217 research outputs found
Pion production from a critical QCD phase
A theoretical scheme which relates multiparticle states generated in
ultrarelativistic nuclear collisions to a QCD phase transition is considered in
the framework of the universality class provided by the 3-D Ising model. Two
different evolution scenarios for the QGP system are examined. The statistical
mechanics of the critical state is accounted for in terms of (critical) cluster
formation consistent with suitably cast effective action functionals, one for
each considered type of expansion. Fractal properties associated with these
clusters, characterizing the density fluctuations near the QCD critical point,
are determined. Monte-Carlo simulations are employed to generate events,
pertaining to the total system, which correspond to signals associated with
unconventional sources of pion production
The Crystallography of Color Superconductivity
We develop the Ginzburg-Landau approach to comparing different possible
crystal structures for the crystalline color superconducting phase of QCD, the
QCD incarnation of the Larkin-Ovchinnikov-Fulde-Ferrell phase. In this phase,
quarks of different flavor with differing Fermi momenta form Cooper pairs with
nonzero total momentum, yielding a condensate that varies in space like a sum
of plane waves. We work at zero temperature, as is relevant for compact star
physics. The Ginzburg-Landau approach predicts a strong first-order phase
transition (as a function of the chemical potential difference between quarks)
and for this reason is not under quantitative control. Nevertheless, by
organizing the comparison between different possible arrangements of plane
waves (i.e. different crystal structures) it provides considerable qualitative
insight into what makes a crystal structure favorable. Together, the
qualitative insights and the quantitative, but not controlled, calculations
make a compelling case that the favored pairing pattern yields a condensate
which is a sum of eight plane waves forming a face-centered cubic structure.
They also predict that the phase is quite robust, with gaps comparable in
magnitude to the BCS gap that would form if the Fermi momenta were degenerate.
These predictions may be tested in ultracold gases made of fermionic atoms. In
a QCD context, our results lay the foundation for a calculation of vortex
pinning in a crystalline color superconductor, and thus for the analysis of
pulsar glitches that may originate within the core of a compact star.Comment: 41 pages, 13 figures, 1 tabl
A versatile method for simulating pp -> ppe+e- and dp -> pne+e-p_spec reactions
We have developed a versatile software package for the simulation of
di-electron production in and collisions at SIS energies. Particular
attention has been paid to incorporate different descriptions of the Dalitz
decay via a common interface. In addition, suitable
parameterizations for the virtual bremsstrahlung process
based on one-boson exchange models have been implemented. Such simulation tools
with high flexibility of the framework are important for the interpretation of
the di-electron data taken with the HADES spectrometer and the design of
forthcoming experiments
Polarization observables in the reaction
We study the reaction slightly above the threshold within an
extended one-boson exchange model which also accounts for knock-out. It
is shown that polarization observables, like the beam-target asymmetry, are
sensible quantities for identifying a admixture in the nucleon wave
function on the few per cent level.Comment: 11 LaTeX pages including 4 ps figure
Monopoles, confinement and deconfinement of (2+1)D compact lattice QED in external fields
The compact Abelian model in three space--time dimensions is studied in the
presence of external electromagnetic fields at finite temperatures. We show
that the deconfinement phase transition is independent on the strength of the
external fields. This result is in agreement with our observation that the
external fields create small--size magnetic dipoles from the vacuum which do
not influence the confining properties of the model. Contrary to the
deconfinement phase, the internal field in the direction of the applied
external field is attenuated in the confinement phase, this screening becomes
stronger with decreasing temperature.Comment: 22 pages, 24 EPS figures, LaTeX uses epsfig.st
The 3-D O(4) universality class and the phase transition in two-flavor QCD
We determine the critical equation of state of the three-dimensional O(4)
universality class. We first consider the small-field expansion of the
effective potential (Helmholtz free energy). Then, we apply a systematic
approximation scheme based on polynomial parametric representations that are
valid in the whole critical regime, satisfy the correct analytic properties
(Griffiths' analyticity), take into account the Goldstone singularities at the
coexistence curve, and match the small-field expansion of the effective
potential. From the approximate representations of the equation of state, we
obtain estimates of several universal amplitude ratios.
The three-dimensional O(4) universality class is expected to describe the
finite-temperature chiral transition of quantum chromodynamics with two light
flavors. Within this picture, the O(4) critical equation of state relates the
reduced temperature, the quark masses, and the condensates around T_c in the
limit of vanishing quark masses.Comment: 19 pages, 5 fig
Spin-triplet superconducting pairing due to local (Hund's rule, Dirac) exchange
We discuss general implications of the local spin-triplet pairing among
fermions induced by local ferromagnetic exchange, example of which is the
Hund's rule coupling. The quasiparticle energy and their wave function are
determined for the three principal phases with the gap, which is momentum
independent. We utilize the Bogolyubov-Nambu-De Gennes approach, which in the
case of triplet pairing in the two-band case leads to the four-components wave
function. Both gapless modes and those with an isotropic gap appear in the
quasiparticle spectrum. A striking analogy with the Dirac equation is briefly
explored. This type of pairing is relevant to relativistic fermions as well,
since it reflects the fundamental discrete symmetry-particle interchange. A
comparison with the local interband spin-singlet pairing is also made.Comment: 16 pages, LaTex, submitted to Phys. Rev.
Two-proton correlations from 158 AGeV Pb+Pb central collisions
The two-proton correlation function at midrapidity from Pb+Pb central
collisions at 158 AGeV has been measured by the NA49 experiment. The results
are compared to model predictions from static thermal Gaussian proton source
distributions and transport models RQMD and VENUS. An effective proton source
size is determined by minimizing CHI-square/ndf between the correlation
functions of the data and those calculated for the Gaussian sources, yielding
3.85 +-0.15(stat.) +0.60-0.25(syst.) fm. Both the RQMD and the VENUS model are
consistent with the data within the error in the correlation peak region.Comment: RevTeX style, 6 pages, 4 figures, 1 table. More discussion are added
about the structure on the tail of the correlation function. The systematic
error is revised. To appear in Phys. Lett.
Event-by-event fluctuations of average transverse momentum in central Pb+Pb collisions at 158 GeV per nucleon
We present first data on event-by-event fluctuations in the average
transverse momentum of charged particles produced in Pb+Pb collisions at the
CERN SPS. This measurement provides previously unavailable information allowing
sensitive tests of microscopic and thermodynamic collision models and to search
for fluctuations expected to occur in the vicinity of the predicted QCD phase
transition. We find that the observed variance of the event-by-event average
transverse momentum is consistent with independent particle production modified
by the known two-particle correlations due to quantum statistics and final
state interactions and folded with the resolution of the NA49 apparatus. For
two specific models of non-statistical fluctuations in transverse momentum
limits are derived in terms of fluctuation amplitude. We show that a
significant part of the parameter space for a model of isospin fluctuations
predicted as a consequence of chiral symmetry restoration in a non-equilibrium
scenario is excluded by our measurement.Comment: 6 pages, 2 figures, submitted to Phys. Lett.
- …