32 research outputs found
Two-Flavor Lattice QCD with a Finite Density of Heavy Quarks: Heavy-Dense Limit and "Particle-Hole" Symmetry
We investigate the properties of the half-filling point in lattice QCD
(LQCD), in particular the disappearance of the sign problem and the emergence
of an apparent particle-hole symmetry, and try to understand where these
properties come from by studying the heavy-dense fermion determinant and the
corresponding strong-coupling partition function (which can be integrated
analytically). We then add in a first step an effective Polyakov loop gauge
action (which reproduces the leading terms in the character expansion of the
Wilson gauge action) to the heavy-dense partition function and try to analyze
how some of the properties of the half-filling point change when leaving the
strong coupling limit. In a second step, we take also the leading
nearest-neighbor fermion hopping terms into account (including gauge
interactions in the fundamental representation) and mention how the method
could be improved further to incorporate the full set of nearest-neighbor
fermion hoppings. Using our mean-field method, we also obtain an approximate
(,T) phase diagram for heavy-dense LQCD at finite inverse gauge coupling
. Finally, we propose a simple criterion to identify the chemical
potential beyond which lattice artifacts become dominant.Comment: 39 pages, 22 figure
Euclidean Dynamical Triangulation revisited: is the phase transition really first order?
The transition between the two phases of 4D Euclidean Dynamical Triangulation
[1] was long believed to be of second order until in 1996 first order behavior
was found for sufficiently large systems [3,4]. However, one may wonder if this
finding was affected by the numerical methods used: to control volume
fluctuations, in both studies [3,4] an artificial harmonic potential was added
to the action; in [4] measurements were taken after a fixed number of accepted
instead of attempted moves which introduces an additional error. Finally the
simulations suffer from strong critical slowing down which may have been
underestimated.
In the present work, we address the above weaknesses: we allow the volume to
fluctuate freely within a fixed interval; we take measurements after a fixed
number of attempted moves; and we overcome critical slowing down by using an
optimized parallel tempering algorithm [6]. With these improved methods, on
systems of size up to 64k 4-simplices, we confirm that the phase transition is
first order.Comment: 7 pages, 5 figures, presented at the 31st International Symposium on
Lattice Field Theory (Lattice 2013), 29 July - 3 August 2013, Mainz, German
Euclidean Dynamical Triangulation revisited: is the phase transition really 1st order? (extended version)
The transition between the two phases of 4D Euclidean Dynamical Triangulation
[1] was long believed to be of second order until in 1996 first order behavior
was found for sufficiently large systems [5,9]. However, one may wonder if this
finding was affected by the numerical methods used: to control volume
fluctuations, in both studies [5,9] an artificial harmonic potential was added
to the action; in [9] measurements were taken after a fixed number of accepted
instead of attempted moves which introduces an additional error. Finally the
simulations suffer from strong critical slowing down which may have been
underestimated. In the present work, we address the above weaknesses: we allow
the volume to fluctuate freely within a fixed interval; we take measurements
after a fixed number of attempted moves; and we overcome critical slowing down
by using an optimized parallel tempering algorithm [12]. With these improved
methods, on systems of size up to 64k 4-simplices, we confirm that the phase
transition is first order.
In addition, we discuss a local criterion to decide whether parts of a
triangulation are in the elongated or crumpled state and describe a new
correspondence between EDT and the balls in boxes model. The latter gives rise
to a modified partition function with an additional, third coupling. Finally,
we propose and motivate a class of modified path-integral measures that might
remove the metastability of the Markov chain and turn the phase transition into
second order.Comment: 26 pages, 21 figures, extended version of arXiv:1311.471
Sampling of General Correlators in Worm Algorithm-based Simulations
Using the complex -model as a prototype for a system which is
simulated by a worm algorithm, we show that not only the charged correlator
or , can be measured at every step of the Monte
Carlo evolution of the worm instead of on closed-worm configurations only. The
method generalizes straightforwardly to other systems simulated by worms, such
as spin or sigma models.Comment: 43 pages, 15 figure
Oscillating propagators in heavy-dense QCD
Using Monte Carlo simulations and extended mean field theory calculations we
show that the -dimensional spin model with complex external fields has
non-monotonic spatial correlators in some regions of its parameter space. This
model serves as a proxy for heavy-dense QCD in dimensions.
Non-monotonic spatial correlators are intrinsically related to a complex mass
spectrum and a liquid-like (or crystalline) behavior. A liquid phase could have
implications for heavy-ion experiments, where it could leave detectable signals
in the spatial correlations of baryons.Comment: 16 pages, 9 figures, updated to match published versio
Bulk-preventing actions for SU(N) gauge theories
We introduce a one-parameter family of SU(N) gauge actions which, when used in combination with an HMC update algorithm, prevent the gauge system from entering an artificial bulk-"phase". We briefly discuss the mechanism behind the bulk-prevention and present test results for different SU(N) gauge groups.Peer reviewe
Bulk-preventing actions for SU(N) gauge theories
We introduce a one-parameter family of SU(N) gauge actions which, when used in combination with an HMC update algorithm, prevent the gauge system from entering an artificial bulk-"phase". We briefly discuss the mechanism behind the bulk-prevention and present test results for different SU(N) gauge groups.Peer reviewe
Nonperturbative Decoupling of Massive Fermions.
SU(2) gauge theory with N_{f}=24 massless fermions is noninteracting at long distances, i.e., it has an infrared fixed point at vanishing coupling. With massive fermions, the fermions are expected to decouple at energy scales below the fermion mass, and the infrared behavior is that of confining SU(2) pure gauge theory. We demonstrate this behavior nonperturbatively with lattice Monte Carlo simulations by measuring the gradient flow running coupling
Nonperturbative Decoupling of Massive Fermions
Funding Information: The support of the Academy of Finland Grants No. 308791, No. 310130, and No. 320123 is acknowledged. Partially funded by the Swiss National Science Foundation (SNSF) through Grant No. 200021_175761. The authors wish to acknowledge CSC—IT Center for Science, Finland, for generous computational resources. Publisher Copyright: © 2022 authors. Published by the American Physical Society.SU(2) gauge theory with Nf=24 massless fermions is noninteracting at long distances, i.e., it has an infrared fixed point at vanishing coupling. With massive fermions, the fermions are expected to decouple at energy scales below the fermion mass, and the infrared behavior is that of confining SU(2) pure gauge theory. We demonstrate this behavior nonperturbatively with lattice Monte Carlo simulations by measuring the gradient flow running coupling.Peer reviewe