3,162 research outputs found
Kinetic decoupling of neutralino dark matter
After neutralinos cease annihilating in the early Universe, they may still
scatter elastically from other particles in the primordial plasma. At some
point in time, however, they will eventually stop scattering. We calculate the
cross sections for neutralino elastic scattering from standard-model particles
to determine the time at which this kinetic decoupling occurs. We show that
kinetic decoupling occurs above a temperature MeV. Thereafter,
neutralinos act as collisionless cold dark matter.Comment: Replaced with revised version, new references adde
QED radiative corrections to the decay pi^0 to e^+e^-
We reconsider QED radiative corrections (RC) to the
decay width. One kind of RC investigated earlier has a renormalization group
origin and can be associated with the final state interaction of electron and
positron. It determines the distribution of lepton pair invariant masses in the
whole kinematic region. The other type of RC has a double-logarithmic character
and is related to almost on-mass-shell behavior of the lepton form factors. The
total effect of RC for the decay is estimated to be
3.2% and for the decay is 4.3%.Comment: 12 pages, 3 figure
Microscopic calculation of 6Li elastic and transition form factors
Variational Monte Carlo wave functions, obtained from a realistic Hamiltonian
consisting of the Argonne v18 two-nucleon and Urbana-IX three-nucleon
interactions, are used to calculate the 6Li ground-state longitudinal and
transverse form factors as well as transition form factors to the first four
excited states. The charge and current operators include one- and two-body
components, leading terms of which are constructed consistently with the
two-nucleon interaction. The calculated form factors and radiative widths are
in good agreement with available experimental data.Comment: 9 pages, 2 figures, REVTeX, submitted to Physical Review Letters,
with updated introduction and reference
Magnetic anisotropy of individual maghemite mesocrystals
Interest in creating magnetic metamaterials has led to methods for growing superstructures of magnetic nanoparticles. Mesoscopic crystals of maghemite (gamma-Fe2O3) nanoparticles can be arranged into highly ordered body-centered tetragonal lattices of up to a few micrometers. Although measurements on disordered ensembles have been carried out, determining the magnetic properties of individual mesoscopic crystals is challenging due to their small total magnetic moment. Here, we overcome these challenges by utilizing sensitive dynamic cantilever magnetometry to study individual micrometer-sized gamma-Fe2O3 mesocrystals. These measurements reveal an unambiguous cubic anisotropy, resulting from the crystalline anisotropy of the constituent maghemite nanoparticles and their alignment within the mesoscopic lattice. The signatures of anisotropy and its origins come to light because we combine the self-assembly of highly ordered mesocrystals with the ability to resolve their individual magnetism. This combination is promising for future studies of the magnetic anisotropy of other nanoparticles, which are too small to investigate individually
On mass corrections to the decays P \to l^+l^-
We use the Mellin-Barnes representation in order to improve the theoretical
estimate of mass corrections to the width of light pseudoscalar meson decay
into a lepton pair, . The full resummation of the terms
and to the
decay amplitude is performed, where is the lepton mass and
is the characteristic scale of the
form factor. The total effect of mass corrections for
the channel is negligible and for the channel its order
is of a few per cent.Comment: 10 pages, 1 figure; one figure is adde
Rare decay \pi^0 \to e^+e^- as a Test of Standard Model
Experimental and theoretical progress concerning the rare decay \pi^0 \to
e^+e^- is briefly reviewed. It includes the latest data from KTeV and a new
model independent estimate of the decay branching which show the deviation
between experiment and theory at the level of . The predictions for
\eta and \eta' decays into lepton pair are presented. We also comment on the
impact on the pion rare decay estimate of the BABAR collaboration on the pion
transition form factor at large momentum transfer.Comment: 11 pages, 2 figures, extended version of the talk given at "New
Physics and Quantum Chromodynamics at External Conditions" conference, May
3-6, 2009, Dnipropetrovsk, Ukrain
Kaluza-Klein Dark Matter, Electrons and Gamma Ray Telescopes
Kaluza-Klein dark matter particles can annihilate efficiently into
electron-positron pairs, providing a discrete feature (a sharp edge) in the
cosmic spectrum at an energy equal to the particle's mass (typically
several hundred GeV to one TeV). Although this feature is probably beyond the
reach of satellite or balloon-based cosmic ray experiments (those that
distinguish the charge and mass of the primary particle), gamma ray telescopes
may provide an alternative detection method. Designed to observe very
high-energy gamma-rays, ACTs also observe the diffuse flux of electron-induced
electromagnetic showers. The GLAST satellite, designed for gamma ray astronomy,
will also observe any high energy showers (several hundred GeV and above) in
its calorimeter. We show that high-significance detections of an
electron-positron feature from Kaluza-Klein dark matter annihilations are
possible with GLAST, and also with ACTs such as HESS, VERITAS or MAGIC.Comment: 10 pages, 2 figure
Energy Dependence of the Delta Resonance: Chiral Dynamics in Action
There is an important connection between the low energy theorems of QCD and
the energy dependence of the Delta resonance in pi-N scattering, as well as the
closely related gamma^{*} N -> pi N reaction. The resonance shape is due not
only to the strong pi-N interaction in the p wave but the small interaction in
the s wave; the latter is due to spontaneous chiral symmetry breaking in QCD
(i.e. the Nambu-Goldstone nature of the pion). A brief overview of experimental
tests of chiral perturbation theory and chiral based models is presentedComment: 11 pages, 6 figures, Festschrift for S.N. yan
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