141 research outputs found
Critical point in the QCD phase diagram for extremely strong background magnetic fields
Lattice simulations have demonstrated that a background (electro)magnetic
field reduces the chiral/deconfinement transition temperature of quantum
chromodynamics for eB < 1 GeV^2. On the level of observables, this reduction
manifests itself in an enhancement of the Polyakov loop and in a suppression of
the light quark condensates (inverse magnetic catalysis) in the transition
region. In this paper, we report on lattice simulations of 1+1+1-flavor QCD at
an unprecedentedly high value of the magnetic field eB = 3.25 GeV^2. Based on
the behavior of various observables, it is shown that even at this extremely
strong field, inverse magnetic catalysis prevails and the transition, albeit
becoming sharper, remains an analytic crossover. In addition, we develop an
algorithm to directly simulate the asymptotically strong magnetic field limit
of QCD. We find strong evidence for a first-order deconfinement phase
transition in this limiting theory, implying the presence of a critical point
in the QCD phase diagram. Based on the available lattice data, we estimate the
location of the critical point.Comment: 22 pages, 20 figures. v2: small changes in title and text; version
accepted for publication in JHE
The nature of the finite temperature QCD transition as a function of the quark masses
The finite temperature QCD transition for physical quark masses is a
crossover. For smaller quark masses a first-order phase transition is expected.
Using Symanzik improved gauge and stout improved fermion action for 2+1 flavour
staggered QCD we give estimates/bounds for the phase line separating the
first-order region from the crossover one. The calculations are carried out on
two different lattice spacings. Our conclusion for the critical mass is for and for lattices.Comment: Talk presented at the XXV International Symposium on Lattice Field
Theory, July 30 - August 4 2007, Regensburg, Germany. 7 pages, 6 figure
QCD phase diagram and equation of state in background electric fields
The phase diagram and the equation of state of QCD is investigated in the
presence of weak background electric fields by means of continuum extrapolated
lattice simulations. The complex action problem at nonzero electric field is
circumvented by a novel Taylor expansion, enabling the determination of the
linear response of the thermal QCD medium to constant electric fields -- in
contrast to simulations at imaginary electric fields, which, as we demonstrate,
involve an infrared singularity. Besides the electric susceptibility of QCD
matter, we determine the dependence of the Polyakov loop on the field strength
to leading order. Our results indicate a plasma-type behavior with a negative
susceptibility at all temperatures, as well as an increase in the transition
temperature as the electric field grows.Comment: 7 pages, 7 figure
Dressed Wilson loops as dual condensates in response to magnetic fields
We introduce dressed Wilson loops as a novel confinement observable. It
consists of closed planar loops of arbitrary geometry but fixed area and its
expectation values decay with the latter. The construction of dressed Wilson
loops is based on chiral condensates in response to magnetic (and electric)
fields, thus linking different physical concepts. We present results for
generalized condensates and dressed Wilson loops on dynamical lattice
configurations and confirm the agreement with conventional Wilson loops in the
limit of large probe mass. We comment on the renormalization of dressed Wilson
loops.Comment: 7 pages; talk presented at the XXIX International Symposium on
Lattice Field Theory (Lattice 2011), July 10-16, 2011, Squaw Valley, Lake
Tahoe, California, US
The equation of state at high temperatures from lattice QCD
We present results for the equation of state upto previously unreachable,
high temperatures. Since the temperature range is quite large, a comparison
with perturbation theory can be done directly.Comment: 7 pages, 5 figures, Lattice 200
Calculation of evapotranspiration from potatoes.
Data from sprinkler irrigation experiments with potatoes were used to calculate the actual and potential evapotranspiration from the crop during the growing season, using standard meteorologie data. During the experiments the moisture extraction from the effective root zone was determined by soil sampling. The water-use by the crop for the different periods was also derived from the water balance and both values were in good agreement in periods without extreme conditions of precipitation, this showing that the derived relations between crop height and surface roughness, between soil cover, light intensity, crop characteristics, soil characteristics and diffusion resistance, and between maturation and internal plant resistance were reasonably established. F.s.-A.G.G.H. (Abstract retrieved from CAB Abstracts by CABI’s permission
Lattice SU(3) thermodynamics and the onset of perturbative behaviour
We present the equation of state (pressure, trace anomaly, energy density and
entropy density) of the SU(3) gauge theory from lattice field theory in an
unprecedented precision and temperature range. We control both finite size and
cut-off effects. The studied temperature window () stretches
from the glueball dominated system into the perturbative regime, which allows
us to discuss the range of validity of these approaches. From the critical
couplings on fine lattices we get T_c/\Lambdamsbar=1.26(7) and use this ratio
to express the perturbative free energy in units. We also determine the
preferred renormalization scale of the Hard Thermal Loop scheme and we fit the
unknown order perturbative coefficient at extreme high temperatures
. We furthermore quantify the nonperturbative contribution to the
trace anomaly using two simple functional forms.Comment: 7 pages, Contribution to the The XXVIII International Symposium on
Lattice Field Theory; June 14 - 19, 2010, Villasimius, Sardinia, Ital
Determination of Freeze-out Conditions from Lattice QCD Calculations
Freeze-out conditions in Heavy Ion Collisions are generally determined by
comparing experimental results for ratios of particle yields with theoretical
predictions based on applications of the Hadron Resonance Gas model. We discuss
here how this model dependent determination of freeze-out parameters may
eventually be replaced by theoretical predictions based on equilibrium QCD
thermodynamics.Comment: presented at the International Conference "Critical Point and Onset
of Deconfinement - CPOD 2011", Wuhan, November 7-11, 201
Thermal momentum distribution from path integrals with shifted boundary conditions
For a thermal field theory formulated in the grand canonical ensemble, the
distribution of the total momentum is an observable characterizing the thermal
state. We show that its cumulants are related to thermodynamic potentials. In a
relativistic system for instance, the thermal variance of the total momentum is
a direct measure of the enthalpy. We relate the generating function of the
cumulants to the ratio of (a) a partition function expressed as a Matsubara
path integral with shifted boundary conditions in the compact direction, and
(b) the ordinary partition function. In this form the generating function is
well suited for Monte-Carlo evaluation, and the cumulants can be extracted
straightforwardly. We test the method in the SU(3) Yang-Mills theory and obtain
the entropy density at three different temperatures.Comment: 4 pages, 1 figure, minor revisions; version accepted in PR
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
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