Weak Decay of Λ\Lambda in Nuclei: Direct Quark Mechanism vs Meson Exchange

Nonmesonic decays of $\Lambda$ in nuclear medium and light hypernuclei are studied by using the $\Lambda N \to NN$ weak transition potential derived from the meson exchange mechanism and the direct quark mechanism. The long range part of the transition potential is described by exchanges of the pseudoscalar mesons ($\pi$, $K$, $\eta$), while the vector mesons ($\rho$, $\omega$, $K^\ast$) may be considered as the medium- and short-range part in the meson exchange picture. We propose the direct quark transition potential as the short range part, which is derived from the matrix elements of the $\Delta S=1$ effective weak Hamiltonian in the two baryon states. The results indicate that the direct quark contribution is significantly large and its behavior is qualitatively different from the vector meson exchanges. We also find that the decay rate is sensitive to the choice of form factor and that a soft cutoff must be used for the pion-baryon verteces so that the strong tensor transition is suppressed. We find that the $\pi + K + DQ$ results are compatible with experiment although the $n/p$ ratio is still too large. The $\pi^+$ decays of light hypernuclei are related to the \DI=3/2 amplitudes of the nonmesonic decay. The role of chiral symmetry for the pionic decays are discussed.Comment: 10 pages, Talk presented by Makoto Oka at the APCTP Workshop on Strangeness Nuclear Physics (SNP'99), February, 199

Second Order Gravitational Effects on CMB Temperature Anisotropy in Lambda dominated flat universes

We study second order gravitational effects of local inhomogeneities on the cosmic microwave background radiation in flat universes with matter and a cosmological constant $\Lambda$. We find that the general relativistic correction to the Newtonian approximation is negligible at second order provided that the size of the inhomogeneous region is sufficiently smaller than the horizon scale. For a spherically symmetric top-hat type quasi-linear perturbation, the first order temperature fluctuation corresponding to the linear integrated Sachs-Wolfe (ISW) effect is enhanced(suppressed) by the second order one for a compensated void(lump). As a function of redshift of the local inhomogeneity, the second order temperature fluctuations due to evolution of the gravitational potential have a peak before the matter-$\Lambda$ equality epoch for a fixed comoving size and a density contrast. The second order gravitational effects from local quasi-linear inhomogeneities at a redshift z~1 may significantly affect the cosmic microwave background.Comment: 16 pages, 8 figures, No major change. Eqs (5.6) and (5.7) are modified to include uncompensating case

Unexplained postoperative retinal hemorrhage after 23-gauge sutureless vitrectomy

We report five cases of unexplained retinal hemorrhage after 23-gauge sutureless vitrectomy. A 23-gauge sutureless vitrectomy was performed for four cases of macular holes (MH) and one case of macular hole retinal detachment (MHRD). Retinal hemorrhages were observed on the first day after surgery and disappeared within several months without leaving any recognizable damage. We speculate that the retinal hemorrhages might have resulted from repeated collapse of the globe through a cannula under air perfusion, but other causes such as retinal vein congestion by face-down positioning are also possible

A direct algorithm for state deadbeat control

The authors propose a novel method for computing state deadbeat feedback gains from systems given in the staircase form. The proposed method uses only manipulations of given matrices, and hence is more direct than the existing method, which requires orthogonal transformations repeatedly. It is shown that the obtained gain is linear quadratic optimal for some weighting matrices </p

A construction of multivariable MRACS with fixed compensator using coprime factorization approach

A multivariable model reference adaptive control system (MRACS) with a fixed compensator is proposed. First, a new two-degree-of-freedom (2DOF) compensator with disturbance estimator is derived. Using this structure, a multivariable MRACS with fixed compensator is constructed. Since the proposed method is based on the 2 DOF structure, the fixed compensator is chosen independently of specifications for reference commands. The boundedness of all signals in the closed-loop system and the convergence of the output error are proved. A design method of the fixed compensator for MRACS with low sensitivity is also given. Finally, numerical examples are illustrated in order to show the effectiveness of the proposed method </p

A direct computation of state deadbeat feedback gains

A method for computing a feedback gain that achieves state deadbeat control is given. From systems given in the staircase form, this method derives the deadbeat gain in a numerically reliable way. It is shown that the gain turns out to be LQ optimal for some weightings

Parametrization of identity interactors and the discrete-time all-pass property

This paper gives a concise parametrization of all identity interactors of a discrete-time multivariable square system. This is performed by means of a state-space description computed from a given particular interactor of the system. The paper then proposes a selection of the parameter which leads to an all-pass closed-loop transfer matrix. This closed-loop system turns out to be equivalent to a certain LQ (linear quadratic) optimal feedback system. A numerical example is given to illustrate the results</p