arXiv subjects

Roberto da Silva

Publications and source records attributed to Roberto da Silva.

At least 55 records · Page 3Linked to original sources

arXiv2018

Investigating the nonequilibrium aspects of long-range Potts model: refinement of critical temperatures and raw exponents

In this work, we analyse the $q-$state Potts model with long-range interactions through nonequilibrium scaling relations commonly used when studying short-range systems. We determine the critical temperature via an optimization method for short-time Monte Carlo simulations. The study takes into consideration two different boundary conditions and three different values of range parameters of the couplings. We also present estimates of some critical exponents, named as raw exponents for systems with long-range interactions, which confirm the non-universal character of the model. Finally, we provide some preliminary results addressing the relations between the raw exponents and the exponents obtained for systems with short-range interactions. The results assert that the methods employed in this work are suitable to study the considered model and can easily be adapted to other systems with long-range interactions.

cond-mat.stat-mech↗

arXiv2018

Unveiling the ZGB model with $CO$ desorption: a single model with two universality classes?

We study the behavior of the phase transitions of the Ziff-Gullari-Barshad (ZGB) model when the $CO$ molecules are adsorbed on the catalytic surface with a rate $y$ and desorbed from the surface with a rate $k$. We employ large-scale nonequilibrium Monte Carlo simulations along with an optimization technique based on the coefficient of determination, in order to obtain an overview of the phase transitions of the model in the whole spectrum of $y$ and $k$: ($0\leq y\leq 1$ and $0\leq k\leq 1$) with precision $Δy=Δk=0.001$. Sucessive refinements reveal a region of points belonging to the directed percolation universality class whereas the exponents $θ$ and $β/ν_{\parallel }$ obtained agree with those of this universality class. On the other hand, the effects of allowing the $CO$ desorption from the lattice on the discontinuous phase transition point of the original ZGB model suggest the emergence of an Ising-like point previously predicted in Ref. \cite{tome1993}. We show that such a point appears after a sequence of two lines of pseudo-critical points which leads to a unique peak of the coefficient of determination curve in $y_{c}=0.554$ and $k_{c}=0.064$. In this point, the exponent $θ$ agrees with the value found for Ising model.

cond-mat.stat-mech↗

arXiv2018

Highly detailed computational study of a surface reaction model with diffusion: four algorithms analyzed via time-dependent and steady-state Monte Carlo simulations

In this work, we present an extensive computational study on the Ziff-Gulari-Barshad (ZGB) model extended in order to include the spatial diffusion of oxygen atoms and carbon monoxide molecules, both adsorbed on the surface. In our approach, we consider two different protocols to implement the diffusion of the atoms/molecules and two different ways to combine the diffusion and adsorption processes resulting in four different algorithms. The influence of the diffusion on the continuous and discontinuous phase transitions of the model is analysed through two very well established methods: the time-dependent Monte Carlo simulations and the steady-state Monte Carlo simulations. We also use an optimization method based on a concept known as coefficient of determination to construct color maps and obtain the phase transitions when the parameters of the model vary. This method was proposed recently to locate nonequilibrium second-order phase transitions and has been successfully used in both systems: with defined Hamiltonian and with absorbing states. The results obtained via time-dependent Monte Carlo simulation along with the coefficient of determination are corroborated by traditional steady-state Monte Carlo simulations also performed for the four algorithms. Finally, we analyse the finite-size effects on the results, as well as, the influence of the number of runs on the reliability of our estimates.

cond-mat.stat-mech↗

arXiv2018

When and how much the altruism impacts your privileged information? Proposing a new paradigm in game theory: The boxers game

In this work, we proposed a new $N$-person game in which the players can bet on two options, for example represented by two boxers. Some of the players have privileged information about the boxers and part of them can provide this information to uninformed players. However, this information may be true if the informed player is altruist or false if he is selfish. So, in this game, the players are divided in three categories: informed and altruist players, informed and selfish players, and uninformed players. By considering the matchings ($N/2$ distinct pairs of randomly chosen players) and that the payoff of the winning group follows aspects captured from two important games, the public goods game and minority game, we showed quantitatively and qualitatively how the altruism can impact on the privileged information. We localized analytically the regions of positive payoffs which were corroborated by numerical simulations performed for all values of information and altruism densities given that we know the information level of the informed players. Finally, in an evolutionary version of the game ,we showed that the gain of the informed players can get worse if we adopted the following procedure: the players increase their investment for situations of positive payoffs, and decrease their investment when negative payoffs occur.

cond-mat.stat-mech↗

arXiv2017

Inferences on the Higgs Boson and Axion Masses through a Maximum Entropy Principle

The Maximum Entropy Principle (MEP) is a method that can be used to infer the value of an unknown quantity in a set of probability functions. In this work we review two applications of MEP: one giving a precise inference of the Higgs boson mass value; and the other one allowing to infer the mass of the axion. In particular, for the axion we assume that it has a decay channel into pairs of neutrinos, in addition to the decay into two photons. The Shannon entropy associated to an initial ensemble of axions decaying into photons and neutrinos is then built for maximization.

hep-ph↗

arXiv2017

Spatial patterns of random walkers under evolution of the attractiveness: persistent nodes, degree distribution, and spectral properties

In this paper we explore the features of a graph generated by random walkers with nodes that have evolutionary attractiveness and Boltzmann-like transition probabilities that depend both on the euclidean distance between the nodes and on the ratio ($β$) of the attractiveness between them. We show that persistent nodes, i.e., nodes that never been reached by random walker in asymptotic times are possible in the stationary case differently from the case where the attractiveness is fixed and equal to one for all nodes ($β=1$). Simultaneously, we also investigate the spectral properties and statistics related to the attractiveness and degree distribution of the evolutionary network. Finally, we study a crossover between persistent phase and no persistent phase and we also show the existence of a special type of transition probability that leads to a power law behaviour for the time evolution of the persistence.

physics.soc-ph↗

arXiv2017

Statistics, distillation, and ordering emergence in a two-dimensional stochastic model of particles in counterflowing streams

In this paper, we proposed a stochastic model which describes two species of particles moving in counterflow. The model generalizes the theoretical framework describing the transport in random systems since particles can work as mobile obstacles, whereas particles of one species move in opposite direction to the particles of the other species, or they can work as fixed obstacles remaining in their places during the time evolution. We conducted a detailed study about the statistics concerning the crossing time of particles, as well as the effects of the lateral transitions on the time required to the system reaches a state of complete geographic separation of species. The spatial effects of jamming were also studied by looking into the deformation of the concentration of particles in the two-dimensional corridor. Finally, we observed in our study the formation of patterns of lanes which reach the steady state regardless the initial conditions used for the evolution. A similar result is also observed in real experiments involving charged colloids motion and simulations of pedestrian dynamics based on Langevin equations, when periodic boundary conditions are considered (particles counterflow in a ring symmetry). The results obtained through Monte Carlo numerical simulations and numerical integrations are in good agreement with each other. However, differently from previous studies, the dynamics considered in this work is not Newton-based, and therefore, even artificial situations of self-propelled objects should be studied in this first-principle modeling.

cond-mat.soft↗

arXiv2017

A new look on the stabilization of inverted pendulum with parametric excitation and large random frequencies: analytical and numerical approaches

In this paper we explore the stability of an inverted pendulum with generalized parametric excitation described by a superposition of $N$ sines with different frequencies and phases. We show that when the amplitude is scaled with the frequency we obtain the stabilization of the real inverted pendulum, i.e., with values of $g$ according to planet Earth ($g\approx 9.8$m/s$^{2}$) for high frequencies. By randomly sorting the frequencies, we obtain a critical amplitude in light of perturbative theory in classical mechanics which is numerically tested by exploring its validity regime in many alternatives. We also analyse the effects when different values of $N$ as well as the pendulum size $l$ are taken into account.

physics.class-ph↗

arXiv2017

Maximum Entropy Inferences on the Axion Mass in Models with Axion-Neutrino Interaction

In this work we use the Maximum Entropy Principle (MEP) to infer the mass of an axion which interacts to photons and neutrinos in an effective low energy theory. The Shannon entropy function to be maximized is suitably defined in terms of the axion branching ratios. We show that MEP strongly constrains the axion mass taking into account the current experimental bounds on the neutrinos masses. Assuming that the axion is massive enough to decay into all the three neutrinos and that MEP fixes all the free parameters of the model, the inferred axion mass is in the interval $0.1\ $eV$\ <m_{A}<0.2$ eV, which can be tested by forthcoming experiments such as IAXO. However, even in the case where MEP fixes just the axion mass and no other parameter, we found that $0.1$ eV $< m_A < 6.3$ eV in the DFSZ model with right-handed neutrinos. Moreover, a light axion, allowed to decay to photons and the lightest neutrino only, is determined by MEP as a viable dark matter candidate.

hep-ph↗

arXiv2016

Time-dependent Monte Carlo simulations of the critical and Lifshitz points of the ANNNI model

In this work, we study the critical behavior of second order points and specifically of the Lifshitz point (LP) of a three-dimensional Ising model with axial competing interactions (ANNNI model), using time-dependent Monte Carlo simulations. First of all, we used a recently developed technique that helps us localize the critical temperature corresponding to the best power law for magnetization decay over time: $\left\langle M\right\rangle _{m_{0}=1}\sim t^{-β/νz}\ $ which is expected of simulations starting from initially ordered states. Secondly, we obtain original results for the dynamic critical exponent $z$, evaluated from the behavior of the ratio $F_{2}(t)=\left\langle M^{2}\right\rangle _{m_{0}=0}/\left\langle M\right\rangle _{m_{0}=1}^{2}\sim t^{3/z}$, along the critical line up to the LP. Finally, we explore all the critical exponents of the LP in detail, including the dynamic critical exponent $θ$ that characterizes the initial slip of magnetization and the global persistence exponent $θ_{g}$ associated to the probability $P(t)$ that magnetization keeps its signal up to time $t$. Our estimates for the dynamic critical exponents at the Lifshitz point are $z=2.34(2)$ and $θ_{g}=0.336(4)$, values very different from the 3D Ising model (ANNNI model without the next-nearest-neighbor interactions at $z$-axis, i.e., $J_{2}=0$) $z\approx 2.07$ and $θ_{g}\approx 0.38$. We also present estimates for the static critical exponents $β$ and $ν$, obtained from extended time-dependent scaling relations. Our results for static exponents are in good agreement with previous works

cond-mat.stat-mech↗

arXiv2016

Towards Rock-paper-scissors patterns in the Optional Public Goods Game under random mobility

Social dilemmas concern a natural conflict between cooperation and self interests among individuals in large populations. The emergence of cooperation and its maintenance is the key for the understanding of fundamental concepts about the evolution of species. In order to understand the mechanisms involved in this framework, here we study the Optional Public Good Games with focus on the effects of diffusive aspects in the emergent patterns of cyclic dominance between the strategies. Differently from other works, we showed that rock-paper-scissors (RPS) patterns occur by introducing a simple kind of random mobility in a lattice sparsely occupied. Such pattern has been revealed to be very important in the conservation of the species in ecological and social environments. The goal of this paper is to show that we do not need more elaborated schemes for construction of the neighbourhood in the game to observe RPS patterns as suggested in the literature. As an interesting additional result, in this contribution we also propose an alternative method to quantify the RPS density in a quantitative context of the game theory which becomes possible to perform a finite size scaling study. Such approach can be very interesting to be applied in other games generically.

physics.soc-ph↗

arXiv2016

An alternative method to characterize first- and second-order phase transitions in surface reaction models

In this work, we revisited the Ziff-Gulari-Barshad (ZGB) model to study its phase transitions and critical exponents through time-dependent Monte Carlo simulations. We used a method proposed recently to locate the non-equilibrium second-order phase transitions and that has been successfully used in systems with defined Hamiltonians and with absorbing states. This method, which is based on optimization of the coefficient of determination of the order parameter, was able to characterize the second-order phase transition of the model, as well as its upper spinodal point, a pseudo-critical point located near the first-order transition. The static critical exponents $β$, $ν_{\parallel }$, and $ν_{\perp }$, as well as the dynamic critical exponents $θ$ and $z$ for the second-order point were also estimated and are in excellent agreement with results found in literature.

cond-mat.stat-mech↗

arXiv2016

Deterministic and stochastic aspects of the stability in an inverted pendulum under a generalized parametric excitation

In this paper, we explore the stability of an inverted pendulum under a generalized parametric excitation described by a superposition of $N$ cosines with different amplitudes and frequencies, based on a simple stability condition that does not require any use of Lyapunov exponent, for example. Our analysis is separated in 3 different cases: $N=1$, $N=2$, and $N$ very large. Our results were obtained via numerical simulations by fourth-order Runge Kutta integration of the non-linear equations. We also calculate the effective potential also for $N>2$. We show then that numerical integrations recover a wider region of stability that are not captured by the (approximated) analytical method. We also analyze stochastic stabilization here: firstly, we look the effects of external noise in the stability diagram by enlarging the variance, and secondly, when $N$ is large, we rescale the amplitude by showing that the diagrams for time survival of the inverted pendulum resembles the exact case for $N=1$. Finally, we find numerically the optimal number of cosines corresponding to the maximal survival probability of the pendulum.

physics.class-ph↗

arXiv2015

Randomness and Arbitrary Coordination in the Reactive Ultimatum Game

Darwin's theory of evolution - as introduced in game theory by Maynard Smith - is not the only important evolutionary aspect in a evolutionary dynamics, since complex interdependencies, competition, and growth should be modeled by, for example, reactive aspects. In the ultimatum game the reciprocity and the fifty-fifty partition seems to be a deviation from rational behavior of the players under the light of the Nash equilibrium concept.Such equilibrium emerges, for example, from the punishment of the responder who generally tends to refuse unfair proposals. In the iterated version of the game, the proposers are able to improve their proposals by adding a value thus making fairer proposals. Such evolutionary aspects are not properly Darwinian-motivated, but they are endowed with a fundamental aspect: they reflect their actions according to value of the offers. Recently, a reactive version of the ultimatum game where acceptance occurs with fixed probability was proposed. In this paper, we aim at exploring this reactive version of the ultimatum game where the acceptance by players depends on the offer. In order to do so, we analyze two situations: (i) mean field and (ii) we consider players inserted within the networks with arbitrary coordination. We then show that the reactive aspect, here studied, thus far not analyzed in the evolutionary game theory literature can unveil an essential feature for the convergence to fifty-fifty split. Moreover we also analyze populations under four different polices ranging from a highly conservative to a moderate one, with respect to decision in changing the proposal based on acceptations. We show that the idea of gaining less more times added to the reciprocity of the players is highly relevant to the "healthy" societies population bargaining concept.

physics.soc-ph↗

arXiv2015

Novel considerations about the non-equilibrium regime of the tricritical point in a metamagnetic model: localization and tricritical exponents

We have investigated the time-dependent regime of a two-dimensional metamagnetic model at its tricritical point via Monte Carlo simulations. First of all, we obtained the temperature and magnetic field corresponding to the tricritical point of the model by using a refinement process based on optimization of the coefficient of determination in the log-log fit of magnetization decay as function of time. With these estimates in hand, we obtained the dynamic tricritical exponents $θ$ and $z$ and the static tricritical exponents $ν$ and $β$ by using the universal power-law scaling relations for the staggered magnetization and its moments at early stage of the dynamic evolution. Our results at tricritical point confirm that this model belongs to the two-dimensional Blume-Capel model universality class for both static and dynamic behaviors, and also they corroborate the conjecture of Janssen and Oerding for the dynamics of tricritical points.

cond-mat.stat-mech↗

arXiv2015

Humans Are Easily Fooled by Digital Images

Digital images are ubiquitous in our modern lives, with uses ranging from social media to news, and even scientific papers. For this reason, it is crucial evaluate how accurate people are when performing the task of identify doctored images. In this paper, we performed an extensive user study evaluating subjects capacity to detect fake images. After observing an image, users have been asked if it had been altered or not. If the user answered the image has been altered, he had to provide evidence in the form of a click on the image. We collected 17,208 individual answers from 383 users, using 177 images selected from public forensic databases. Different from other previously studies, our method propose different ways to avoid lucky guess when evaluating users answers. Our results indicate that people show inaccurate skills at differentiating between altered and non-altered images, with an accuracy of 58%, and only identifying the modified images 46.5% of the time. We also track user features such as age, answering time, confidence, providing deep analysis of how such variables influence on the users' performance.

cs.GR↗

arXiv2015

Universality, Correlations, and Rankings in the Brazilian Universities National Admission Examinations

The scores obtained by students that have performed the ENEM exam, the Brazilian High School National Examination used to admit students at the Brazilian universities, is analyzed. The average high school's scores are compared between different disciplines through the Pearson correlation coefficient. The results show a very large correlation between the performance in the different subjects. Even thought the students' scores in the ENEM due to the standardization form a Gaussian, we show that the high schools' scores form a bimodal distribution that can not be used to evaluate and compare performance over time. We also show that this high schools distribution reflects the correlation between school performance and economic level of the students. The ENEM's scores are compared with a Brazilian non standardized exam, the entrance exam at the Universidade Federal do Rio Grande do Sul. The comparison of the performance of the same individuals in both tests is compared showing that the two tests not only select different abilities but chooses a different set of individuals. Our results indicates that standardized exams might be an interesting tool to compare performance over the years but only of individuals and not of institutions.

physics.soc-ph↗

arXiv2014

A study of the influence of the mobility on the phase transitions of the synchronous SIR model

By using an appropriate version of the synchronous SIR model, we studied the effects of dilution and mobility on the critical immunization rate. We showed that, by applying time-dependent Monte Carlo (MC) simulations at criticality, and taking into account the optimization of the power law for the density of infected individuals, the critical immunization necessary to block the epidemic in two-dimensional lattices decreases as dilution increases with a logarithmic dependence. On the other hand, the mobility minimizes such effects and the critical immunizations is greater when the probability of movement of the individuals increases.

physics.soc-ph↗