6 research outputs found
Quasi-relativistic behavior of cold atoms in light fields
We study the influence of three laser beams on the center of mass motion of
cold atoms with internal energy levels in a tripod configuration. We show that
similar to electrons in graphene the atomic motion can be equivalent to the
dynamics of ultra-relativistic two-component Dirac fermions. We propose and
analyze an experimental setup for observing such a quasi-relativistic motion of
ultracold atoms. We demonstrate that the atoms can experience negative
refraction and focussing by Veselago-type lenses. We also show how the chiral
nature of the atomic motion manifests itself as an oscillation of the atomic
internal state population which depends strongly on the direction of the center
of mass motion. For certain directions an atom remains in its initial state,
whereas for other directions the populations undergo oscillations between a
pair of internal states.Comment: 4 pages, updated version, Phys. Rev. A 77, (R)011802 (2008
Formalism for dilepton production via virtual photon bremsstrahlung in hadronic reactions
We derive a set of new formulas for various distributions in dilepton
production via virtual photon bremsstrahlung from pseudoscalar mesons and
unpolarized spin-one-half fermions. These formulas correspond to the leading
and sub-leading terms in the Low-Burnett-Kroll expansion for real photon
bremsstrahlung. The relation of our leading-term formulas to previous works is
also shown. Existing formulas are examined in the light of Lorentz covariance
and gauge invariance. Numerical comparison is made in a simple example, where
an "exact" formula and real photon data exist. The results reveal large
discrepancies among different bremsstrahlung formulas. Of all the leading-term
bremsstrahlung formulas, the one derived in this work agrees best with the
exact formula. The issues of M_T-scaling and event generators are also
addressed.Comment: 37 pages, RevTeX, epsf.sty, 10 embedded figure
Charged current cross section for massive cosmological neutrinos impinging on radioactive nuclei
We discuss the cross section formula both for massless and massive neutrinos
on stable and radioactive nuclei. The latter could be of interest for the
detection of cosmological neutrinos whose observation is one of the main
challenges of modern cosmology. We analyze the signal to background ratio as a
function of the ratio of the neutrino mass over the detector resolution and
show that an energy resolution less than 0.5 eV would be required for sub-eV
neutrino masses, independently of the gravitational neutrino clustering.
Finally we mention the non-resonant character of neutrino capture on
radioactive nuclei.Comment: 5 pages, 2 figure
Effective permittivity of mixtures of anisotropic particles
We use a new approach to derive dielectric mixing rules for macroscopically
homogeneous and isotropic multicomponent mixtures of anisotropic inhomogeneous
dielectric particles. Two factors of anisotropy are taken into account, the
shape of the particles and anisotropy of the dielectric parameters of the
particles' substances. Our approach is based upon the notion of macroscopic
compact groups of particles and the procedure of averaging of the fields over
volumes much greater than the typical scales of these groups. It enables us to
effectively sum up the contributions from multiple interparticle reemission and
short-range correlation effects, represented by all terms in the infinite
iterative series for the electric field strength and induction. The expression
for the effective permittivity can be given the form of the Lorentz-Lorenz
type, which allows us to determine the effective polarizabilities of the
particles in the mixture. These polarizabilities are found as integrals over
the regions occupied by the particles and taken of explicit functions of the
principal components of the permittivity tensors of the particles' substances
and the permittivity of the host medium. The case of a mixture of particles of
the ellipsoidal shape is considered in detail to exemplify the use of general
formulas. As another example, Bruggeman-type formulas are derived under
pertinent model assumptions. The ranges of validity of the results obtained are
discussed as well.Comment: 9 pages, 4 figure