1,221 research outputs found
Analytic Methods in Nonperturbative QCD
Recently developed analytic methods in the framework of the Field Correlator
Method are reviewed in this series of four lectures and results of calculations
are compared to lattice data and experiment. Recent lattice data demonstrating
the Casimir scaling of static quark interaction strongly support the FCM and
leave very little space for all other theoretical models, e.g. instanton
gas/liquid model. Results of calculations for mesons, baryons, quark-gluon
plasma and phase transition temperature demonstrate that new analytic methods
are a powerful tool of nonperturbative QCD along with lattice simulations.Comment: LaTeX, 34 pages; Lectures given at the 13th Indian-Summer School
"Understanding the Structure of Hadrons", August 28 - September 1, 2000,
Prague, Czech Republi
Bethe--Salpeter equation in QCD
We extend to regular QCD the derivation of a confining
Bethe--Salpeter equation previously given for the simplest model of scalar QCD
in which quarks are treated as spinless particles. We start from the same
assumptions on the Wilson loop integral already adopted in the derivation of a
semirelativistic heavy quark potential. We show that, by standard
approximations, an effective meson squared mass operator can be obtained from
our BS kernel and that, from this, by expansion the
corresponding Wilson loop potential can be reobtained, spin--dependent and
velocity--dependent terms included. We also show that, on the contrary,
neglecting spin--dependent terms, relativistic flux tube model is reproduced.Comment: 23 pages, revte
Magnetic string contribution to hadron dynamics in QCD
Dynamics of a light quark in the field of static source (heavy-light meson)
is studied using the nonlinear Dirac equation, derived recently. Special
attention is paid to the contribution of magnetic correlators and it is found
that it yields a significant increase of string tension at intermediate
distances. The spectrum of heavy-light mesons is computed with account of this
contribution and compared to experimental and lattice data.Comment: 10 pages Revte
Parton Saturation-An Overview
The idea of partons and the utility of using light-cone gauge in QCD are
introduced. Saturation of quark and gluon distributions are discussed using
simple models and in a more general context. The Golec-Biernat W\usthoff model
and some simple phenomenology are described. A simple, but realistic, equation
for unitary, the Kovchegov equation, is discussed, and an elementary derivation
of the JIMWLK equation is given.Comment: Cargese Lectures, 34 pages, 19 figure
The three-dimensional randomly dilute Ising model: Monte Carlo results
We perform a high-statistics simulation of the three-dimensional randomly
dilute Ising model on cubic lattices with . We choose a
particular value of the density, x=0.8, for which the leading scaling
corrections are suppressed. We determine the critical exponents, obtaining , , , and ,
in agreement with previous numerical simulations. We also estimate numerically
the fixed-point values of the four-point zero-momentum couplings that are used
in field-theoretical fixed-dimension studies. Although these results somewhat
differ from those obtained using perturbative field theory, the
field-theoretical estimates of the critical exponents do not change
significantly if the Monte Carlo result for the fixed point is used. Finally,
we determine the six-point zero-momentum couplings, relevant for the
small-magnetization expansion of the equation of state, and the invariant
amplitude ratio that expresses the universality of the free-energy
density per correlation volume. We find .Comment: 34 pages, 7 figs, few correction
Chiral Lagrangian with confinement from the QCD Lagrangian
An effective Lagrangian for the light quark in the field of a static source
is derived systematically using the exact field correlator expansion. The
lowest Gaussian term is bosonized using nonlocal colorless bosonic fields and a
general structure of effective chiral Lagrangian is obtained containing all set
of fields. The new and crucial result is that the condensation of scalar
isoscalar field which is a usual onset of chiral symmetry breaking and is
constant in space-time, assumes here the form of the confining string and
contributes to the confining potential, while the rest bosonic fields describe
mesons with the q\bar q quark structure and pseudoscalars play the role of
Nambu-Goldstone fields. Using derivative expansion the effective chiral
Lagrangian is deduced containing both confinement and chiral effects for
heavy-light mesons. The pseudovector quark coupling constant is computed to be
exactly unity in the local limit,in agreement with earlier large N_c arguments.Comment: LaTeX2e, 17 page
Magnetic field strength and orientation effects on co-fe discontinuous multilayers close to percolation
International audienceMagnetization and magnetoresistance in function of the magnitude and orientation of applied magnetic field were studied in Co-Fe discontinuous multilayers close to their structural percolation. The high pulsed magnetic fields up to 33 T were used in the 120â310 K temperature range. Comparison between longitudinal and transverse with respect to the film plane field configurations was made in the low-field and high-field regimes in order to clarify the nature of the measured negative magnetoresistance. Coexistence of two distinct magnetic fractions, superparamagnetic SPM, consisting of small spherical Co-Fe granules and superferromagnetic SFM, by bigger Co-Fe clusters, was established in this system. These fractions were shown to have different relevance for the system magnetization and magnetotransport. While the magnetization is almost completely up to 97% defined by the SFM contribution and practically independent of temperature in this range, the magnetoresistance experiences a crossover from a regime dominated by Langevin correlations suppressed with temperature between neighbor SPM and SFM moments at low fields, to that dominated by spin scattering enhanced with temperature of charge carriers within SFM clusters at high fields. Also, the demagnetizing effects, sensitive to the field orientation, were found to essentially define the low-field behavior and characteristic crossover field
Nonlinear atom optics and bright gap soliton generation in finite optical lattices
We theoretically investigate the transmission dynamics of coherent matter
wave pulses across finite optical lattices in both the linear and the nonlinear
regimes. The shape and the intensity of the transmitted pulse are found to
strongly depend on the parameters of the incident pulse, in particular its
velocity and density: a clear physical picture for the main features observed
in the numerical simulations is given in terms of the atomic band dispersion in
the periodic potential of the optical lattice. Signatures of nonlinear effects
due the atom-atom interaction are discussed in detail, such as atom optical
limiting and atom optical bistability. For positive scattering lengths, matter
waves propagating close to the top of the valence band are shown to be subject
to modulational instability. A new scheme for the experimental generation of
narrow bright gap solitons from a wide Bose-Einstein condensate is proposed:
the modulational instability is seeded in a controlled way starting from the
strongly modulated density profile of a standing matter wave and the solitonic
nature of the generated pulses is checked from their shape and their
collisional properties
Quantitative Treatment of Decoherence
We outline different approaches to define and quantify decoherence. We argue
that a measure based on a properly defined norm of deviation of the density
matrix is appropriate for quantifying decoherence in quantum registers. For a
semiconductor double quantum dot qubit, evaluation of this measure is reviewed.
For a general class of decoherence processes, including those occurring in
semiconductor qubits, we argue that this measure is additive: It scales
linearly with the number of qubits.Comment: Revised version, 26 pages, in LaTeX, 3 EPS figure
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