367 research outputs found
Direct SUSY dark matter detection-Theoretical rates due to the spin
The recent WMAP data have confirmed that exotic dark matter together with the
vacuum energy (cosmological constant) dominate in the flat Universe. Thus the
direct dark matter detection, consisting of detecting the recoiling nucleus, is
central to particle physics and cosmology. Supersymmetry provides a natural
dark matter candidate, the lightest supersymmetric particle (LSP). The relevant
cross sections arise out of two mechanisms: i) The coherent mode, due to the
scalar interaction and ii) The spin contribution arising from the axial
current. In this paper we will focus on the spin contribution, which is
expected to dominate for light targets.
For both modes it is possible to obtain detectable rates, but in most models
the expected rates are much lower than the present experimental goals. So one
should exploit two characteristic signatures of the reaction, namely the
modulation effect and, in directional experiments, the correlation of the event
rates with the sun's motion.
In standard non directional experiments the modulation is small, less than
two per cent. In the case of the directional event rates we like to suggest
that the experiments exploit two features, of the process, which are
essentially independent of the SUSY model employed, namely: 1) The
forward-backward asymmetry, with respect to the sun's direction of motion, is
very large and 2) The modulation is much larger, especially if the observation
is made in a plane perpendicular to the sun's velocity. In this case the
difference between maximum and minimum can be larger than 40 per cent and the
phase of the Earth at the maximum is direction dependent.Comment: 16 Latex pages, 15 figures, 3 table
Scaling law for the electromagnetic form factors of the proton
The violation of the scaling law for the electric and magnetic form factors
of the proton are examined within the cloudy bag model. The suppression of the
ratio of the electric and magnetic form factors is natural in the bag model.
The pion cloud plays a moderate role in understanding the recent data from
TJNAF.Comment: 8 pages, REVTeX, 2 figures include
Mass corrections in decay and the role of distribution amplitudes
We consider mass correction effects on the polar angular distribution of a
baryon--antibaryon pair created in the chain decay process , generalizing a previous analysis of Carimalo. We show the
relevance of the features of the baryon distribution amplitudes and estimate
the electromagnetic corrections to the QCD results.Comment: 26 pages + 3 figures, REVTEX 3.0, figures appended as uuencoded,
tar-compressed postscript fil
Delta--Excitation and Exchange Corrections for NN--Bremsstrahlung
The role of the relativistic amplitudes for a number of
processes usually neglected in potential model calculations of
NN--bremsstrahlung is investigated. In particular, we consider the
--excitation pole contributions related to the one--pion and one--rho
exchange and in addition include the exchange contributions induced by the
radiative decays. The contributions are
calculated from relativistic Born amplitudes fitted to --production and
absorption data in the energy range up to 1 GeV and then used to supplement
potential model and soft photon calculations for nucleon--nucleon
bremsstrahlung. The effects on --observables, although moderate in
general, are found to be important in some kinematic domains.Comment: 15 pages in LaTex, using Revtex, 6 figures as uufile'd, compressed
Postscript file included, TRIUMF preprint TRI-PP-94-9
On the Nucleon Distribution Amplitude: The Heterotic Solution
We present a new nucleon distribution amplitude which amalgamates features of
the Chernyak-Ogloblin-Zhitnitsky model with those of the Gari-Stefanis model.
This "heterotic" solution provides the possibility to have asymptotically a
small ratio \hbox{}, while fulfilling
most of the sum-rule requirements up to the third order. Using this nucleon
distribution amplitude we calculate the electromagnetic and weak nucleon form
factors, the transition form factor and the decay widths
of the charmonium levels , , and into .
The agreement with the available data is remarkable in all cases.Comment: 15 pages, RUB-TPII-21/92 Preprin
New calculations of the PNC Matrix Element for the 0 doublet in N
A new calculation of the predominantly isoscalar PNC matrix element between
the (E 8.7 MeV) states in N has
been carried out in a (0+1+2+3+4) model space with the
Warburton-Brown interaction. The magnitude of the PNC matrix element of 0.22 to
0.34 eV obtained with the DDH PNC interaction is substantially suppressed
compared with previous calculations in smaller model spaces but shows agreement
with the preliminary Seattle experimental data. The calculated sign is opposite
to that obtained experimentally, and the implications of this are discussed.Comment: REVTEX, 28 page
Lithium-6: A Probe of the Early Universe
I consider the synthesis of 6Li due to the decay of relic particles, such as
gravitinos or moduli, after the epoch of Big Bang Nucleosynthesis. The
synthesized 6Li/H ratio may be compared to 6Li/H in metal-poor stars which, in
the absence of stellar depletion of 6Li, yields significantly stronger
constraints on relic particle densities than the usual consideration of
overproduction of 3He. Production of 6Li during such an era of non-thermal
nucleosynthesis may also be regarded as a possible explanation for the
relatively high 6Li/H ratios observed in metal-poor halo stars.Comment: final version, Physical Review Letters, additional figure giving
limits on relic decaying particle
Parity Mixed Doublets in A = 36 Nuclei
The -circular polarizations () and asymmetries
() of the parity forbidden M1 + E2 -decays: MeV) and MeV)
MeV) are investigated theoretically. We use the recently proposed
Warburton-Becker-Brown shell-model interaction. For the weak forces we discuss
comparatively different weak interaction models based on different assumptions
for evaluating the weak meson-hadron coupling constants. The results determine
a range of values from which we find the most probable values:
= for and = for .Comment: RevTeX, 17 pages; to appear in Phys. Rev.
Nucleon Charge and Magnetization Densities from Sachs Form Factors
Relativistic prescriptions relating Sachs form factors to nucleon charge and
magnetization densities are used to fit recent data for both the proton and the
neutron. The analysis uses expansions in complete radial bases to minimize
model dependence and to estimate the uncertainties in radial densities due to
limitation of the range of momentum transfer. We find that the charge
distribution for the proton is significantly broad than its magnetization
density and that the magnetization density is slightly broader for the neutron
than the proton. The neutron charge form factor is consistent with the Galster
parametrization over the available range of Q^2, but relativistic inversion
produces a softer radial density. Discrete ambiguities in the inversion method
are analyzed in detail. The method of Mitra and Kumari ensures compatibility
with pQCD and is most useful for extrapolating form factors to large Q^2.Comment: To appear in Phys. Rev. C. Two new figures and accompanying text have
been added and several discussions have been clarified with no significant
changes to the conclusions. Now contains 47 pages including 21 figures and 2
table
On isovector meson exchange currents in the Bethe-Salpeter approach
We investigate the nonrelativistic reduction of the Bethe-Salpeter amplitude
for the deuteron electrodisintegration near threshold energies. To this end,
two assumptions have been used in the calculations: 1) the static approximation
and 2) the one iteration approximation. Within these assumptions it is possible
to recover the nonrelativistic result including a systematic extension to
relativistic corrections. We find that the so-called pair current term can be
constructed from the -wave contribution of the deuteron Bethe-Salpeter
amplitude. The form factor that enters into the calculation of the pair current
is constrained by the manifestly gauge independent matrix elements.Comment: 15 pages, incl. 3 figures, to be published Phys. Rev.
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