1,736 research outputs found
Instanton fermionic zero mode at finite temperature and chemical potential
In QCD the spontaneous breaking of chiral symmetry and the U(1) axial anomaly
can be understood considering instantons as the gauge configurations mediating
quark-quark interaction. The existence of an exact zero mode solution of the
Dirac equation in the field of a single instanton is the fundamental ingredient
of this analysis. Explicit expressions for psi_0 are available for T different
from 0 and mu=0, and mu different from 0 and T=0. In this paper we derive the
solution for the most general case T different from 0 and mu different from 0.
This new result opens the possibility of investigating the QCD dynamics
associated with instantons in the full phase diagram. As a first step in this
direction we will study the dependence of the instanton density from the
thermodynamic coordinates.Comment: 12 pages, 4 figures, Final version, accepted for publication on Phys.
Rev.
Exploring the transition into the Chiral Regime of QCD using the Interacting Instanton Liquid Model
The non-perturbative quark-gluon interaction depends significantly on the
value of the quark mass. In particular, in the light quark mass regime,
correlations are strongly influenced by dynamics associated to chiral symmetry
breaking. We use the Interacting Instanton Liquid Model (IILM) as a tool to
investigate the microscopic dynamical mechanisms which underly the dependence
on the quark mass and drive the transition into the chiral regime of QCD. To
ensure the validity of the model, we first verify that the dependence on the
quark mass for several observables calculated in the IILM agrees well with the
predictions of chiral perturbation theory and with lattice simulations. We then
show that a quark mass m*~80 MeV emerging naturally from the model specifies
the mass scale above which the dynamics associated with low-lying eigenmodes of
the Direac operator becomes sub-leading and the contribution of the fermion
determinant is suppressed.Comment: contribution to XXV International Symposium on Lattice Field Theory,
July 2007, Regensbur
Metropolis Monte Carlo on the Lefschetz thimble: application to a one-plaquette model
We propose a new algorithm based on the Metropolis sampling method to perform
Monte Carlo integration for path integrals in the recently proposed formulation
of quantum field theories on the Lefschetz thimble. The algorithm is based on a
mapping between the curved manifold defined by the Lefschetz thimble of the
full action and the flat manifold associated with the corresponding quadratic
action. We discuss an explicit method to calculate the residual phase due to
the curvature of the Lefschetz thimble. Finally, we apply this new algorithm to
a simple one-plaquette model where our results are in perfect agreement with
the analytic integration. We also show that for this system the residual phase
does not represent a sign problem
Influence of augmented humans in online interactions during voting events
The advent of the digital era provided a fertile ground for the development
of virtual societies, complex systems influencing real-world dynamics.
Understanding online human behavior and its relevance beyond the digital
boundaries is still an open challenge. Here we show that online social
interactions during a massive voting event can be used to build an accurate map
of real-world political parties and electoral ranks. We provide evidence that
information flow and collective attention are often driven by a special class
of highly influential users, that we name "augmented humans", who exploit
thousands of automated agents, also known as bots, for enhancing their online
influence. We show that augmented humans generate deep information cascades, to
the same extent of news media and other broadcasters, while they uniformly
infiltrate across the full range of identified groups. Digital augmentation
represents the cyber-physical counterpart of the human desire to acquire power
within social systems.Comment: 11 page
Sparse Predictive Structure of Deconvolved Functional Brain Networks
The functional and structural representation of the brain as a complex
network is marked by the fact that the comparison of noisy and intrinsically
correlated high-dimensional structures between experimental conditions or
groups shuns typical mass univariate methods. Furthermore most network
estimation methods cannot distinguish between real and spurious correlation
arising from the convolution due to nodes' interaction, which thus introduces
additional noise in the data. We propose a machine learning pipeline aimed at
identifying multivariate differences between brain networks associated to
different experimental conditions. The pipeline (1) leverages the deconvolved
individual contribution of each edge and (2) maps the task into a sparse
classification problem in order to construct the associated "sparse deconvolved
predictive network", i.e., a graph with the same nodes of those compared but
whose edge weights are defined by their relevance for out of sample predictions
in classification. We present an application of the proposed method by decoding
the covert attention direction (left or right) based on the single-trial
functional connectivity matrix extracted from high-frequency
magnetoencephalography (MEG) data. Our results demonstrate how network
deconvolution matched with sparse classification methods outperforms typical
approaches for MEG decoding
Thermodynamics of a three-flavor nonlocal Polyakov--Nambu--Jona-Lasinio model
The present work generalizes a nonlocal version of the Polyakov loop-extended
Nambu and Jona-Lasinio (PNJL) model to the case of three active quark flavors,
with inclusion of the axial U(1) anomaly. Gluon dynamics is incorporated
through a gluonic background field, expressed in terms of the Polyakov loop.
The thermodynamics of the nonlocal PNJL model accounts for both chiral and
deconfinement transitions. Our results obtained in mean-field approximation are
compared to lattice QCD results for quark flavors. Additional
pionic and kaonic contributions to the pressure are calculated in random phase
approximation. Finally, this nonlocal 3-flavor PNJL model is applied to the
finite density region of the QCD phase diagram. It is confirmed that the
existence and location of a critical point in this phase diagram depends
sensitively on the strength of the axial U(1) breaking interaction.Comment: 31 pages, 15 figures, minor changes compared to v
Thermodynamics and quark susceptibilities: a Monte-Carlo approach to the PNJL model
The Monte-Carlo method is applied to the Polyakov-loop extended
Nambu--Jona-Lasinio (PNJL) model. This leads beyond the saddle-point
approximation in a mean-field calculation and introduces fluctuations around
the mean fields. We study the impact of fluctuations on the thermodynamics of
the model, both in the case of pure gauge theory and including two quark
flavors. In the two-flavor case, we calculate the second-order Taylor expansion
coefficients of the thermodynamic grand canonical partition function with
respect to the quark chemical potential and present a comparison with
extrapolations from lattice QCD. We show that the introduction of fluctuations
produces only small changes in the behavior of the order parameters for chiral
symmetry restoration and the deconfinement transition. On the other hand, we
find that fluctuations are necessary in order to reproduce lattice data for the
flavor non-diagonal quark susceptibilities. Of particular importance are pion
fields, the contribution of which is strictly zero in the saddle point
approximation
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