84 research outputs found
Is there Quark Matter in (Low-Mass) Pulsars?
The effect of the QCD phase transition is studied for the mass-radius
relation of compact stars and for hot and dense matter at a given proton
fraction used as input in core-collapse supernova simulations. The phase
transitions to the 2SC and CFL color superconducting phases lead to stable
hybrid star configurations with a pure quark matter core. In supernova
explosions quark matter could be easily produced due to -equilibrium,
small proton fractions and nonvanishing temperatures. A low critical density
for the phase transition to quark matter is compatible with present pulsar mass
measurements.Comment: 4 pages, 3 figures, talk given at the QM2008 conference, Jaipur,
India, February 4-10, 2008, JPG in pres
Pulsar kicks by anisotropic neutrino emission from quark matter
We discuss an acceleration mechanism for pulsars out of their supernova
remnants based on asymmetric neutrino emission from quark matter in the
presence of a strong magnetic field. The polarized electron spin fixes the
neutrino emission from the direct quark Urca process in one direction along the
magnetic field. We calculate the magnetic field strength which is required to
polarize the electron spin as well as the required initial proto-neutron star
temperature for a successfull acceleration mechanism. In addition we discuss
the neutrino mean free paths in quark as well as in neutron matter which turn
out to be very small. Consequently, the high neutrino interaction rates will
wash out the asymmetry in neutrino emission. As a possible solution to this
problem we take into account effects from colour superconductivity.Comment: 6 pages, 3 figures, poster contribution at the conference "Nuclear
Physics in Astrophysics III",Dresden,March 26-31,200
Normal ground state of dense relativistic matter in a magnetic field
The properties of the ground state of relativistic matter in a magnetic field
are examined within the framework of a Nambu-Jona-Lasinio model. The main
emphasis of this study is the normal ground state, which is realized at
sufficiently high temperatures and/or sufficiently large chemical potentials.
In contrast to the vacuum state, which is characterized by the magnetic
catalysis of chiral symmetry breaking, the normal state is accompanied by the
dynamical generation of the chiral shift parameter . In the chiral
limit, the value of determines a relative shift of the longitudinal
momenta (along the direction of the magnetic field) in the dispersion relations
of opposite chirality fermions. We argue that the chirality remains a good
approximate quantum number even for massive fermions in the vicinity of the
Fermi surface and, therefore, the chiral shift is expected to play an important
role in many types of cold dense relativistic matter, relevant for applications
in compact stars. The qualitative implications of the revealed structure of the
normal ground state on the physics of protoneutron stars are discussed. A
noticeable feature of the parameter is that it is insensitive to
temperature when , where is the chemical potential, and
{\it increases} with temperature for . The latter implies that the
chiral shift parameter is also generated in the regime relevant for heavy ion
collisions.Comment: 28 pages, 6 figures; v2: title changed in journa
Strange matter in core-collapse supernovae
We discuss the possible impact of strange quark matter on the evolution of
core-collapse supernovae with emphasis on low critical densities for the
quark-hadron phase transition. For such cases the hot proto-neutron star can
collapse to a more compact hybrid star configuration hundreds of milliseconds
after core-bounce. The collapse triggers the formation of a second shock wave.
The latter leads to a successful supernova explosion and leaves an imprint on
the neutrino signal. These dynamical features are discussed with respect to
their compatibility with recent neutron star mass measurements which indicate a
stiff high density nuclear matter equation of state.Comment: 8 pages, 3 figures, Invited talk at the "Strangeness in Quark Matter"
conference, 18-24 September 2011, Polish Academy of Arts and Sciences,
Cracow, Polan
Strange Exotic States and Compact Stars
We discuss the possible appearance of strange exotic multi-quark states in
the interior of neutron stars and signals for the existence of strange quark
matter in the core of compact stars. We show how the in-medium properties of
possible pentaquark states are constrained by pulsar mass measurements. The
possibility of generating the observed large pulsar kick velocities by
asymmetric emission of neutrinos from strange quark matter in magnetic fields
is outlined.Comment: 10 pages, invited talk given at the International Conference on
Strangeness in Quark Matter 2006 (SQM2006), UCLA, USA, March 26-31, 2006,
Journal of Physics G in press, refs. adde
Strangeness in Astrophysics and Cosmology
Some recent developments concerning the role of strange quark matter for
astrophysical systems and the QCD phase transition in the early universe are
addressed. Causality constraints of the soft nuclear equation of state as
extracted from subthreshold kaon production in heavy-ion collisions are used to
derive an upper mass limit for compact stars. The interplay between the
viscosity of strange quark matter and the gravitational wave emission from
rotation-powered pulsars are outlined. The flux of strange quark matter nuggets
in cosmic rays is put in perspective with a detailed numerical investigation of
the merger of two strange stars. Finally, we discuss a novel scenario for the
QCD phase transition in the early universe, which allows for a small
inflationary period due to a pronounced first order phase transition at large
baryochemical potential.Comment: 8 pages, invited talk given at the International Conference on
Strangeness in Quark Matter (SQM2009), Buzios, Brasil, September 28 - October
2, 200
Efficient Learning of Accurate Surrogates for Simulations of Complex Systems
Machine learning methods are increasingly used to build computationally
inexpensive surrogates for complex physical models. The predictive capability
of these surrogates suffers when data are noisy, sparse, or time-dependent. As
we are interested in finding a surrogate that provides valid predictions of any
potential future model evaluations, we introduce an online learning method
empowered by optimizer-driven sampling. The method has two advantages over
current approaches. First, it ensures that all turning points on the model
response surface are included in the training data. Second, after any new model
evaluations, surrogates are tested and "retrained" (updated) if the "score"
drops below a validity threshold. Tests on benchmark functions reveal that
optimizer-directed sampling generally outperforms traditional sampling methods
in terms of accuracy around local extrema, even when the scoring metric favors
overall accuracy. We apply our method to simulations of nuclear matter to
demonstrate that highly accurate surrogates for the nuclear equation of state
can be reliably auto-generated from expensive calculations using a few model
evaluations.Comment: 13 pages, 6 figures, submitted to Nature Machine Intelligenc
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