172 research outputs found

    Comprehensive Interpretation of Thermal Dileptons at the SPS

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    Employing thermal dilepton rates based on medium-modified electromagnetic correlation functions we show that recent dimuon spectra of the NA60 collaboration in central In-In collisions at the CERN-SPS can be understood in terms of radiation from a hot and dense hadronic medium. Earlier calculated \rho-meson spectral functions, as following from hadronic many-body theory, provide an accurate description of the data up to dimuon invariant masses of about M\simeq 0.9 GeV, with good sensitivity to details of the predicted \rho-meson line shape. This, in particular, identifies baryon-induced effects as the prevalent ones. We show that a reliable description of the \rho contribution opens the possibility to study further medium effects: at higher masses (M \simeq 0.9-1.5 GeV) 4-pion type annihilation is required to account for the experimentally observed excess indicating precursor effects of chiral symmetry restoration (``chiral mixing''), while remaining structures in the \omega and \phi region are suggestive for modifications in their line shapes as well.Comment: 4 pages, 4 figures, v2: slightly improved estimate of four-pion contributions; accepted for publication in Phys. Rev. Let

    Production of Light Nuclei at Thermal Freezeout in Heavy-Ion Collisions

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    We revisit the problem of the production of light atomic nuclei in ultrarelativistic heavy-ion collisions. While their production systematics is well produced by hadro-chemical freezeout at temperatures near the QCD pseudo-critical temperature, their small binding energies of a few MeV per nucleon suggest that they cannot survive as bound states under these conditions. Here, we adopt the concept of effective chemical potentials in the hadronic evolution from chemical to thermal freezeout (at typically TfoT_{\rm fo}\simeq100\,MeV), which, despite frequent elastic rescatterings in hadronic matter, conserves the effective numbers of particles which are stable under strong interactions, most notably pions, kaons and nucleons. It turns out that the large chemical potentials that build up for antibaryons result in thermal abundances of light nuclei and antinuclei, formed at thermal freezeout, which essentially agree with the ones evaluated at chemical freezeout. Together with their transverse-momentum spectra, which also indicate a kinetic freezeout near TfoT_{\rm fo}, this provides a natural explanation for their production systematics without postulating their survival at high temperatures.Comment: 5 pages, 7 figures, v2: "Note added" correcte

    Chiral Symmetry Restoration in the Instanton Liquid at Finite Density

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    The properties of the QCD partition function at finite chemical potential are studied within the instanton liquid model. It is shown that the density dependence of the quark-induced instanton-antiinstanton (I-A) interaction leads to the formation of topologically neutral I-A pairs ('molecules'), resulting in a first order chiral phase transition at a critical chemical potential μqc310\mu_q^c\simeq 310 MeV. At somewhat higher densities (μq360\mu_q\ge360 MeV), the quark Fermi surface becomes instable with respect to diquark condensation (Cooper pairs) generating BCS-type energy gaps of order 50 MeV.Comment: 7 pages LaTeX, 4 eps-figures and espcrc1.sty included, to appear in the Proc. of the 'QCD at Finite Baryon Density'-Workshop (Bielefeld, 27.-30.04.98

    Thermal Dileptons as Fireball Thermometer and Chronometer

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    Thermal dilepton radiation from the hot fireballs created in high-energy heavy-ion collisions provides unique insights into the properties of the produced medium. We first show how the predictions of hadronic many-body theory for a melting ρ\rho meson, coupled with QGP emission utilizing a modern lattice-QCD based equation of state, yield a quantitative description of dilepton spectra in heavy-ion collisions at the SPS and the RHIC beam energy scan program. We utilize these results to systematically extract the excess yields and their invariant-mass spectral slopes to predict the excitation function of fireball lifetimes and (early) temperatures, respectively. We thereby demonstrate that future measurements of these quantities can yield unprecedented information on basic fireball properties. Specifically, our predictions quantify the relation between the measured and maximal fireball temperatures, and the proportionality of excess yields and total lifetime. This information can serve as a "caloric" curve to search for a first-order QCD phase transition, and to detect non-monotonous lifetime variations possibly related to critical phenomena.Comment: 4 pages 3 figure
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