51 research outputs found
Local CP-violation and electric charge separation by magnetic fields from lattice QCD
We study local CP-violation on the lattice by measuring the local correlation
between the topological charge density and the electric dipole moment of
quarks, induced by a constant external magnetic field. This correlator is found
to increase linearly with the external field, with the coefficient of
proportionality depending only weakly on temperature. Results are obtained on
lattices with various spacings, and are extrapolated to the continuum limit
after the renormalization of the observables is carried out. This
renormalization utilizes the gradient flow for the quark and gluon fields. Our
findings suggest that the strength of local CP-violation in QCD with physical
quark masses is about an order of magnitude smaller than a model prediction
based on nearly massless quarks in domains of constant gluon backgrounds with
topological charge. We also show numerical evidence that the observed local
CP-violation correlates with spatially extended electric dipole structures in
the QCD vacuum.Comment: 19 pages, 7 figures. Additional lattice results about the induced
electric dipole structure, extended model description, specified terminology.
Version published in JHE
QCD-Thermodynamics using 5-dim Gravity
We calculate the critical temperature and free energy of the gluon plasma
using the dilaton potential arXiv:0911.0627[hep-ph] in the gravity theory of
AdS/QCD. The finite temperature observables are calculated in two ways: first,
from the Page-Hawking computation of the free energy, and secondly using the
Bekenstein-Hawking proportionality of the entropy with the area of the horizon.
Renormalization is well defined, because the T=0 theory has asymptotic freedom.
We further investigate the change of the critical temperature with the number
of flavours induced by the change of the running coupling constant in the
quenched theory. The finite temperature behaviour of the speed of sound,
spatial string tension and vacuum expectation value of the Polyakov loop follow
from the corresponding string theory in AdS_5.Comment: 38 pages, 12 figure
Transition temperature and the equation of state from lattice QCD, Wuppertal-Budapest results
The QCD transition is studied on lattices up to . The chiral
condensate is presented as a function of the temperature, and the corresponding
transition temperature is extracted. The equation of state is determined on
lattices with and at some temperature values with . The
pressure and the trace anomaly are presented as functions of the temperature in
the range 100 ...1000 MeV . Using the same configurations we determine the
continuum extrapolated phase diagram of QCD on the plane for small to
moderate chemical potentials. Two transition lines are defined with two
quantities, the chiral condensate and the strange quark number susceptibility.Comment: 4 pages, 2 figures, Proceedings for Quark Matter 201
Thermodynamic properties of QCD in external magnetic fields
We consider the effect of strong external electromagnetic fields on
thermodynamic observables in QCD, through lattice simulations with 1+1+1
flavors of staggered quarks at physical quark masses. Continuum extrapolated
results are presented for the light quark condensates and for their tensor
polarizations, as functions of the temperature and the magnetic field. We find
the light condensates to undergo inverse magnetic catalysis in the transition
region, in a manner that the transition temperature decreases with growing
magnetic field. We also compare the results to other approaches and lattice
simulations. Furthermore, we relate the tensor polarization to the spin part of
the magnetic susceptibility of the QCD vacuum, and show that this contribution
is diamagnetic.Comment: 13 pages, 9 figures, talks presented by FB and GE at Xth Quark
Confinement and the Hadron Spectrum, 8-12 October 2012, TUM Campus Garching,
Munich, German
QCD quark condensate in external magnetic fields
We present a comprehensive analysis of the light condensates in QCD with
1+1+1 sea quark flavors (with mass-degenerate light quarks of different
electric charges) at zero and nonzero temperatures of up to 190 MeV and
external magnetic fields B<1 GeV^2/e. We employ stout smeared staggered
fermions with physical quark masses and extrapolate the results to the
continuum limit. At low temperatures we confirm the magnetic catalysis scenario
predicted by many model calculations, while around the crossover the condensate
develops a complex dependence on the external magnetic field, resulting in a
decrease of the transition temperature.Comment: slight changes in the text, version accepted for publication in PRD.
5 pages, 5 figure
Magnetic susceptibility of QCD at zero and at finite temperature from the lattice
The response of the QCD vacuum to a constant external (electro)magnetic field is studied through the tensor polarization of the chiral condensate and the magnetic susceptibility at zero and at finite temperature. We determine these quantities using lattice configurations generated with the tree-level Symanzik improved gauge action and N-f 1 + 1 + 1 flavors of stout smeared staggered quarks with physical masses. We carry out the renormalization of the observables under study and perform the continuum limit both at T > 0 and at T = 0, using different lattice spacings. Finite size effects are studied by using various spatial lattice volumes. The magnetic susceptibilities chi(f) reveal a spin-diamagnetic behavior; we obtain at zero temperature chi(u) = -(2.08 +/- 0.08) GeV-2, chi(d) = -(2.02 +/- 0.09) GeV-2 and chi(s) = -(3.4 +/- 1.4) GeV-2 for the up, down and strange quarks, respectively, in the (MS) over bar scheme at a renormalization scale of 2 GeV. We also find the polarization to change smoothly with the temperature in the confinement phase and then to drastically reduce around the transition region
The QCD phase diagram for external magnetic fields
The effect of an external (electro)magnetic field on the finite temperature
transition of QCD is studied. We generate configurations at various values of
the quantized magnetic flux with flavors of stout smeared staggered
quarks, with physical masses. Thermodynamic observables including the chiral
condensate and susceptibility, and the strange quark number susceptibility are
measured as functions of the field strength. We perform the renormalization of
the studied observables and extrapolate the results to the continuum limit
using and 10 lattices. We also check for finite volume effects using
various lattice volumes. We find from all of our observables that the
transition temperature significantly decreases with increasing magnetic
field. This is in conflict with various model calculations that predict an
increasing . From a finite volume scaling analysis we find that the
analytic crossover that is present at B=0 persists up to our largest magnetic
fields , and that the transition strength
increases mildly up to this .Comment: 22 pages, 13 figure
An effective chiral Hadron-Quark Equation of State
We construct an effective model for the QCD equation of state, taking into
account chiral symmetry restoration as well as the deconfinement phase
transition. The correct asymptotic degrees of freedom at the high and low
temperature limits are included (quarks hadrons). The model
shows a rapid crossover for both order parameters, as is expected from lattice
calculations. We then compare the thermodynamic properties of the model at
which turn out to be in qualitative agreement with lattice data,
while apparent quantitative differences can be attributed to hadronic
contributions and excluded volume corrections. Furthermore we discuss the
effects of a repulsive vector type quark interaction at finite baryon number
densities on the resulting phase diagram of the model. Our current model is
able to reproduce a first-order liquid gas phase transition as expected, but
does not show any signs of a first order deconfinement or chiral phase
transition. Both transitions rather appear as a very wide crossover in which
heavily medium modified hadron coexist with free quarks.Comment: 19 pages, 13 figures Version accepted by J. Phys.
DGP brane cosmology and quark-hadron phase transition
In the standard picture of cosmology it is predicted that a phase transition,
associated with chiral symmetry breaking after the electroweak transition, has
occurred at approximately 10 \mu seconds after the Big Bang to convert a plasma
of free quarks and gluons into hadrons. We consider the quark-hadron phase
transition in a DGP brane world scenario within an effective model of QCD. We
study the evolution of the physical quantities useful for the study of the
early universe, namely, the energy density, temperature and the scale factor
before, during, and after the phase transition. Also, due to the high energy
density in the early universe, we consider the quadratic energy density term
that appears in the Friedmann equation. In DGP brane models such a term
corresponds to the negative branch (\epsilon=-1) of the Friedmann equation when
the Hubble radius is much smaller than the crossover length in 4D and 5D
regimes. We show that for different values of the cosmological constant on a
brane, \lambda, phase transition occurs and results in decreasing the effective
temperature of the quark-gluon plasma and of the hadronic fluid. We then
consider the quark-hadron transition in the smooth crossover regime at high and
low temperatures and show that such a transition occurs along with decreasing
the effective temperature of the quark-gluon plasma during the process of the
phase transition.Comment: 15 pages, 5 figures, two sections and references added. arXiv admin
note: substantial text overlap with arXiv:1103.007
Impact of resonance decays on critical point signals in net-proton fluctuations
The non-monotonic beam energy dependence of the higher cumulants of
net-proton fluctuations is a widely studied signature of the conjectured
presence of a critical point in the QCD phase diagram. In this work we study
the effect of resonance decays on critical fluctuations. We show that resonance
effects reduce the signatures of critical fluctuations, but that for reasonable
parameter choices critical effects in the net-proton cumulants survive. The
relative role of resonance decays has a weak dependence on the order of the
cumulants studied with a slightly stronger suppression of critical effects for
higher-order cumulants
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