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Fred Cooper

Publications and source records attributed to Fred Cooper.

At least 109 records · Page 6Linked to original sources
arXiv1999

A Two-dimensional Model with Chiral Condensates and Cooper Pairs having QCD-like Phase Structure

We generalize our previous model to an O(N) symmetric two-dimensional model which possesses chiral symmetry breaking and superconducting (Cooper pair condensates) phases at large-N. At zero temperature and density, the model can be solved analytically in the large-N limit. We perform the renormalization explicitly and obtain a closed form expression of the effective potential. There exists a renormalization group invariant parameter $δ$ that determines which of the condensates exist in the vacuum. At finite temperatures and densities, we map out the phase structure of the model by a detailed numerical analysis of the renormalized effective potential. For $δ$ positive and sufficiently large, the phase diagram in the $μ$-$T$ (chemical potential-temperature) plane exactly mimics the features expected for QCD with two light flavors of quarks. At low temperatures there exists low-$μ$ chiral symmetry breaking and high-$μ$ Cooper pair condensate regions which are separated by a first-order phase transition. At high $μ$, when the temperature is raised, the system undergoes a second-order phase transition from the superconducting phase to an unbroken phase in which both condensates vanish. For a range of values of $δ$ the theory possesses a tricritical point ($μ_{tc}$ and $T_{tc}$); for $μ> μ_{tc}$ ($μ< μ_{tc}$) the phase transition from the low temperature chiral symmetry breaking phase to unbroken phase is first-order (second-order). For the range of $δ$ in which the system mimics QCD, we expect the model to be useful for the investigation of dynamical aspects of nonequilibrium phase transitions, and to provide information relevant to the study of relativistic heavy ion collisions and the dense interiors of neutron stars.

hep-ph↗
arXiv1999

Variational Ansatz for PT-Symmetric Quantum Mechanics

A variational calculation of the energy levels of a class of PT-invariant quantum mechanical models described by the non-Hermitian Hamiltonian H= p^2 - (ix)^N with N positive and x complex is presented. Excellent agreement is obtained for the ground state and low lying excited state energy levels and wave functions. We use an energy functional with a three parameter class of PT-symmetric trial wave functions in obtaining our results.

quant-ph↗
arXiv1999

Competing Condensates in Two Dimensions

We generalize our previous 2-dimensional model in which a pairing condensate psi-psi was generated at large N. In the present case, we allow for both psi-psi and a chiral condensate psibar-psi to exist. We construct the effective potential to leading order in 1/N, and derive the gap equations at finite density and temperature. We study the zero density and temperature situation analytically. We perform the renormalization explicitly and we show that the physics is controlled by a parameter related to the relative strengths of the interactions in the pairing and chiral channels. We show that although a solution to the gap equations exists in which both condensates are non-vanishing, the global minimum of the effective potential always occurs for the case when one or the other condensate vanishes.

hep-ph↗
arXiv1998

Cooper pairing at large N in a 2-dimensional model

We study a 2-dimensional model of Fermi fields that is closely related to the Gross-Neveu model, and show that to leading order in 1/N a condensate forms. This effect is independent of the chemical potential, a peculiarity that we expect to be specific to 2 dimensions. We also expect the condensate to be unstable against corrections at higher orders in 1/N. We compute the Green's functions associated with the composite psi-psi, and show that the Fermion acquires a Majorana mass proportional to the gap, and that a massless Goldstone pole appears.

hep-ph↗
arXiv1998

Inclusive Dilepton Production at RHIC

jWe review recent research on Initial Value Problems in Quantum Field Theory. We pay special attention to the chiral phase transition in the linear sigma model. We discuss conditions for the development of Disoriented Chiral Condensates and show how their presence affects both the pion inclusive distribution and the dilepton distribution. We discuss the use of Schwinger's closed time path formalism for determining inclusive distribution functions.

hep-ph↗
arXiv1998

The quantum roll in d-dimensions and the large-d expansion

We investigate the quantum roll for a particle in a $d$-dimensional ``Mexican hat'' potential in quantum mechanics, comparing numerical simulations in $d$-dimensions with the results of a large-$d$ expansion, up to order $1/d$, of the coupled closed time path (CTP) Green's function equations, as well as to a post-Gaussian variational approximation in $d$-dimensions. The quantum roll problem for a set of $N$ coupled oscillators is equivalent to a $(d=N)$-dimensional spherically symmetric quantum mechanics problem. For this problem the large-N expansion is equivalent to an expansion in $1/d$ where $d$ is the number of dimensions. We use the Schwinger-Mahanthappa-Keldysh CTP formalism to determine the causal update equations to order $1/d$. We also study the quantum fluctuations $ $ as a function of time and find that the $1/d$ corrections improve the agreement with numerical simulations at short times (over one or two oscillations) but beyond two oscillations, the approximation fails to correspond to a positive probability function. Using numerical methods, we also study how the long time behavior of the motion changes from its asymptotic ($d \to \infty$) harmonic behavior as we reduce $d$.

hep-ph↗
arXiv1997

Order 1/N corrections to the time-dependent Hartree approximation for a system of N+1 oscillators

We solve numerically to order 1/N the time evolution of a quantum dynamical system of N oscillators of mass m coupled quadratically to a massless dynamic variable. We use Schwinger's closed time path (CTP) formalism to derive the equations. We compare two methods which differ by terms of order 1/N^2. The first method is a direct perturbation theory in 1/N using the path integral. The second solves exactly the theory defined by the effective action to order 1/N. We compare the results of both methods as a function of N. At N=1, where we expect the expansion to be quite innacurate, we compare our results to an exact numerical solution of the Schroedinger equation. In this case we find that when the two methods disagree they also diverge from the exact answer. We also find at N=1 that the 1/N corrected evolutions track the exact answer for the expectation values much longer than the mean field (N= \infty) result.

hep-ph↗
arXiv1997

Compacton Solutions in a Class of Generalized Fifth Order Korteweg-de Vries Equations

We study a class of generalized fifth order Korteweg-de Vries (KdV) equations which are derivable from a Lagrangian L(p,m,n,l) which has variable powers of the first and second derivatives of the field with powers given by the parameters p,m,n,l. The resulting field equation has solitary wave solutions of both the usual (non-compact) and compact variety ("compactons"). For the particular case that p=m=n+l, the solitary wave solutions have compact support and the feature that their width is independent of the amplitude. We discuss the Hamiltonian structure of these theories and find that mass, momentum, and energy are conserved. We find in general that these are not completely integrable systems. Numerical simulations show that an arbitrary compact initial wave packet whose width is wider than that of a compacton breaks up into several compactons all having the same width. The scattering of two compactons is almost elastic, with the left over wake eventually turning into compacton-anticompacton pairs. When there are two different compacton solutions for a single set of parameters the wider solution is stable, and this solution is a minimum of the Hamiltonian.

patt-sol↗
arXiv1996

Chaos in Time Dependent Variational Approximations to Quantum Dynamics

Dynamical chaos has recently been shown to exist in the Gaussian approximation in quantum mechanics and in the self-consistent mean field approach to studying the dynamics of quantum fields. In this study, we first show that any variational approximation to the dynamics of a quantum system based on the Dirac action principle leads to a classical Hamiltonian dynamics for the variational parameters. Since this Hamiltonian is generically nonlinear and nonintegrable, the dynamics thus generated can be chaotic, in distinction to the exact quantum evolution. We then restrict attention to a system of two biquadratically coupled quantum oscillators and study two variational schemes, the leading order large N (four canonical variables) and Hartree (six canonical variables) approximations. The chaos seen in the approximate dynamics is an artifact of the approximations: this is demonstrated by the fact that its onset occurs on the same characteristic time scale as the breakdown of the approximations when compared to numerical solutions of the time-dependent Schrodinger equation.

quant-ph↗
arXiv1996

Nonequilibrium Dynamics of Symmetry Breaking in Lambda Phi^4 Field Theory

The time evolution of O(N) symmetric lambda Phi^4 scalar field theory is studied in the large N limit. In this limit the mean field and two-point correlation function evolve together as a self-consistent closed Hamiltonian system, characterized by a Gaussian density matrix. The static part of the effective Hamiltonian defines the True Effective Potential U_eff for configurations far from thermal equilibrium. Numerically solving the time evolution equations for energy densities corresponding to a quench in the unstable spinodal region, we find results quite different from what might be inferred from the equilibrium free energy ``effective'' potential F. Typical time evolutions show effectively irreversible energy flow from the coherent mean fields to the quantum fluctuating modes, due to the creation of massless Goldstone bosons near threshold. The plasma frequency and collisionless damping rate of the mean fields are calculated in terms of the particle number density by a linear response analysis and compared with the numerical results. Dephasing of the fluctuations leads also to the growth of an effective entropy and the transition from quantum to classical behavior of the ensemble. In addition to casting some light on fundamental issues of nonequilibrium quantum statistical mechanics, the general framework presented in this work may be applied to a study of the dynamics of second order phase transitions in a wide variety of Landau-Ginsburg systems described by a scalar order parameter.

hep-ph↗
arXiv1996

Time evolution of the chiral phase transition during a spherical expansion

We examine the non-equilibrium time evolution of the hadronic plasma produced in a relativistic heavy ion collision, assuming a spherical expansion into the vacuum. We study the $O(4)$ linear sigma model to leading order in a large-$N$ expansion. Starting at a temperature above the phase transition, the system expands and cools, finally settling into the broken symmetry vacuum state. We consider the proper time evolution of the effective pion mass, the order parameter $\langle σ\rangle$, and the particle number distribution. We examine several different initial conditions and look for instabilities (exponentially growing long wavelength modes) which can lead to the formation of disoriented chiral condensates (DCCs). We find that instabilities exist for proper times which are less than 3 fm/c. We also show that an experimental signature of domain growth is an increase in the low momentum spectrum of outgoing pions when compared to an expansion in thermal equilibrium. In comparison to particle production during a longitudinal expansion, we find that in a spherical expansion the system reaches the ``out'' regime much faster and more particles get produced. However the size of the unstable region, which is related to the domain size of DCCs, is not enhanced.

hep-th↗
arXiv1995

SPHERICALLY SYMMETRIC RANDOM WALKS I. REPRESENTATION IN TERMS OF ORTHOGONAL POLYNOMIALS

Spherically symmetric random walks in arbitrary dimension $D$ can be described in terms of Gegenbauer (ultraspherical) polynomials. For example, Legendre polynomials can be used to represent the special case of two-dimensional spherically symmetric random walks. In general, there is a connection between orthogonal polynomials and semibounded one-dimensional random walks; such a random walk can be viewed as taking place on the set of integers $n$, $n=0,~1,~2,~\ldots$, that index the polynomials. This connection allows one to express random-walk probabilities as weighted inner products of the polynomials. The correspondence between polynomials and random walks is exploited here to construct and analyze spherically symmetric random walks in $D$-dimensional space, where $D$ is {\sl not} restricted to be an integer. The weighted inner-product representation is used to calculate exact closed-form spatial and temporal moments of the probability distribution associated with the random walk. The polynomial representation of spherically symmetric random walks is also used to calculate the two-point Green's function for a rotationally symmetric free scalar quantum field theory.

hep-lat↗
arXiv1994

Semiquantum Chaos and the Large N Expansion

We consider the dynamical system consisting of a quantum degree of freedom $A$ interacting with $N$ quantum oscillators described by the Lagrangian \bq L = {1\over 2}\dot{A}^2 + \sum_{i=1}^{N} \left\{{1\over 2}\dot{x}_i^2 - {1\over 2}( m^2 + e^2 A^2)x_i^2 \right\}. \eq In the limit $N \rightarrow \infty$, with $e^2 N$ fixed, the quantum fluctuations in $A$ are of order $1/N$. In this limit, the $x$ oscillators behave as harmonic oscillators with a time dependent mass determined by the solution of a semiclassical equation for the expectation value $\VEV{A(t)}$. This system can be described, when $\VEV{x(t)}= 0$, by a classical Hamiltonian for the variables $G(t) = \VEV{x^2(t)}$, $\dot{G}(t)$, $A_c(t) = \VEV{A(t)}$, and $\dot{A_c}(t)$. The dynamics of this latter system turns out to be chaotic. We propose to study the nature of this large-$N$ limit by considering both the exact quantum system as well as by studying an expansion in powers of $1/N$ for the equations of motion using the closed time path formalism of quantum dynamics.

chao-dyn↗
arXiv1994

Non-Equilibrium Quantum Fields in the Large N Expansion

An effective action technique for the time evolution of a closed system consisting of one or more mean fields interacting with their quantum fluctuations is presented. By marrying large $N$ expansion methods to the Schwinger-Keldysh closed time path (CTP) formulation of the quantum effective action, causality of the resulting equations of motion is ensured and a systematic, energy conserving and gauge invariant expansion about the quasi-classical mean field(s) in powers of $1/N$ developed. The general method is exposed in two specific examples, $O(N)$ symmetric scalar $ł\F^4$ theory and Quantum Electrodynamics (QED) with $N$ fermion fields. The $ł\F^4$ case is well suited to the numerical study of the real time dynamics of phase transitions characterized by a scalar order parameter. In QED the technique may be used to study the quantum non-equilibrium effects of pair creation in strong electric fields and the scattering and transport processes in a relativistic $e^+e^-$ plasma. A simple renormalization scheme that makes practical the numerical solution of the equations of motion of these and other field theories is described.

hep-ph↗
arXiv1994

Supersymmetry and Quantum Mechanics

In the past ten years, the ideas of supersymmetry have been profitably applied to many nonrelativistic quantum mechanical problems. In particular, there is now a much deeper understanding of why certain potentials are analytically solvable and an array of powerful new approximation methods for handling potentials which are not exactly solvable. In this report, we review the theoretical formulation of supersymmetric quantum mechanics and discuss many applications. Exactly solvable potentials can be understood in terms of a few basic ideas which include supersymmetric partner potentials, shape invariance and operator transformations. Familiar solvable potentials all have the property of shape invariance. We describe new exactly solvable shape invariant potentials which include the recently discovered self-similar potentials as a special case. The connection between inverse scattering, isospectral potentials and supersymmetric quantum mechanics is discussed and multi-soliton solutions of the KdV equation are constructed. Approximation methods are also discussed within the framework of supersymmetric quantum mechanics and in particular it is shown that a supersymmetry inspired WKB approximation is exact for a class of shape invariant potentials. Supersymmetry ideas give particularly nice results for the tunneling rate in a double well potential and for improving large $N$ expansions. We also discuss the problem of a charged Dirac particle in an external magnetic field and other potentials in terms of supersymmetric quantum mechanics. Finally, we discuss structures more general than supersymmetric quantum mechanics such as parasupersymmetric quantum mechanics in which there is a symmetry between a boson and a para-fermion of order $p$.

hep-th↗