39,154 research outputs found
Relative fixed-width stopping rules for Markov chain Monte Carlo simulations
Markov chain Monte Carlo (MCMC) simulations are commonly employed for
estimating features of a target distribution, particularly for Bayesian
inference. A fundamental challenge is determining when these simulations should
stop. We consider a sequential stopping rule that terminates the simulation
when the width of a confidence interval is sufficiently small relative to the
size of the target parameter. Specifically, we propose relative magnitude and
relative standard deviation stopping rules in the context of MCMC. In each
setting, we develop sufficient conditions for asymptotic validity, that is
conditions to ensure the simulation will terminate with probability one and the
resulting confidence intervals will have the proper coverage probability. Our
results are applicable in a wide variety of MCMC estimation settings, such as
expectation, quantile, or simultaneous multivariate estimation. Finally, we
investigate the finite sample properties through a variety of examples and
provide some recommendations to practitioners.Comment: 24 page
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
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
Giant increase in critical current density of KxFe2-ySe2 single crystals
By using post-annealing and quenching technique, we show that the critical
current density Jc,ab of KxFe2-ySe2 single crystals can be enhanced more than
one order of magnitude up to ~ 2.1 \times 10^4 A/cm^2 at 1.8 K. The scaling law
between normalized pinning force and reduced field for all measured
temperatures was observed, reflecting the presence of only one dominant pinning
mechanism. Analysis indicates presence of 3D point-like normal core pinning
sources in quenched KxFe2-ySe2 samples whereas dominant vortex interaction with
pinning centers is via spatial variations in Tc ("deltaTc pinning").Comment: 3 figures, 4 page
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