g to ibar i), gluon fusion (g g to ibar i), and quark-antiquark annihilation (jbar j to ibar i), where i and j represent quark species. We use the thermal quark masses, $m_i^2(T)\simeq m_i^2 + (2g^2/9)T^2$, 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.
I calculate the dispersion relation for quarks of mass $m$ and momentum $k$ in a quark gluon plasma at temperature $T$, in the limit $m^2+k^2 \gg (gT)^2$, where $g$ is the strong coupling constant. I find three contributions to the dispersion relation: one that depends on $T$ but not $m$ or $k$, one that depends on $m$ and $T$ but not $k$, and third contribution that depends on all three (and is opposite in sign to the other two).
Regression with $χ^2$ constructed from the covariance matrix 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 $χ^2$ 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.
Requirements for correlation measurements in high--multiplicity events are discussed. Attention is focussed 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 production rates for massless $(u,d)$ and massive $(s,c,b)$ quarks in pure glue and quark gluon plasmas to leading order in the strong coupling constant $g$. The leading contribution comes from gluon decay into $q\bar q$ pairs, using a thermal gluon propagator with finite thermal mass and damping rate. The rate behaves as $α_S^2(\ln 1/α_S)^2 T^4$ when $m, α_S \rightarrow 0$ and depends linearly on the transverse gluon damping rate for all values of the quark mass $m$. The light quark ($u$, $d$, $s$) chemical equilibration time is approximately 10-100 $T^{-1}$ for $g=$2-3, so that quarks are likely to remain far from chemical equilibrium in ultrarelativistic nuclear collisions.
The dilepton mass distribution from pre-equilibrium matter in ultrarelativistic nuclear collisions is indistinguishable from a thermally produced distribution.
I estimate the degree of local chemical equilibration for $π$, $ρ$ and $ω$ mesons in ultra--relativistic nuclear collisions. The $π$ and $ρ$ mesons remain near chemical equilibrium in all cases, while the $ω$ meson density is typically 30--50\% higher than the equilibrium value for O+Ag collisions at $\sqrt{s}=20$ GeV. If chemical reactions are turned off, the $ω$ meson density is much larger, 2.5--3 times its equilibrium value. Thus, $ω$ mesons may provide the most sensitive tests for the degree of chemical equilibration in nuclear collisions.
I discuss methods for identifying and quantifying phase transitions in particle collisions, concentrating on two techniques for use in ultra-relativistic nuclear collisions. The first technique is to use rapidity correlation measurements to determine the correlation length, while the second is to use the transverse mass distribution of dileptons in the rho-omega peak to determine the transition temperature.