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ABSTRACT: We analyse an artificial neural network which deviates from biological behaviour in two aspects. First, the process of activation of a generic neuron is not described by a monotone increasing output function. This means that, while artificial neurons modelled on biological behaviour are active when the sum of the postsynaptic potentials is larger than a given threshold and quiescent in the opposite case, in our model the state of the neuron depends non-monotonically on its argument, i.e. the activation function is not an increasing function, but has a more complicated behaviour which reduces to the usual (step or sigmoid) function for some particular values of the parameters describing its shape. Second, they assume as a learning rule of the network a modification of the Hebb rule, namely they choose an iterative algorithm (Edinburgh algorithm) that constructs a synaptic matrix with a given set of stored memories and given retrieval properties, i.e. given domains of attraction of the stored patterns. The non-monotonicity of the output function results, for some values of the parameters, in a larger storage capacity than in conventional models, whereas the choice of the learning rule allows to control the domains of attraction of the memories.
07/2009; 4(1):1-9.
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ABSTRACT: We study the phase diagram and the pion modes in the electrically neutral two flavor Polyakov–Nambu–Jona-Lasinio model. One of the main results of this paper is that in the model with massive quarks, when electrical neutrality is required, pions do not condense in the ground state of the model: the isospin chemical potential μI=-μe/2 is always smaller than the value required for pion condensation to occur. Moreover we investigate on the pions and σ mass spectra. We find that the qualitative behavior of the masses resembles that obtained in the Nambu–Jona-Lasinio model. We close this paper by studying the intriguing possibility that bound states with the quantum numbers of the pions can be formed above the chiral phase transition.
Phys. Rev. D. 07/2008; 78(1).
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ABSTRACT: We study the interplay between the chiral and the deconfinement transitions, both at high temperature and high quark chemical potential, by a non local Nambu-Jona Lasinio model with the Polyakov loop in the mean field approximation and requiring neutrality of the ground state. We consider three forms of the effective potential of the Polyakov loop: two of them with a fixed deconfinement scale, cases I and II, and the third one with a $\mu$ dependent scale, case III. In the cases I and II, at high chemical potential $\mu$ and low temperature $T$ the main contribution to the free energy is due to the Z(3)-neutral three-quark states, mimicking the quarkyonic phase of the large $N_c$ phase diagram. On the other hand in the case III the quarkyonic window is shrunk to a small region. Finally we comment on the relations of these results to lattice studies and on possible common prospects. We also briefly comment on the coexistence of quarkyonic and color superconductive phases.
06/2008;
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ABSTRACT: We study how the charge neutrality affects the phase structure of the three-flavor Polyakov-loop Nambu–Jona-Lasinio (PNJL) model. We point out that, within the conventional PNJL model at finite density, the color neutrality is missing because the Wilson line serves as an external colored field coupled to dynamical quarks. In this paper we heuristically assume that the model may still be applicable. To get color neutrality, one has then to allow nonvanishing color chemical potentials. We study how the quark matter phase diagram in (T,ms2/μ)-plane is affected by imposing neutrality and by including the Polyakov-loop dynamics. Although these two effects are correlated in a nonlinear way, the impact of the Polyakov loop turns out to be significant in the T direction, while imposing neutrality brings a remarkable effect in the ms2/μ direction. In particular, we find a novel unlocking transition, when the temperature is increased, even in the chiral SU(3) limit. We clarify how and why this is possible once the dynamics of the colored Polyakov loop is taken into account. Also we succeed in giving an analytic expression for Tc for the transition from two-flavor pairing (2SC) to unpaired quark matter in the presence of the Polyakov loop.
Phys. Rev. D. 04/2008; 77(7).
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ABSTRACT: We study how the charge neutrality affects the phase structure of three-flavor PNJL model. We point out that, within the conventional PNJL model at finite density the color neutrality is missing because the Wilson line serves as an external ``colored'' field coupled to dynamical quarks. In this paper we heuristically assume that the model may still be applicable. To get color neutrality one has then to allow non vanishing color chemical potentials. We study how the quark matter phase diagram in $(T,m_s^2/\mu)$-plane is affected by imposing neutrality and by including the Polyakov loop dynamics. Although these two effects are correlated in a nonlinear way, the impact of the Polyakov loop turns out to be significant in the $T$ direction, while imposing neutrality brings a remarkable effect in the $m_s^2/\mu$ direction. In particular, we find a novel unlocking transition, when the temperature is increased, even in the chiral SU(3) limit. We clarify how and why this is possible once the dynamics of the colored Polyakov loop is taken into account. Also we succeed in giving an analytic expression for $T_c$ for the transition from two-flavor pairing (2SC) to unpaired quark matter in the presence of the Polyakov loop.
03/2008;
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ABSTRACT: We study the phase diagram and the pion modes in the electrically neutral two flavor PNJL model. One of the main result of this paper is that when electrical neutrality is required, pions do not condense in the ground state of the model: the isospin chemical potential $\mu_I = -\mu_e/2$ is always smaller than the value required for pion condensation to occur. Moreover we investigate on the pions and $\sigma$ mass spectra. We find that the qualitative behavior of the masses resembles that obtained in the NJL model. We close this paper by studying the intriguing possibility that a bound state with the quantum numbers of the pions can be formed above the chiral phase transition.
02/2008;
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ABSTRACT: We study the phase diagram of the three flavor Polyakov-Nambu-Jona Lasinio (PNJL) model and in particular the interplay between chiral symmetry restoration and deconfinement crossover. We compute chiral condensates, quark densities and the Polyakov loop at several values of temperature and chemical potential. Moreover we investigate on the role of the Polyakov loop dynamics in the transition from nuclear matter to quark matter.
12/2007;
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ABSTRACT: AdS/QCD correspondence predicts the structure of quark-antiquark potential in the static limit. We use this piece of information together with the Salpeter equation (Schr\"odinger equation with relativistic kinematics) and a short range hyperfine splitting potential to determine quark masses and the quark potential parameters from the meson spectrum. The agreement between theory and experimental data is satisfactory provided one considers only mesons comprising at least one heavy quark. We use the same potential (in the one-gluon-exchange approximation) and these data to estimate constituent diquark masses. Using these results as an input we compute tetraquark masses using a diquark-antidiquark model. The masses of the states X(3872) or Y(3940) are predicted rather accurately. We also compute tetraquark masses with open charm and strangeness. Our result is that tetraquark candidates such as D_s(2317), D_s(2457) or X(2632) can hardly be interpreted as diquark-antidiquark states within the present approach.
12/2007;
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ABSTRACT: We study the bosonic excitations in the favorite cubic three flavor crystalline LOFF phases of QCD. We calculate in the Ginzburg-Landau approximation the masses of the eight pseudo Nambu-Goldstone Bosons (NGB) present in the low energy theory. We also compute the decay constants of the massless NGB Goldstones associated to superfluidity as well as those of the eight pseudo NGB. Differently from the corresponding situation in the Color-Flavor-Locking phase, we find that meson condensation phases are not expected in the present scenario.
07/2007;
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ABSTRACT: We study the phase diagram of a generalized Winfree model. The modification is such that the coupling depends on the fraction of synchronized oscillators, a situation which has been noted in some experiments on coupled Josephson junctions and mechanical systems. We let the global coupling k be a function of the Kuramoto order parameter r through an exponent z such that z=1 corresponds to the standard Winfree model, z<1 strengthens the coupling at low r (low amount of synchronization), and at z>1 , the coupling is weakened for low r . Using both analytical and numerical approaches, we find that z controls the size of the incoherent phase region and that one may make the incoherent behavior less typical by choosing z<1 . We also find that the original Winfree model is a rather special case; indeed, the partial locked behavior disappears for z>1 . At fixed k and varying gamma , the stability boundary of the locked phase corresponds to a transition that is continuous for z<1 and first order for z>1 . This change in the nature of the transition is in accordance with a previous study of a similarly modified Kuramoto model.
Physical Review E 05/2007; 75(5 Pt 1):051104. · 2.26 Impact Factor
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ABSTRACT: We present a self-consistent evaluation of the constituent quark masses in the three flavor Larkin-Ovchinnikov-Fulde-Ferrell (LOFF) phases of QCD, employing an improved Nambu-Jona Lasinio model. This result allows to determine the window for values of the baryonic chemical potential where the LOFF state is energetically favored. Comment: 17 pages, 5 figures, JHEP3 style. A second treatment of the ultraviolet cutoff added. Three figures adapted to the new cutoff. Two references added. Version accepted for publication on JHEP
01/2007;
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ABSTRACT: We present an analysis of two-body B decays with a pseudoscalar (P) and an axial-vector meson (A) in the final state using factorization. We employ as inputs a limited number of experimental data, i.e. results for the B→K1γ, and B→K*γ radiative decays and the branching ratios for B→πρ, πK*, Kρ, Kπ nonleptonic decays. In this way we are able to compare our predictions with recent data from the Belle and BABAR collaborations on B→a1π and make predictions on several other B→PA decay channels, which might be used as a guide for experimental researches and as tests of factorization.
Physical Review D 09/2006; 74(5):054035. · 4.56 Impact Factor
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ABSTRACT: We discuss specific heat and neutrino emissivity due to direct Urca processes for quark matter in the color superconductive Larkin-Ovchinnikov-Fulde-Ferrell (LOFF) phase of Quantum-Chromodynamics. We assume that the three light quarks $u, d, s$ are in a color and electrically neutral state and interact by a four fermion Nambu-Jona Lasinio coupling. We study a LOFF state characterized by a single plane wave for each pairing. From the evaluation of neutrino emissivity and fermionic specific heat, the cooling rate of simplified models of compact stars with a quark core in the LOFF state is estimated. Comment: 16 pages, 5 figures, revtex4 style. Version accepted for publication in Phys. Rev. D
07/2006;
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ABSTRACT: We present an analysis of two-body B decays with a pseudoscalar (P) and an axial-vector meson (A) in the final state using naive factorization. We employ as inputs a limited number of experimental data, i.e. results for the B -> K1 gamma and B -> K* gamma radiative decays and the branching ratios for B -> pion+rho, pion+K*, K+rho, K+pion non leptonic decays. In this way we are able to make predictions on several B -> P A decays that might be used as a guide for experimental researches of these decays and as tests of factorization. Comment: Some entries in Tables V and VI modified due to new data for B->K rho decays. One reference and a comment on K1b weak coupling added. Two predictions for the channels with a kaon and a positively charged axial vector presented
Physical Review D 02/2006; 74(5):054035. · 4.56 Impact Factor
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Physics Letters B 02/2006; 634:565-566. · 3.95 Impact Factor
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Physics Letters B 05/2005; 615:297-297. · 3.95 Impact Factor
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ABSTRACT: The large branching ratio for B-> K1 gamma recently measured at Belle implies a large B -> K1 transition form factor and large branching ratios for non leptonic B decays involving an axial-vector meson. In this paper we present an analysis of two-body B decays with an axial-vector meson in the final state using naive factorization and the B -> K1 form factors obtained from the measured radiative decays. We find that the predicted B -> J/psi K1 branching ratio is in agreement with experiment. We also suggest that the decay rates of B -> K1 pi, B -> a1 K and B -> b1 K could be used to test the factorization ansatz. Comment: 8 pages; 7 new references included and a comment on K2(1430) in the final state added
05/2005;
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ABSTRACT: We present an approximate gap equation for different crystalline structures of the Larkin-Ovchinnikov-Fulde-Ferrel phase of high density QCD at T=0. This equation is derived by using an effective condensate term obtained by averaging the inhomogeneous condensate over distances of the order of the crystal lattice size. The approximation is expected to work better far off any second-order phase transition. As a function of the difference of the chemical potentials of the up and down quarks, δμ, we get that the octahedron is energetically favored from δμ=Δ0/√2 to 0.95Δ0, where Δ0 is the gap for the homogeneous phase, while in the range 0.95Δ0–1.32Δ0 the face-centered cube prevails. At δμ=1.32Δ0 a first-order phase transition to the normal phase occurs.
Phys. Rev. D. 09/2004; 70(5).
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L Angelini,
M de Tommaso,
M Guido,
K Hu,
P Ch Ivanov,
D Marinazzo, G Nardulli,
L Nitti,
M Pellicoro,
C Pierro,
S Stramaglia
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ABSTRACT: We investigate phase synchronization in EEG recordings from migraine patients. We use the analytic signal technique, based on the Hilbert transform, and find that migraine brains are characterized by enhanced alpha band phase synchronization in the presence of visual stimuli. Our findings show that migraine patients have an overactive regulatory mechanism that renders them more sensitive to external stimuli.
Physical Review Letters 08/2004; 93(3):038103. · 7.37 Impact Factor
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ABSTRACT: We study the phase-synchronization properties of systolic and diastolic arterial pressure in healthy subjects. We find that delays in the oscillatory components of the time series depend on the frequency bands that are considered, in particular we find a change of sign in the phase shift going from the very low frequency band to the high frequency band. This behavior should reflect a collective behavior of a system of nonlinear interacting elementary oscillators. We prove that some models describing such systems, e.g., the Winfree and the Kuramoto models, offer a clue to this phenomenon. For these theoretical models there is a linear relationship between phase shifts and the difference of natural frequencies of oscillators and a change of sign in the phase shift naturally emerges.
Physical Review E 07/2004; 69(6 Pt 1):061923. · 2.26 Impact Factor
