114 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
Bose-Einstein Condensation of strongly interacting bosons: from liquid He to QCD monopoles
Starting from classic work of Feynman on the -point of liquid
Helium, we show that his idea of universal action per particle at the BEC
transition point is much more robust that it was known before. Using a simple
"moving string model" for supercurrent and calculating the action, both
semiclassically and numerically, we show that the critical action is the same
for noninteracting and strongly interacting systems such as liquid He.
Inversely, one can obtain accurate dependence of critical temperature on
density: one important consequence is that high density (solid) He cannot be a
BEC state of He atoms, with upper density accurately matching the observations.
We then use this model for the deconfinement phase transition of QCD-like gauge
theories, treated as BEC of (color)magnetic monopoles. We start with
Feynman-like approach without interaction, estimating the monopole mass at
. Then we include monopole's Coulomb repulsion, and formulate a relation
between the mass, density and coupling which should be fulfilled at the
deconfinement point. We end up proposing various ways to test on the lattice
whether it is indeed the BEC point for monopoles
Are there hadronic bound states above the QCD transition temperature?
Recent lattice QCD calculations, at physical pion masses and small lattice
spacings that approach the continuum limit, have revealed that non-diagonal
quark correlators above the critical temperature are finite up to about 2
. Since the transition from hadronic to free partonic degrees of freedom
is merely an analytic cross-over, it is likely that, in the temperature regime
between 1-2 , quark and gluon quasiparticles and pre-hadronic bound states
can coexist. The correlator values, in comparison to PNJL model calculations
beyond mean-field, indicate that at least part of the mixed phase resides in
color-neutral bound states. A similar effect was postulated for the in-medium
fragmentation process, i.e. for partons which do not thermalize with the system
and thus constitute the non-equilibrium component of the particle emission
spectrum from a deconfined plasma phase. Here, for the first time we
investigate the likelihood of forming bound states also in the equilibrated,
parton dominated phase above which is described by lattice QCD.Comment: 15 pages, 4 Figure
Exploring the transition into the chiral regime of QCD using the interacting instanton liquid model
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