Search results for "Field theory"
showing 10 items of 1188 documents
Computation of Amplitudes in the Discretized Approach to String Field Theory
1988
An approach to Witten string field theory based on the discretization of the world sheet is adopted. We use it to calculate tree amplitudes with the formulation of the theory based on string functionals. The results are evaluated numerically and turn out to be very accurate, giving, for a string approximated by 600 points, values within 0.02% of the prediction of the dual model. The method opens a way to calculate amplitudes in string field theory using nonflat backgrounds as well as compactified dimensions.
Unequal rapidity correlators in the dilute limit of JIMWLK
2019
We study unequal rapidity correlators in the stochastic Langevin picture of Jalilian-Marian-Iancu-McLerran-Weigert-Leonidov-Kovner (JIMWLK) evolution in the Color Glass Condensate effective field theory. We discuss a diagrammatic interpretation of the long-range correlators. By separately evolving the Wilson lines in the direct and complex conjugate amplitudes, we use the formalism to study two-particle production at large rapidity separations. We show that the evolution between the rapidities of the two produced particles can be expressed as a linear equation, even in the full nonlinear limit. We also show how the Langevin formalism for two-particle correlations reduces to a BFKL picture i…
Many-body Landau-Zener effect at fast sweep
2005
The asymptotic staying probability P in the Landau-Zener effect with interaction is analytically investigated at fast sweep, epsilon = pi Delta^2/(2 hbar v) << 1. We have rigorously calculated the value of I_0 in the expansion P =~ 1 - epsilon + epsilon^2/2 + epsilon^2 I_0 for arbitrary couplings and relative resonance shifts of individual tunneling particles. The results essentially differ from those of the mean-field approximation. It is shown that strong long-range interactions such as dipole-dipole interaction (DDI) generate huge values of I_0 because flip of one particle strongly influences many others. However, in the presence of strong static disorder making resonance for indiv…
Measuring the magnetic dipole transition of single nanorods by spectroscopy and Fourier microscopy
2020
International audience; Rare-earth doped nanocrystals possess optical transitions with significant either electric or magnetic dipole characters. They are of considerable interest for understanding and engineering light-matter interactions at the nanoscale with numerous applications in nanophotonics. Here, we study the 5 D 0 → 7 F 1 transition dipole vector in individual NaYF 4 : Eu 3+ nanorod crystals by Fourier and confocal micro-scopies. A single-crystal host matrix leads to narrow emission lines at room temperature that permit separation of the Stark sublevels resulting from the crystal-field splitting. We observe a fully magnetic transition and low variability of the transition dipole …
Nonlocal density correlations as a signature of Hawking radiation from acoustic black holes
2008
We have used the analogy between gravitational systems and nonhomogeneous fluid flows to calculate the density-density correlation function of an atomic Bose-Einstein condensate in the presence of an acoustic black hole. The emission of correlated pairs of phonons by Hawking-like process results into a peculiar long-range density correlation. Quantitative estimations of the effect are provided for realistic experimental configurations.
A model of CPT violation for neutrinos
2002
Any local relativistic quantum field theory of Dirac-Weyl fermions conserves CPT. Here we examine whether a simple nonlocal field theory can violate CPT. We construct a new relativistic field theory of fermions, which we call ``homeotic'', which is nonlocal but causal and Lorentz invariant. The free homeotic theory is in fact equivalent to free Dirac theory. We show that a homeotic theory with a suitable nonlocal four-fermion interaction is causal and as a result has a well-defined perturbative S-matrix. By coupling a right-handed homeotic fermion to a left-handed Dirac-Weyl fermion, we obtain a causal theory of CPT-violating neutrino oscillations.
Metallic and Insulating Phases of Repulsively Interacting Fermions in a 3D Optical Lattice
2008
The fermionic Hubbard model plays a fundamental role in the description of strongly correlated materials. Here we report on the realization of this Hamiltonian using a repulsively interacting spin mixture of ultracold $^{40}$K atoms in a 3D optical lattice. We have implemented a new method to directly measure the compressibility of the quantum gas in the trap using in-situ imaging and independent control of external confinement and lattice depth. Together with a comparison to ab-initio Dynamical Mean Field Theory calculations, we show how the system evolves for increasing confinement from a compressible dilute metal over a strongly-interacting Fermi liquid into a band insulating state. For …
Complex-mass renormalization in chiral effective field theory
2009
We consider a low-energy effective field theory of vector mesons and Goldstone bosons using the complex-mass renormalization. As an application we calculate the mass and the width of the $\rho$ meson.
Low-energy excitations from interacting tunneling units in the mean-field approximation
1991
Abstract The low-energy excitation spectrum of dilute concentrations of interacting tunneling quadrupoles randomly distributed in a non-polar medium was studied in the mean-field approximation. In particular the case of six-orientational tunneling quadrupoles (TQs) with a r−3 (elastic) interaction was considered. Because of the random position of the TQs, the internal field in a random variable and for relatively low concentrations has a Lorenzian probability distribution. The low-energy density of states is a constant and the low-energy excitations arise from the large internal fields, i.e. strongly interacting tunneling quadrupoles. The low-energy excitations were compared with those obta…
Magnetism of metallacrown single-molecule magnets: From a simplest model to realistic systems
2018
Electronic and magnetic properties of molecular nanomagnets are determined by competing energy scales due to the crystal field splitting, the exchange interactions between transition metal atoms, and relativistic effects. We present a comprehensive theory embracing all these phenomena based on first-principles calculations. In order to achieve this goal, we start from the ${\mathrm{FeNi}}_{4}$ cluster as a paradigm. The system can be accurately described on the ab initio level yielding all expected electronic states in a range of multiplicities from 1 to 9, with a ferromagnetic ground state. By adding the spin-orbit coupling between them we obtain the zero-field splitting. This allows to in…