101,184 research outputs found
Determinations of |V_ub| and |V_cb| from measurements of B -> X_u,c\ell\nu differential decay rates
Methods are described in the framework of light-cone expansion which allow
one to determine the Cabibbo-Kobayashi-Maskawa matrix elements |V_ub| and
|V_cb| from measurements of the differential decay rates as a function of the
scaling variables in the inclusive semileptonic decays of B mesons. By these
model-independent methods the dominant hadronic uncertainties can be avoided
and the B -> X_u\ell\nu decay can be very efficiently differentiated from the B
-> X_c\ell\nu decay, which may lead to precise determinations of |V_ub| and
|V_cb|.Comment: 11 pages, 2 figures, version as published in Mod. Phys. Lett. A, more
discussion, references added, title chang
Comprehensive analysis of large yields observed in Li induced reactions
Background: Large yields have been reported over the years in
reactions with Li and Li projectiles. Previous theoretical analyses
have shown that the elastic breakup (EBU) mechanism (i.e., projectile breakup
leaving the target in its ground state) is able to account only for a small
fraction of the total inclusive breakup cross sections, pointing
toward the dominance of non-elastic breakup (NEB) mechanisms.
Purpose: We aim to provide a systematic study of the inclusive cross
sections observed in nuclear reactions induced by Li projectiles. In
addition to estimating the total singles cross sections, it is our
goal to evaluate angular and energy distributions of these particles
and compare with experimental data, when available.
Method: We compute separately the EBU and NEB components of the inclusive
breakup cross sections. For the former, we use the continuum-discretized
coupled-channels (CDCC) method, which treats this mechanism to all orders. For
the NEB part, we employ the the model proposed in the eighties by Ichimura,
Austern and Vincent [Phys. Rev. C32, 432 (1982)], within the DWBA
approximation.
Results: Overall, the sum of the computed EBU and NEB cross sections is found
to reproduce very well the measured singles cross sections. In all cases
analyzed, we find that the inclusive breakup cross section is largely dominated
by the NEB component.
Conclusions: The presented method provides a global and systematic
description of inclusive breakup reactions induced by Li projectiles. It
provides also a natural explanation of the previously observed underestimation
of the measured yields by CDCC calculations. The method used here can
be extended to other weakly-bound projectiles, including halo nuclei.Comment: 11 pages, 16 figures, fig. 12 updated; some typos correcte
Feedback-stabilization of an arbitrary pure state of a two-level atom
Unit-efficiency homodyne detection of the resonance fluorescence of a
two-level atom collapses the quantum state of the atom to a stochastically
moving point on the Bloch sphere. Recently,Hofmann, Mahler, and Hess [Phys.
Rev. A {\bf 57}, 4877 (1998)] showed that by making part of the coherent
driving proportional to the homodyne photocurrent can stabilize the state to
any point on the bottom half of the sphere. Here we reanalyze their proposal
using the technique of stochastic master equations, allowing their results to
be generalized in two ways. First, we show that any point on the upper or lower
half, but not the equator, of the sphere may be stabilized. Second, we consider
non-unit-efficiency detection, and quantify the effectiveness of the feedback
by calculating the maximal purity obtainable in any particular direction in
Bloch space.Comment: 9 pages, 7 figures, Physical Review
The puzzle of complete fusion suppression in weakly-bound nuclei: a Trojan Horse effect?
Experimental studies of nuclear collisions involving light weakly-bound
nuclei show a systematic suppression of the complete fusion cross section by
30\% with respect to the expectation for tightly bound nuclei, at
energies above the Coulomb barrier. Although it is widely accepted that the
phenomenon is related to the weak binding of these nuclei, the origin of this
suppression is not fully understood. In here, we present a novel approach that
provides the complete fusion for weakly bound nuclei and relates its
suppression to the competition between the different mechanisms contributing to
the reaction cross section. The method is applied to the Li+Bi
reactions, where we find that the suppression of complete fusion is mostly
caused by the flux associated with non-elastic breakup modes, such as the
partial capture of the projectile (incomplete fusion), whereas the elastic
breakup mode is found to play a minor role. Finally, we demonstrate that the
large yields observed in these reactions can be naturally explained as
a consequence of a {\it Trojan Horse} mechanism.Comment: 6 pages, 4 figures. Accepted for publication in Phys. Rev. Letter
Random consensus protocol in large-scale networks
One of the main performance issues for consensus
protocols is the convergence speed. In this paper, we focus on the
convergence behavior of discrete-time consensus protocols over
large-scale sensor networks with uniformly random deployment,
which are modelled as Poisson random graphs. Instead of
using the random rewiring procedure, we introduce a deterministic
principle to locate certain “chosen nodes” in the network
and add “virtual” shortcuts among them so that the number
of iterations to achieve average consensus drops dramatically.
Simulation results are presented to verify the efficiency of this
approach. Moreover, a random consensus protocol is proposed,
in which virtual shortcuts are implemented by random routes
Bounded perturbation resilience of projected scaled gradient methods
We investigate projected scaled gradient (PSG) methods for convex
minimization problems. These methods perform a descent step along a diagonally
scaled gradient direction followed by a feasibility regaining step via
orthogonal projection onto the constraint set. This constitutes a generalized
algorithmic structure that encompasses as special cases the gradient projection
method, the projected Newton method, the projected Landweber-type methods and
the generalized Expectation-Maximization (EM)-type methods. We prove the
convergence of the PSG methods in the presence of bounded perturbations. This
resilience to bounded perturbations is relevant to the ability to apply the
recently developed superiorization methodology to PSG methods, in particular to
the EM algorithm.Comment: Computational Optimization and Applications, accepted for publicatio
Random induced subgraphs of Cayley graphs induced by transpositions
In this paper we study random induced subgraphs of Cayley graphs of the
symmetric group induced by an arbitrary minimal generating set of
transpositions. A random induced subgraph of this Cayley graph is obtained by
selecting permutations with independent probability, . Our main
result is that for any minimal generating set of transpositions, for
probabilities where , a random induced subgraph has a.s. a unique
largest component of size , where
is the survival probability of a specific branching process.Comment: 18 pages, 1 figur
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