54 research outputs found
Thermal QCD in a non-uniform magnetic background
Off-central heavy-ion collisions are known to feature magnetic fields with
magnitudes and characteristic gradients corresponding to the scale of the
strong interactions. In this work, we employ equilibrium lattice simulations of
the underlying theory, QCD, involving similar inhomogeneous magnetic field
profiles to achieve a better understanding of this system. We simulate three
flavors of dynamical staggered quarks with physical masses at a range of
magnetic fields and temperatures, and extrapolate the results to the continuum
limit. Analyzing the impact of the field on the quark condensate and the
Polyakov loop, we find non-trivial spatial features that render the QCD medium
qualitatively different as in the homogeneous setup, especially at temperatures
around the transition. In addition, we construct leading-order chiral
perturbation theory for the inhomogeneous background and compare its prediction
to our lattice results at low temperature. Our findings will be useful to
benchmark effective theories and low-energy models of QCD for a better
description of peripheral heavy-ion collisions.Comment: 24 pages, 15 figure
The order of the quantum chromodynamics transition predicted by the standard model of particle physics
We determine the nature of the QCD transition using lattice calculations for
physical quark masses. Susceptibilities are extrapolated to vanishing lattice
spacing for three physical volumes, the smallest and largest of which differ by
a factor of five. This ensures that a true transition should result in a
dramatic increase of the susceptibilities.No such behaviour is observed: our
finite-size scaling analysis shows that the finite-temperature QCD transition
in the hot early Universe was not a real phase transition, but an analytic
crossover (involving a rapid change, as opposed to a jump, as the temperature
varied). As such, it will be difficult to find experimental evidence of this
transition from astronomical observations.Comment: 7 pages, 4 figure
The QCD phase diagram at nonzero quark density
We determine the phase diagram of QCD on the \mu-T plane for small to
moderate chemical potentials. Two transition lines are defined with two
quantities, the chiral condensate and the strange quark number susceptibility.
The calculations are carried out on N_t =6,8 and 10 lattices generated with a
Symanzik improved gauge and stout-link improved 2+1 flavor staggered fermion
action using physical quark masses. After carrying out the continuum
extrapolation we find that both quantities result in a similar curvature of the
transition line. Furthermore, our results indicate that in leading order the
width of the transition region remains essentially the same as the chemical
potential is increased.Comment: 12 pages, 6 figure
QCD equation of state at nonzero chemical potential: continuum results with physical quark masses at order mu^2
We determine the equation of state of QCD for nonzero chemical potentials via
a Taylor expansion of the pressure. The results are obtained for N_f=2+1
flavors of quarks with physical masses, on various lattice spacings. We present
results for the pressure, interaction measure, energy density, entropy density,
and the speed of sound for small chemical potentials. At low temperatures we
compare our results with the Hadron Resonance Gas model. We also express our
observables along trajectories of constant entropy over particle number. A
simple parameterization is given (the Matlab/Octave script parameterization.m,
submitted to the arXiv along with the paper), which can be used to reconstruct
the observables as functions of T and mu, or as functions of T and S/N.Comment: 14 pages, 15 figures, version accepted for publication in JHE
Introducing the Dirac-Milne universe
The \Lambda CDM standard model, although an excellent parametrization of the
present cosmological data, requires two as yet unobserved components, Dark
Matter and Dark Energy, for more than 95% of the Universe. Faced to this
unsatisfactory situation, we study an unconventional cosmology, the Dirac-Milne
universe, a matter-antimatter symmetric cosmology, in which antimatter is
supposed to present a negative active gravitational mass. The main feature of
this cosmology is the linear evolution of the scale factor with time which
directly solves the age and horizon problems of a matter-dominated universe. We
study the concordance of this model to the cosmological test of Type Ia
Supernov\ae\ distance measurements and calculate the theoretical primordial
abundances of light elements for this cosmology. We also show that the acoustic
scale of the Cosmic Microwave Background naturally emerges at the degree scale
despite an open geometry.Comment: Replaced to match published versio
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