2,401 research outputs found
Angular distribution of positrons in coherent pair production in deformed crystals
We investigate the angular distribution of positrons in the coherent process
electronpositron pair creation process by high-energy photons in a periodically
deformed single crystal with a complex base. The formula for the corresponding
differential cross-section is derived for an arbitrary deformation field. The
case is considered in detail when the photon enters into the crystal at small
angles with respect to a crystallographic axis. The results of the numerical
calculations are presented for and diamond single crystals
and Moliere parameterization of the screened atomic potentials in the case of
the deformation field generated by the acoustic wave of S-type.Comment: 11 pages, 4 figures, figures and references adde
On the theory of coherent pair production in crystals in presence of acoustic waves
The influence of hypersonic waves excited in a single crystal is investigated
on the process of electron-positron pair creation by high-energy photons. The
coherent part of the corresponding differential cross-section is derived as a
function of the amplitude and wave number of the hypersound. The values of the
parameters are specified for which the latter affects remarkably on the pair
creation cross-section. It is shown that under certain conditions the presence
of hypersonic waves can result in enhancement of the process cross-section.Comment: 10 pages, 3 EPS figure
Little Groups of Preon Branes
Little groups for preon branes (i.e. configurations of branes with maximal
(n-1)/n fraction of survived supersymmetry) for dimensions d=2,3,...,11 are
calculated for all massless, and partially for massive orbits. For massless
orbits little groups are semidirect product of d-2 translational group
on a subgroup of (SO(d-2) R-invariance) group. E.g. at d=9
the subgroup is exceptional group. It is also argued, that 11d Majorana
spinor invariants, which distinguish orbits, are actually invariant under
d=2+10 Lorentz group. Possible applications of these results include
construction of field theories in generalized space-times with brane charges
coordinates, different problems of group's representations decompositions,
spin-statistics issues.Comment: LaTeX, 11 page
Representations and BPS states of 10+2 superalgebra
The 12d supersymmetry algebra is considered, and classification of BPS states
for some canonical form of second-rank central charge is given. It is shown,
that possible fractions of survived supersymmetry can be 1/16, 1/8, 3/16, 1/4,
5/16 and 1/2, the values 3/8, 7/16 cannot be achieved in this way. The
consideration of a special case of non-zero sixth-rank tensor charge also is
included.Comment: Minor changes.Accepted for publication in Mod.Phys.Lett.A.Latex fil
Coherent pair production in deformed crystals with a complex base
We investigate the coherent electron-positron pair creation by high-energy
photons in a periodically deformed single crystal with a complex base. The
formula for the corresponding differential cross-section is derived for an
arbitrary deformation field. The conditions are specified under which the
influence of the deformation is considerable. The case is considered in detail
when the photon enters into the crystal at small angles with respect to a
crystallographic axis. The results of the numerical calculations are presented
for single crystal and Moliere parametrization of the
screened atomic potentials in the case of the deformation field generated by
the acoustic wave of type. In dependence of the parameters, the presence of
deformation can either enhance or reduce the pair creation cross-section. This
can be used to control the parameters of the positron sources for storage rings
and colliders.Comment: 10 pages, 4 figures, misprint in the numerical coefficients in figure
captions is correcte
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