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Axisymmetric accretion disks in vicinity of a central compact body are studied. In the case of non-viscous disk it is proven that all solutions for the midplane circular velocity are unstable. Hence, the pure hydrodynamic turbulence in accretion disks is possible. It is disproved the well-known arguments that an inviscid accretion disk must be sub-Keplerian. It is also demonstrated that the regular asymptotic solutions, often used in astrophysics, can lead to erroneous conclusions. It is proven that a laminar viscous disk can be approximated with a great precision by the vortex motion. Assuming that a turbulent gas tends to flow with minimal losses, we have shown that a turbulent disk tends to be Keplerian., Comment: 38 pages

Two-dimensional electron systems with fascinating properties exist in multilayers of standard semiconductors, on helium surfaces, and in oxides. Compared to the two-dimensional (2D) electron gases of semiconductors, the 2D electron systems in oxides are typically more strongly correlated and more sensitive to the microscopic structure of the hosting lattice. This sensitivity suggests that the oxide 2D systems are highly tunable by hydrostatic pressure. Here we explore the effects of hydrostatic pressure on the well-characterized 2D electron system formed at LaAlO$_{3}$ -SrTiO$_{3}$ interfaces[1] and measure a pronounced, unexpected response. Pressure of $\sim$2 GPa reversibly doubles the 2D carrier density $n_{s}$ at 4 K. Along with the increase of $n_{s}$, the conductivity and mobility are reduced under pressure. First-principles pressure simulations reveal the same behavior of the carrier density and suggest a possible mechanism of the mobility reduction, based on the dielectric properties of both materials and their variation under external pressure., Comment: 13 pages and 10 figures main text. 6 pages and 10 figures supplementary information. Phys Rev B (in press)

The charge-exchange reaction K^+ Xe --> K^0 p Xe' is investigated using the data of the DIANA experiment. The distribution of the pK^0 effective mass shows a prominent enhancement near 1538 MeV formed by \sim 80 events above the background, whose width is consistent with being entirely due to the experimental resolution. Under the selections based on a simulation of K^+Xe collisions, the statistical significance of the signal reaches 5.5\sigma. We interpret this observation as strong evidence for formation of a pentaquark baryon with positive strangeness, \Theta^+(uudd\bar{s}), in the charge-exchange reaction K^+ n --> K^0 p on a bound neutron. The mass of the \Theta^+ baryon is measured as m(\Theta^+) = 1538+-2 MeV. Using the ratio between the numbers of resonant and non-resonant charge-exchange events in the peak region, the intrinsic width of this baryon resonance is determined as \Gamma(\Theta^+) = 0.34+-0.10 MeV., Comment: 19 pages, 8 figures

Axisymmetric accretion disks in vicinity of a central compact body are studied. For the simple models such as vertically isothermal disks as well as adiabatic ones the exact solutions to the steady-state MHD (magneto-hydrodynamic) system were found under the assumption that the radial components of velocity and magnetic field are negligible. On the basis of the exact solution one may conclude that vertically isothermal disks will be totally isothermal. The exact solution for the case of adiabatic disk corroborates the view that thin disk accretion must be highly non-adiabatic. An intermediate approach, that is between the above-listed two, for the modeling of thin accretion disks is developed. In the case of non-magnetic disk, this approach enables to prove, with ease, that all solutions for the mid-plane circular velocity are unstable provided the disk is non-viscous. Hence, this approach enables to demonstrate that the pure hydrodynamic turbulence in accretion disks is possible. It is interesting that a turbulent magnetic disk tends to be Keplerian. This can easily be shown by assuming that the turbulent gas tends to flow with minimal losses, i.e. to have the Euler number as small as possible., Comment: All found misprints are corrected. A few examples are added for the sake of illustration. Instability of non-magnetic disks is considered in more detail

The stability of nonlinear explicit difference schemes with not, in general, open domains of the scheme operators are studied. For the case of path-connected, bounded, and Lipschitz domains, we establish the notion that a multi-level nonlinear explicit scheme is stable iff (if and only if) the corresponding scheme in variations is stable. A new modification of the central Lax-Friedrichs (LxF) scheme is developed to be of the second order accuracy. The modified scheme is based on nonstaggered grids. A monotone piecewise cubic interpolation is used in the central scheme to give an accurate approximation for the model in question. The stability of the modified scheme is investigated. Some versions of the modified scheme are tested on several conservation laws, and the scheme is found to be accurate and robust. As applied to hyperbolic conservation laws with, in general, stiff source terms, it is constructed a second order nonstaggered central scheme based on operator-splitting techniques., Comment: 45 pages, 7 figures

The data on the reactions K^+Xe --> K^0 \gamma X and K^+Xe --> K^+ \gamma X, obtained with the bubble chamber DIANA, have been analyzed for possible radiative decays of the \Theta^+(1540) baryon: \Theta^+ --> K^0 p \gamma and \Theta^+ --> K^+ n \gamma. No signals have been observed, and we derive the upper limits \Gamma(\Theta^+ --> K^0 p \gamma) / \Gamma(\Theta^+ --> K^0 p) < 0.032 and \Gamma(\Theta^+ --> K^+ n \gamma) / \Gamma(\Theta^+ --> K^+ n) < 0.041 which, using our previous measurement of \Gamma(\Theta^+ --> KN) = (0.39+-0.10) MeV, translate to \Gamma(\Theta^+ --> K^0 p \gamma) < 8 keV and \Gamma(\Theta^+ --> K^+ n \gamma) < 11 keV at 90% confidence level. We have also measured the cross sections of K^+ -induced reactions involving emission of a neutral pion: \sigma(K^+n --> K^0 p \pi^0) = (68+-18) \mub and \sigma(K^+N --> K^+ N \pi^0) = (30+-8) \mub for incident K^+ momentum of 640 MeV., Comment: 8 pages

The data on the charge-exchange reaction K^+Xe --> K^0 p Xe, obtained with the bubble chamber DIANA, are reanalyzed using increased statistics and updated selections. Our previous evidence for formation of a narrow pK^0 resonance with mass near 1538 MeV is confirmed and reinforced. The statistical significance of the signal reaches some 8\sigma (6\sigma) when estimated as S/\sqrt{B} (S/\sqrt{B+S}). The mass and intrinsic width of the \Theta^+ baryon are measured as m = (1538+-2) MeV and \Gamma = (0.39+-0.10) MeV., Comment: 13 pages, 9 figures

The stability of difference schemes for, in general, hyperbolic systems of conservation laws with source terms are studied. The basic approach is to investigate the stability of a non-linear scheme in terms of its cor- responding scheme in variations. Such an approach leads to application of the stability theory for linear equation systems to establish stability of the corresponding non-linear scheme. It is established the notion that a non-linear scheme is stable if and only if the corresponding scheme in variations is stable. A new modification of the central Lax-Friedrichs (LxF) scheme is developed to be of the second order accuracy. A monotone piecewise cubic interpolation is used in the central schemes to give an accurate approximation for the model in question. The stability of the modified scheme are investigated. Some versions of the modified scheme are tested on several conservation laws, and the scheme is found to be accurate and robust. As applied to hyperbolic conservation laws with, in general, stiff source terms, it is constructed a second order nonstaggered central scheme based on operator-splitting techniques., Comment: 33 pages, 7 figures

The monotonicity and stability of difference schemes for, in general, hyperbolic systems of conservation laws with source terms are studied. The basic approach is to investigate the stability and monotonicity of a non-linear scheme in terms of its corresponding scheme in variations. Such an approach leads to application of the stability theory for linear equation systems to establish stability of the corresponding non-linear scheme. The main methodological innovation is the theorems establishing the notion that a non-linear scheme is stable (and monotone) if the corresponding scheme in variations is stable (and, respectively, monotone). Criteria are developed for monotonicity and stability of difference schemes associated with the numerical analysis of systems of partial differential equations. The theorem of Friedrichs (1954) is generalized to be applicable to variational schemes with non-symmetric matrices. A new modification of the central Lax-Friedrichs (LxF) scheme is developed to be of the second order accuracy. A monotone piecewise cubic interpolation is used in the central schemes to give an accurate approximation for the model in question. The stability and monotonicity of the modified scheme are investigated. Some versions of the modified scheme are tested on several conservation laws, and the scheme is found to be accurate and robust. As applied to hyperbolic conservation laws with, in general, stiff source terms, it is constructed a second order scheme based on operator-splitting techniques., Comment: 48 pages, 6 figures. Found inexactitudes are corrected. A second order scheme based on operator-splitting techniques is constructed

We have continued our investigation of the charge-exchange reaction K^+ Xe --> K^0 p Xe' in the bubble chamber DIANA. In agreement with our previous results based on part of the present statistics, formation of a narrow p K^0 resonance with mass of 1537+-2 MeV/c^2 is observed in the elementary transition K^+ n --> K^0 p on a neutron bound in the Xenon nucleus. Visible width of the peak is consistent with being entirely due to instrumental resolution and allows to place an upper limit on its intrinsic width: \Gamma < 9 MeV/c^2. A more precise estimate of the resonance intrinsic width, \Gamma = 0.36+-0.11 MeV/c^2, is obtained from the ratio between the numbers of resonant and non-resonant charge-exchange events. The signal is observed in a restricted interval of incident K^+ momentum, that is consistent with smearing of a narrow p K^0 resonance by Fermi motion of the target neutron. Statistical significance of the signal is some 7.3, 5.3, and 4.3 standard deviations for the estimators S/sqrt{B}, S/sqrt{S+B}, and S/sqrt{S+2B}, respectively. This observation confirms and reinforces our earlier results, and offers strong evidence for formation of a pentaquark baryon with positive strangeness in the charge-exchange reaction K^+ n --> K^0 p on a bound neutron., Comment: 13 pages, 8 figures, some chenges in text and references, more precise estimate of Theta(1540) to add, submitted to Phys.Atom.Nucl(Yad.Fiz.)

The status of our investigation of low-energy $K^+$Xe collisions in the Xenon bubble chamber DIANA is reported. In the charge-exchange reaction $K^+Xe \to K^0 p Xe'$ the spectrum of $K^0 p$ effective mass shows a resonant enhancement with $M = 1539 \pm 2$ MeV/c$^2$ and $\Gamma \le 9 MeV/c$^2$. The statistical significance of the enhancement is near $4.4\sigma$. The mass and width of the observed resonance are consistent with expectations for the lightest member of the anti-decuplet of exotic pentaquark baryons, as predicted in the framework of the chiral soliton model., Comment: 9 pages, 4 figures

This is a collection of proposals of experiments at hadronic beams of 10 GeV accelerator, situated at the Institute of Theoretical and Experimental Physics (Moscow, Russia). The proposals were presented by the leaders of several ITEP experimental groups and include selected problems of hadron-hadron (exotic states, scalar mesons, electomagnetic form factors, polarization phenomena etc.) and hadron-nucleon interactions (flucton and nuclear matter properties, hadron properties inside nuclei, interactions at small distances). Most of proposals are united by an idea of a common magnetic spectrometer with LiH(D) and polarized targets, wich could be built on the base of an existing 3m magnet. Some of them also refer to the intrinsic accelerator beam and to the new ion ring., Comment: 66 pages, 26 figures, in Russian