36,260 research outputs found
The Non-Mesonic Weak Decay of Double-Lambda Hypernuclei: A Microscopic Approach
The non--mesonic weak decay of double-- hypernuclei is studied
within a microscopic diagrammatic approach. Besides the nucleon--induced
mechanism, , widely studied in single-- hypernuclei,
additional hyperon--induced mechanisms, ,
and , are
accessible in double-- hypernuclei and are investigated here. As in
previous works on single-- hypernuclei, we adopt a nuclear matter
formalism extended to finite nuclei via the local density approximation and a
one--meson exchange weak transition potential (including the ground state
pseudoscalar and vector octets mesons) supplemented by correlated and
uncorrelated two--pion--exchange contributions. The weak decay rates are
evaluated for hypernuclei in the region of the experimentally accessible light
hypernuclei Be and B. Our
predictions are compared with a few previous evaluations. The rate for the
decay is dominated by --, -- and
--exchange and turns out to be about 2.5\% of the free decay
rate, , while the total rate for the and decays, dominated by
--exchange, amounts to about 0.25\% of . The
experimental measurement of these decays would be essential for the beginning
of a systematic study of the non--mesonic decay of strangeness
hypernuclei. This field of research could also shed light on the possible
existence and nature of the --dibaryon.Comment: 17 pages, 2 figure
Soft-Collinear Messengers: A New Mode in Soft-Collinear Effective Theory
It is argued that soft-collinear effective theory for processes involving
both soft and collinear partons, such as exclusive B-meson decays, should
include a new mode in addition to soft and collinear fields. These
"soft-collinear messengers" can interact with both soft and collinear particles
without taking them far off-shell. They thus can communicate between the soft
and collinear sectors of the theory. The relevance of the new mode is
demonstrated with an explicit example, and the formalism incorporating the
corresponding quark and gluon fields into the effective Lagrangian is
developed.Comment: 22 pages, 5 figures. Extended Section 6, clarifying the relevance of
different types of soft-collinear interaction
Phase Transitions in a Two-Component Site-Bond Percolation Model
A method to treat a N-component percolation model as effective one component
model is presented by introducing a scaled control variable . In Monte
Carlo simulations on , , and simple cubic
lattices the percolation threshold in terms of is determined for N=2.
Phase transitions are reported in two limits for the bond existence
probabilities and . In the same limits, empirical formulas
for the percolation threshold as function of one
component-concentration, , are proposed. In the limit a new
site percolation threshold, , is reported.Comment: RevTeX, 5 pages, 5 eps-figure
Combination of a magnetic Feshbach resonance and an optical bound-to-bound transition
We use laser light near resonant with an optical bound-to-bound transition to
shift the magnetic field at which a Feshbach resonance occurs. We operate in a
regime of large detuning and large laser intensity. This reduces the
light-induced atom-loss rate by one order of magnitude compared to our previous
experiments [D.M. Bauer et al. Nature Phys. 5, 339 (2009)]. The experiments are
performed in an optical lattice and include high-resolution spectroscopy of
excited molecular states, reported here. In addition, we give a detailed
account of a theoretical model that describes our experimental data
Power Counting in the Soft-Collinear Effective Theory
We describe in some detail the derivation of a power counting formula for the
soft-collinear effective theory (SCET). This formula constrains which operators
are required to correctly describe the infrared at any order in the
Lambda_QCD/Q expansion (lambda expansion). The result assigns a unique
lambda-dimension to graphs in SCET solely from vertices, is gauge independent,
and can be applied independent of the process. For processes with an OPE the
lambda-dimension has a correspondence with dynamical twist.Comment: 12 pages, 1 fig, journal versio
Enhanced nonperturbative effects in jet distributions
We consider the triple differential distribution
d\Gamma/(dE_J)(dm_J^2)(d\Omega_J) for two-jet events at center of mass energy
M, smeared over the endpoint region m_J^2 << M^2, |2 E_J -M| ~ \Delta, \lqcd <<
\Delta << M. The leading nonperturbative correction, suppressed by
\lqcd/\Delta, is given by the matrix element of a single operator. A similar
analysis is performed for three jet events, and the generalization to any
number of jets is discussed. At order \lqcd/\Delta, non-perturbative effects in
four or more jet events are completely determined in terms of two matrix
elements which can be measured in two and three jet events.Comment: Significant changes made. The first moment does not vanish--the paper
has been modified to reflect this. Relations between different numbers of
jets still hol
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