63 research outputs found
The curvature of the QCD phase transition line
We determine the curvature of the phase transition line in the mu-T plane
through an analysis of various observables, including the Polyakov loop, the
quark number susceptibilities and the susceptibility of the chiral condensate.
The second derivative of these quantities with respect to mu was calculated.
The measurements were carried out on N_T = 4,6,8 and 10 lattices generated with
a Symanzik improved gauge and stout-link improved 2+1 flavour staggered fermion
action using physical quark masses.Comment: Talk presented at the XXVI International Symposium on Lattice Field
Theory, July 14 - 19, 2008, Williamsburg, Virginia, USA. 7 pages, 6 figure
Magnetized baryons and the QCD phase diagram: NJL model meets the lattice
We determine the baryon spectrum of 1 + 1 + 1-flavor QCD in the presence of
strong background magnetic fields using lattice simulations at physical quark
masses for the first time. Our results show a splitting within multiplets
according to the electric charge of the baryons and reveal, in particular, a
reduction of the nucleon masses for strong magnetic fields. This
first-principles input is used to define constituent quark masses and is
employed to set the free parameters of the Polyakov loop-extended
Nambu-Jona-Lasinio (PNJL) model in a magnetic field-dependent manner. The so
constructed model is shown to exhibit inverse magnetic catalysis at high
temperatures and a reduction of the transition temperature as the magnetic
field grows - in line with non-perturbative lattice results. This is contrary
to the naive variant of this model, which gives incorrect results for this
fundamental phase diagram. Our findings demonstrate that the magnetic field
dependence of the PNJL model can be reconciled with the lattice findings in a
systematic way, employing solely zero-temperature first-principles input.Comment: 14 pages, 7 figures, updatet some figures, included new references
and a table of key result
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
Meson masses in electromagnetic fields with Wilson fermions
We determine the light meson spectrum in QCD in the presence of background magnetic fields using quenched Wilson fermions. Our continuum extrapolated results indicate a monotonous reduction of the connected neutral pion mass as the magnetic field grows. The vector meson mass is found to remain nonzero, a finding relevant for the conjectured rho-meson condensation at strong magnetic fields. The continuum extrapolation was facilitated by adding a novel magnetic field-dependent improvement term to the additive quark mass renormalization. Without this term, sizable lattice artifacts that would deceptively indicate an unphysical rise of the connected neutral pion mass for strong magnetic fields are present. We also investigate the impact of these lattice artifacts on further observables like magnetic polarizabilities and discuss the magnetic field-induced mixing between rho-mesons and pions. We also derive Ward-Takashi identities for QCD thorn QED both in the continuum formulation and for ( order a-improved) Wilson fermions
Weak Decay of Magnetized Pions
The leptonic decay of charged pions is investigated in the presence of background magnetic fields. In this situation, Lorentz symmetry is broken, and new fundamental decay constants need to be introduced, associated with the decay via the vector part of the electroweak current. We calculate the magnetic field dependence of both the usual and a new decay constant nonperturbatively on the lattice. We employ both Wilson and staggered quarks and extrapolate the results to the continuum limit. With this nonperturbative input, we calculate the tree level electroweak amplitude for the full decay rate in strong magnetic fields. We find that the muonic decay of the charged pion is enhanced drastically by the magnetic field. We comment on possible astrophysical implications
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
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
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