On Heteropolymer Shape Dynamics

We investigate the time evolution of the heteropolymer model introduced by Iori, Marinari and Parisi to describe some of the features of protein folding mechanisms. We study how the (folded) shape of the chain evolves in time. We find that for short times the mean square distance (squared) between chain configurations evolves according to a power law, $D \sim t ^\nu$. We discuss the influence of the quenched disorder (represented by the randomness of the coupling constants in the Lennard-Jones potential) on value of the critical exponent. We find that $\nu$ decreases from $\frac{2}{3}$ to $\frac{1}{2}$ when the strength of the quenched disorder increases.Comment: 12 pages, very simple LaTeX file, 6 figures not included, sorry. SCCS 33

Centralized vs Decentralized Markets in the Laboratory: The Role of Connectivity

This paper compares the performance of centralized and decentralized markets experimentally. We constrain trading exchanges to happen on an exogenously predetermined network, representing the trading relationships in markets with differing levels of connectivity. Our experimental results show that, despite having lower trading volumes, decentralized markets are generally not less efficient. Although information can propagate quicker through highly connected markets, we show that higher connectivity also induces informed traders to trade faster and exploit further their information advantages before the information becomes fully incorporated into prices. This not only reduces market efficiency, but it increases wealth inequality. We show that, in more connected markets, informed traders trade not only relatively quicker, but also more, in the right direction, despite not doing it at better prices

Off-Equilibrium Dynamics of a 4D Spin Glass with Asymmetric Couplings

We study the off-equilibrium dynamics of the Edwards-Anderson spin glass in four dimensions under the influence of a non-hamiltonian perturbation. We find that for small asymmetry the model behaves as the hamiltonian one, while for large asymmetry the behaviour of the model can be well described by an interrupted aging scenario. The autocorrelation function C(t_w+\tau,t_w) scales as \tau/t_w^\beta, with \beta a function of the asymmetry. For very long waiting times the previous regime crosses over to a time translational invariant regime (TTI) with stretched exponential relaxation. The model does not show signs of reaching a TTI regime for weak asymmetry, but in the aging regime the exponent \beta is always different from one, showing a non trivial aging scenario.Comment: Latex, 12 pages, 9 figure

The Apparent Madness of Crowds: Irrational collective behavior emerging from interactions among rational agents

Standard economic theory assumes that agents in markets behave rationally. However, the observation of extremely large fluctuations in the price of financial assets that are not correlated to changes in their fundamental value, as well as the extreme instance of financial bubbles and crashes, imply that markets (at least occasionally) do display irrational behavior. In this paper, we briefly outline our recent work demonstrating that a market with interacting agents having bounded rationality can display price fluctuations that are {\em quantitatively} similar to those seen in real markets.Comment: 4 pages, 1 figure, to appear in Proceedings of International Workshop on "Econophysics of Stock Markets and Minority Games" (Econophys-Kolkata II), Feb 14-17, 200

A fitness model for the Italian Interbank Money Market

We use the theory of complex networks in order to quantitatively characterize the formation of communities in a particular financial market. The system is composed by different banks exchanging on a daily basis loans and debts of liquidity. Through topological analysis and by means of a model of network growth we can determine the formation of different group of banks characterized by different business strategy. The model based on Pareto's Law makes no use of growth or preferential attachment and it reproduces correctly all the various statistical properties of the system. We believe that this network modeling of the market could be an efficient way to evaluate the impact of different policies in the market of liquidity.Comment: 5 pages 5 figure

Simulated Tempering: A New Monte Carlo Scheme

We propose a new global optimization method ({\em Simulated Tempering}) for simulating effectively a system with a rough free energy landscape (i.e. many coexisting states) at finite non-zero temperature. This method is related to simulated annealing, but here the temperature becomes a dynamic variable, and the system is always kept at equilibrium. We analyze the method on the Random Field Ising Model, and we find a dramatic improvement over conventional Metropolis and cluster methods. We analyze and discuss the conditions under which the method has optimal performances.Comment: 12 pages, very simple LaTeX file, figures are not included, sorr

Study of the 27 Al(d, α) 25 Mg and 27 Al(d, p) 28 al nuclear reactions at 2 MeV deuteron energy

Differential excitation functions and angular distributions from 30° to 150° have been measured for the27Al(d, α)26Mg and27Al(d, p)28Al reactions leading to the ground state and to the first excited states of the residual nuclei25Mg and28Al, in the deuteron energy range (1.4≪2.3) MeV. The overall energy resolution was 10 keV for the differential excitation functions and 2 0 keV for the angular distributions. Fluctuations occurring in the differential excitation functions as well as in the integrated cross-sections have been analysed on the basis of the statistical theory. In particular, with the use of correlation functions, values of Γ=(40±10)keV and θ0≃30° have been deduced for the « coherence » energy and the « coherence » angle, respectively. The theoretical values of Γ and the average values of the integrated cross-sections have been evaluated using a consistent set of parameters. The analysis showed that the predictions of the statistical model agree with the experimental results for the27Al(d, α)25Mg reactions. Both statistical and nonstatistioal effects have been found to contribute to the27Al(d, p)28Al reaction for the proton transitions leading to the low-lying levels of the residual nucleus

The level width of atomic nuclei in the continuum energy region: Comparison with the statistical-model previsions

In this paper the theoretical expression of the « coherence energy » which characterizes the Ericson flctuations of an excitation function is derived in the framework of the statistical model. It is given by a weighted average over spins, parity and energy of the widths of the levels of the compound nucleus interested in the reaction. The case of purely statistical reactions and the case in which a nonstatistical effect is present are analised