13,573 research outputs found
Phase Structure of Confining Theories on R^3 x S^1
Recent work on QCD-like theories on R^3 x S^1 has revealed that a confined
phase can exist when the circumference L of S^1 is sufficiently small. Adjoint
QCD and double-trace deformation theories with certain conditions are such
theories, and we present some new results for their phase diagrams. First we
show the connection between the large-L and small-L confined regions in the
phase diagram of SU(3) adjoint QCD using Polyakov-Nambu-Jona Lasinio models.
Then we consider an SU(2) double-trace deformation theory with adjoint scalars
and study conflicts between the Higgs and small-L confined phase.Comment: 3 pages, 2 figures. Talk given at the IX International Conference on
Quark Confinement and Hadron Spectrum - Madrid, Spain, 30 Aug 2010 - 03 Sep
201
Complex saddle points in QCD at finite temperature and density
The sign problem in QCD at finite temperature and density leads naturally to
the consideration of complex saddle points of the action or effective action.
The global symmetry of the finite-density action, where
is charge conjugation and is complex conjugation,
constrains the eigenvalues of the Polyakov loop operator at a saddle point
in such a way that the action is real at a saddle point, and net color charge
is zero. The values of and at the saddle point,
are real but not identical, indicating the different free energy cost
associated with inserting a heavy quark versus an antiquark into the system. At
such complex saddle points, the mass matrix associated with Polyakov loops may
have complex eigenvalues, reflecting oscillatory behavior in color-charge
densities. We illustrate these properties with a simple model which includes
the one-loop contribution of gluons and massless quarks moving in a constant
Polyakov loop background. Confinement-deconfinement effects are modeled
phenomenologically via an added potential term depending on the Polyakov loop
eigenvalues. For sufficiently large and , the results obtained reduce
to those of perturbation theory at the complex saddle point. These results may
be experimentally relevant for the CBM experiment at FAIR.Comment: 13 pages, 3 figures. Additional references and minor revision
PNJL model for adjoint fermions
Recent work on QCD-like theories has shown that the addition of adjoint
fermions obeying periodic boundary conditions to gauge theories on R^3 X S^1
can lead to a restoration of center symmetry and confinement for sufficiently
small circumference L of S^1. At small L, perturbation theory may be used
reliably to compute the effective potential for the Polyakov loop P in the
compact direction. Periodic adjoint fermions act in opposition to the gauge
fields, which by themselves would lead to a deconfined phase at small L. In
order for the fermionic effects to dominate gauge field effects in the
effective potential, the fermion mass must be sufficiently small. This
indicates that chiral symmetry breaking effects are potentially important. We
develop a Polyakov-Nambu-Jona Lasinio (PNJL) model which combines the known
perturbative behavior of adjoint QCD models at small L with chiral symmetry
breaking effects to produce an effective potential for the Polyakov loop P and
the chiral order parameter psi-bar psi. A rich phase structure emerges from the
effective potential. Our results are consistent with the recent lattice
simulations of Cossu and D'Elia, which found no evidence for a direct
connection between the small-L and large-L confining regions. Nevertheless, the
two confined regions are connected indirectly if an extended field theory model
with an irrelevant four-fermion interaction is considered. Thus the small-L and
large-L regions are part of a single confined phase.Comment: 6 pages, 4 figures; presented at INPC 201
Production of 92Nb, 92Mo, and 146Sm in the gamma-process in SNIa
The knowledge of the production of extinct radioactivities like 92Nb and
146Sm by photodisintegration processes in ccSN and SNIa models is essential for
interpreting abundances in meteoritic material and for Galactic Chemical
Evolution (GCE). The 92Mo/92Nb and 146Sm/144Sm ratios provide constraints for
GCE and production sites. We present results for SNIa with emphasis on nuclear
uncertainties.Comment: 6 pages, 4 figures, Proceedings of the 13th Symposium on Nuclei in
the Cosmos (NIC XIII), July 2014, Debrecen, Hungar
Code dependencies of pre-supernova evolution and nucleosynthesis in massive stars: Evolution to the end of core helium burning
Massive stars are key sources of radiative, kinetic and chemical feedback in the Universe. Grids of massive star models computed by different groups each using their own codes, input physics choices and numerical approximations, however, lead to inconsistent results for the same stars. We use three of these 1D codes – genec, kepler and mesa – to compute non-rotating stellar models of 15, 20 and 25 M⊙ and compare their nucleosynthesis. We follow the evolution from the main sequence until the end of core helium burning. The genec and kepler models hold physics assumptions used in large grids of published models. The mesa code was set up to use convective core overshooting such that the CO core masses are consistent with those obtained by genec. For all models, full nucleosynthesis is computed using the NuGrid post-processing tool mppnp. We find that the surface abundances predicted by the models are in reasonable agreement. In the helium core, the standard deviation of the elemental overproduction factors for Fe to Mo is less than 30 per cent – smaller than the impact of the present nuclear physics uncertainties. For our three initial masses, the three stellar evolution codes yield consistent results. Differences in key properties of the models, e.g. helium and CO core masses and the time spent as a red supergiant, are traced back to the treatment of convection and, to a lesser extent, mass loss. The mixing processes in stars remain the key uncertainty in stellar modelling. Better constrained prescriptions are thus necessary to improve the predictive power of stellar evolution models
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