173 research outputs found
Collective mechanism of dilepton production in high-energy nuclear collisions
Collective bremsstrahlung of vector meson fields in relativistic nuclear
collisions is studied within the time-dependent Walecka model. Mutual
deceleration of the colliding nuclei is described by introducing the effective
stopping time and average rapidity loss of baryons. It is shown that
electromagnetic decays of virtual omega-mesons produced by bremsstrahlung
mechanism can provide a substantial contribution to the soft dilepton yield at
the SPS bombarding energies. In particular, it may be responsible for the
dilepton enhancement observed in 160 AGev central Pb+Au collisions. Suggestions
for future experiments to estimate the relative contribution of the collective
mechanism are given.Comment: 6 page
Dilepton production by bremsstrahlung of meson fields in nuclear collisions
We study the bremsstrahlung of virtual omega mesons due to the collective
deceleration of nuclei at the initial stage of an ultrarelativistic heavy-ion
collision. It is shown that electromagnetic decays of these mesons may give an
important contribution to the observed yields of dileptons. Mass spectra of
positron-electron and muon pairs produced in central Au+Au collisions are
calculated under some simplifying assumptions on the space-time variation of
the baryonic current in a nuclear collision process. Comparison with the CERES
data for 160 AGev Pb+Au collisions shows that the proposed mechanism gives a
noticeable fraction of the observed lepton pairs in the intermediate region of
invariant masses. Sensitivity of the dilepton yield to the in-medium
modification of masses and widths of vector mesons is demonstrated.Comment: 14 page
Color plasma oscillation in strangelets
The dispersion relation and damping rate of longitudinal color plasmons in
finite strange quark matter (strangelets) are evaluated in the limits of weak
coupling, low temperature, and long wavelength. The property of the QCD vacuum
surrounding a strangelet makes the frequency of the plasmons nearly the same as
the color plasma frequency of bulk matter. The plasmons are damped by their
coupling with individual excitations of particle-hole pairs of quarks, of which
the energy levels are discretized by the boundary. For strangelets of
macroscopic size, the lifetime of the plasmons is found to be proportional to
the size, as in the case of the usual plasma oscillations in metal
nanoparticles.Comment: 9 pages (REVTeX), 2 Postscript figures, to be published in Phys. Rev.
Particle Production and Effective Thermalization in Inhomogeneous Mean Field Theory
As a toy model for dynamics in nonequilibrium quantum field theory we
consider the abelian Higgs model in 1+1 dimensions with fermions. In the
approximate dynamical equations, inhomogeneous classical (mean) Bose fields are
coupled to quantized fermion fields, which are treated with a mode function
expansion. The effective equations of motion imply e.g. Coulomb scattering, due
to the inhomogeneous gauge field. The equations are solved numerically. We
define time dependent fermion particle numbers with the help of the single-time
Wigner function and study particle production starting from inhomogeneous
initial conditions. The particle numbers are compared with the Fermi-Dirac
distribution parametrized by a time dependent temperature and chemical
potential. We find that the fermions approximately thermalize locally in time.Comment: 16 pages + 6 eps figures, some clarifications and two references
added, typos corrected; to appear in Phys.Rev.
The Council of Europe's Approach towards Ageism
In this chapter, I examine the degree of interest in ageism among Council of
Europe members, and the degree of interest in its elimination through the Council
of Europe forum. I also examine the interpretation of the concept of ageism by various
Council of Europe institutions. Finally, I explore the Councilâs willingness and
ability to eliminate or at least mitigate ageism effect
Physics and Astrophysics of Strange Quark Matter
3-flavor quark matter (strange quark matter; SQM) can be stable or metastable
for a wide range of strong interaction parameters. If so, SQM can play an
important role in cosmology, neutron stars, cosmic ray physics, and
relativistic heavy-ion collisions. As an example of the intimate connections
between astrophysics and heavy-ion collision physics, this Chapter gives an
overview of the physical properties of SQM in bulk and of small-baryon number
strangelets; discusses the possible formation, destruction, and implications of
lumps of SQM (quark nuggets) in the early Universe; and describes the structure
and signature of strange stars, as well as formation and detection of
strangelets in cosmic rays. It is concluded, that astrophysical and laboratory
searches are complementary in many respects, and that both should be pursued to
test the intriguing possibility of a strange ground state for hadronic matter,
and (more generally) to improve our knowledge of the strong interactions.Comment: 45 pages incl. figures. To appear in "Hadrons in Dense Matter and
Hadrosynthesis", Lecture Notes in Physics, Springer Verlag (ed. J.Cleymans
Thermal charm production by massive gluons and quarks
We investigate charm production in an equilibrated quark-gluon plasma
produced in heavy-ion collisions at RHIC and LHC. Effective quark and gluon
masses are introduced from thermal QCD calculations. Assuming a Bjorken-type
longitudinal expansion and including the influence of temperature dependent
masses on the expansion, we determine the total number of c\bar{c} pairs
produced in the quark-gluon plasma phase. We calculate the charm production
rate at leading order with massive gluons and quarks and compare our result to
charm production by massless partons. We consider two different scenarios for
the initial conditions, a parton gas with a rather long kinetic equilibration
time and a minijet gas with a short equilibration time. In a parton gas,
assuming m_c=1.2 GeV, we obtain a substantial enhancement over the thermal
charm rate from massless quarks and gluons, up to 4.9 secondary charm quark
pairs in Au+Au collisions at RHIC and 245 charm pairs in Pb+Pb collisions at
the LHC.Comment: 19 pages in Latex and 9 Postscript figure
Human Rights and the Pink Tide in Latin America : Which Rights Matter?
Latin America witnessed the election of ânew Leftâ governments in the early 21 st century that, in different ways, sought to open a debate about alternatives to paradigms of neoliberal development. What has this meant for the way that human rights are understood and for patterns of human rights compliance? Using qualitative and quantitative evidence, this article discusses how human rights are imagined and the compliance records of new Left governments through the lens of the three âgenerationsâ of human rights â political and civil, social and economic, and cultural and environmental rights. The authors draw in particular on evidence from Andean countries and the Southern Cone. While basic civil and individual liberties are still far from guaranteed, especially in the Andean region, new Left countries show better overall performances in relation to socio-economic rights compared to the past and to other Latin American countries. All new Left governments also demonstrate an increasing interest in âthird generationâ (cultural and environmental) rights, though this is especially marked in the Andean Left. The authors discuss the tensions around interpretations and categories of human rights, reflect on the stagnation of first generation rights and note the difficulties associated with translating second and third generation rights into policy
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