Symmetry analysis of ordered phases of the lower tungsten carbide W2C

The channels of disorder-order phase transitions through which there can arise orthorhombic (space group Pbcn) β′-W₂C, rhombohedral (space group P $ \bar 3 $ \bar 3 m1)β″-W₂C, and trigonal (space group P $ \bar 3 $ \bar 3 1m)ɛ-W₂C superstructures of the disordered lower hexagonal carbide β-W₂C have been determined. The distribution functions of carbon atoms in the superstructures under consideration have been calculated, and the physically admissible sequence of transformations associated with the ordering of the lower carbide β-W₂C that occur with a decrease in the temperature has been established.     

Confinement singularities in two-pion decays of mesons

We consider as an example the two-pion decay of the ρ meson, the ³ S ₁ q $ \bar q $ \bar q state of the constituent quarks—the decay being determined by the transition q $ \bar q $ \bar q → ππ contains information about confinement interactions. One can specify in this decay two types of transitions: (i) the bremsstrahlung radiation of a pion q → q + π (or $ \bar q $ \bar q → $ \bar q $ \bar q + π) with a subsequent fusion q $ \bar q $ \bar q → π, and (ii) the direct transition q $ \bar q $ \bar q → ππ. We demonstrate how in the amplitudes of the corresponding transitions the quark singularities have to disappear, i.e., what is the way the quark confinement at relatively short distances can be realized. We calculate and estimate the contributions of processes with bremsstrahlung radiation of the pion and of the direct transition q $ \bar q $ \bar q → ππ. The estimates demonstrate that the processes involving the direct transition q $ \bar q $ \bar q → ππ are necessary, but they cannot be determined unambiguously by the decay ρ(775) → ππ. We conclude that for the determination of the q $ \bar q $ \bar q → ππ transition more complete data on the resonance decays into the ππ channels are needed than those available at the moment.     

Coupling running through the looking-glass of dimensional reduction

The dimensional reduction, in a form of transition from four to two dimensions, was used in the 90s of the past century in a context of the HE Regge scattering. Recently, it has got a new impetus in quantum gravity where it opens the way to renormalizability and finite short-distance behaviour. We consider a QFT model gφ⁴ with running coupling defined in both domains of different dimensionality; the $ \bar g $ \bar g (q ²) evolutions being duly correlated at the reduction scale q ∼ M. Beyond this scale, in the deep UV 2-dimensional region, the running coupling does not increase any more. Instead, it slightly decreases and tends to a finite value $ \bar g $ \bar g ₂(∞) < $ \bar g $ \bar g ₂(M ²) from above. As a result, the global evolution picture looks quite peculiar and proposes a base for the modified scenario of gauge couplings behavior with UV fixed points provided by dimensional reduction instead of leptoquarks.     

Vorticity waves in problems of hydrodynamic stability

The known problem of flow transition near a circular cylinder at Re = 40 from a symmetrical form to the Karman vortex street can be considered as the problem of vortex wave development and intensification. Development of three bundles of vortex waves of low intensity is observed in a wake of a cylinder; these bundles are easily visualized as the structures of relative vorticity $ \bar \Omega $ \bar \Omega = Ω(t ₁) − Ω(t ₀): difference of vorticity Ω at two time moments, t ₀ being fixed. In the field of $ \bar \Omega $ \bar \Omega the alternating structure of quadrupoles is characterized by linear parameter l = h/d: the ratio of the width of the central bundle of vortex waves to the distance between the centers of quadrupoles of a “single sign”. When l = 0.281 is achieved, which coincides with the value of the same parameter of a stable Karman street, the transition from symmetrical streamlining by viscous incompressible liquid to the vortex street occurs.     

Effect of the orientation of strain axes on the creep in Ni<Subscript>3</Subscript>Ge alloy single crystals

The creep in Ni₃Ge alloy single crystals with strain axis orientations of [001], [$ \bar 1 $ \bar 1 39], [$ \bar 2 $ \bar 2 34], and [$ \bar 1 $ \bar 1 22] was investigated. It was found that changes in strain axis orientation that lead to cubic slip increase the creep resistance of Ni₃Ge alloy single crystals.     

Mimicking diffuse supernova antineutrinos with the sun as a source

Measuring the $ \bar \nu _e $ \bar \nu _e component of the cosmic diffuse supernova neutrino background (DSNB) is the next ambitious goal for low-energy neutrino astronomy. The largest flux is expected in the lowest accessible energy bin. However, for E ≲ 15 MeV a possible signal can be mimicked by a solar $ \bar \nu _e $ \bar \nu _e flux that originates from the usual ⁸B neutrinos by spin-flavor oscillations. We show that such an interpretation is possible within the allowed range of neutrino electromagnetic transition moments and solar turbulent field strengths and distributions. Therefore, an unambiguous detection of the DSNB requires a significant number of events at E ≳ 15 MeV.     

Directed flow of identified particles from Au+Au collisions at RHIC

We present the measurement of directed flow (v ₁) for the identified particles, namely, Λ, $ \bar \Lambda $ \bar \Lambda and K _s ⁰, as a function of rapidity and centrality in Au+Au collisions at $ \sqrt {s_{NN} } $ \sqrt {s_{NN} } = 200 GeV and 62.4 GeV. The measurement is based on the run IV data obtained by the STAR experiment at RHIC. In order to enhance event plane resolution, we use tracks reconstructed from the Forward Time Projection Chambers (FTPCs), together with the sideward deflection of spectator neutrons measured by the STAR’s Shower Maximum Detector at Zero Degree Calorimeters (ZDC-SMDs). We find that for 200 GeV, proton and antiproton v ₁ is less than 1%, the K _s ⁰ Λ, $ \bar \Lambda $ \bar \Lambda v ₁ is less than 2%; for 62 GeV, proton v ₁ is less than 1% and antiproton is less than 2%, v ₁ for K _s ⁰, Λ, $ \bar \Lambda $ \bar \Lambda is less than 2% in Au+Au collisions at 200 GeV.     

Anisotropy of elastic strains and specific features of the defect structure of <Emphasis Type="Italic">a</Emphasis>-plane GaN epitaxial films grown on <Emphasis Type="Italic">r</Emphasis>-plane sapphire

The structural state of GaN epitaxial layers grown on r-plane sapphire through metal-organic vapor phase epitaxy has been investigated using X-ray diffraction. The interplanar spacings in two directions in the (11$ \bar 2 $ \bar 2 0) plane of the interface and in the direction perpendicular to it, as well as the diffraction peaks in the ϑ and ϑ-2ϑ scan modes in the Bragg and Laue geometries, are measured on double- and triple-crystal diffractometers. The intensity distribution maps for asymmetric Bragg reflections are constructed for two azimuthal positions of the sample. An analysis of the data obtained has demonstrated that the elastic strain is anisotropic and that the X-ray diffraction pattern parallel to the interface plane is broadened. The layers are contracted in the [1$ \bar 1 $ \bar 1 00] direction and unstrained in the [0001] direction. The broadening of the Bragg reflections in the [1$ \bar 1 $ \bar 1 00] direction is considerably larger than that in the [0001] direction. It is shown using the Williamson-Hall plots for the Bragg and Laue reflections that these broadenings are not related to different degrees of mosaicity but are determined by the local dilatations and misorientations around defects. The data obtained are analyzed, and the conclusions regarding the dislocation structure of the samples are drawn.     

Anomalous structure of the crust and mantle of mars

This study determined the contribution of Martian topography and the density jump at the Mohorovicic discontinuity (M) to the gravity in a quadratic approximation. It also resolved the problem of determining the possible depths of compensation for topography harmonics of various degrees and orders. It shows that all the topography compensation is within the depth range from 0 to 1400 km. Different topographic irregularities are most likely to be compensated at depths that correspond to the upper crust ($ \bar d = 4.5 \pm 3.7{\text{ km}} $ \bar d = 4.5 \pm 3.7{\text{ km}} ), crust-mantle transition layer ($ \bar d = 78 \pm 24{\text{ km}} $ \bar d = 78 \pm 24{\text{ km}} ), lithospheric boundary ($ \bar d = 200 \pm 34{\text{ km}} $ \bar d = 200 \pm 34{\text{ km}} ), upper-middle mantle transition layer ($ \bar d = 400 \pm 70{\text{ km}} $ \bar d = 400 \pm 70{\text{ km}} ), or middle-lower mantle transition layer ($ \bar d = 1120 \pm 180{\text{ km}} $ \bar d = 1120 \pm 180{\text{ km}} ). This paper presents the lateral distributions of compensation masses for these depths and the respective maps. According to calculations, stresses in the Martian crust and mantle may be as high as 10⁸ Pa. This paper shows that the topographic anomalies of the Tharsis volcanic plateau and the symmetric formation in the eastern hemisphere could have originated and be dynamically maintained by two plumes of melted mantle substance enriched with fluids; these plumes may have their origin at the boundary of the lower mantle.     

Energy gaps at neutrality point in bilayer graphene in a magnetic field

Utilizing the Baym-Kadanoff formalism with the polarization function calculated in the random phase approximation, the dynamics of the ν = 0 quantum Hall state in bilayer graphene is analyzed. Two phases with nonzero energy gap, the ferromagnetic and layer asymmetric ones, are found. The phase diagram in the plane ($ \bar \Delta _0 $ \bar \Delta _0 , B), where $ \bar \Delta _0 $ \bar \Delta _0 is a top-bottom gates voltage imbalance, is described. It is shown that the energy gap scales linearly, ΔE ∼ 14B [T] K, with magnetic field.     

Internal waves in a compressible two-layer model atmosphere: Hamiltonian description

We consider slow, compared to the speed of sound, motions of an ideal compressible fluid (gas) in a gravitational field in the presence of two isentropic layers with a small specific-entropy difference between them. Assuming the flow to be potential in each of the layers (v _{1, 2} = ▿ϕ_{1, 2}) and neglecting the acoustic degrees of freedom (div($ \bar \rho $ \bar \rho (z)▿ϕ_{1, 2}) ≈ 0, where $ \bar \rho $ \bar \rho (z) is the average equilibrium density), we derive the equations of motion for the boundary in terms of the shape of the surface z = η(x, y, t) itself and the difference between the boundary values of the two velocity field potentials: ψ(x, y, t) = ψ₁ − ψ₂. We prove the Hamilto nian structure of the derived equations specified by a Lagrangian of the form ℒ = ∫$ \bar \rho $ \bar \rho (η)η _t ψdxdy − ℋ{η, ψ}. The system under consideration is the simplest theoretical model for studying internal waves in a sharply stratified atmosphere in which the decrease in equilibrium gas density due to gas compressibility with increasing height is essentially taken into account. For plane flows, we make a generalization to the case where each of the layers has its own constant potential vorticity. We investigate a system with a model dependence $ \bar \rho $ \bar \rho (z) ∝ e ^−2αz with which the Hamiltonian ℋ{η, ψ} can be represented explicitly. We consider a long-wavelength dynamic regime with dispersion corrections and derive an approximate nonlinear equation of the form u _t + auu _x − b[−$ \hat \partial _x^2 $ \hat \partial _x^2 + α²]^1/2 u _x = 0 (Smith’s equation) for the slow evolution of a traveling wave.     

Mechanism of the production of the <Emphasis Type="Italic">a</Emphasis><Subscript>0</Subscript>(980) resonance in the γγ → π<Superscript>0</Superscript>η reaction

High-statistics Belle data on the γγ → π⁰η reaction have been analyzed in order to reveal the mechanism of two-photon production and the nature of the a ₀(980) resonance. The solution obtained for the γγ → π⁰η amplitude is in agreement with the predictions of the chiral theory for the πη-scattering length; with the strong coupling of the a ₀(980) resonance with the πη, K $ \bar K $ \bar K , and πη′ channels; and with the key role of the a ₀(980) → (K $ \bar K $ \bar K + π⁰η + π⁰η′) → γγ rescattering mechanisms in the a ₀(980) → γγ decay. This picture is much in favor of the q ² $ \bar q $ \bar q ² nature of a ₀(980) resonance and is consistent with the properties of its partners, σ₀(600) and f ₀(980) resonances, in particular, with those manifested in the γγ → ππ reactions. The important role of vector exchanges in the formation of the nonresonant background in the γγ → π⁰η reaction has been revealed. Preliminary information on the π⁰η → π⁰η reaction has been obtained.     

The model-independent analysis indications for spectroscopy of scalar mesons. Proof for the K0* (900)

In a model-independent approach the data on ππ → ππ, K $ \bar K $ \bar K , ηη, ηη′ in the I ^G J ^PC = 0⁺0⁺⁺ channel and on the Kπ scattering in the $ I\left( {J^P } \right) = \frac{1} {2}\left( {0^ + } \right) $ I\left( {J^P } \right) = \frac{1} {2}\left( {0^ + } \right) channel are analyzed jointly for studying the status and QCD nature of the f ₀- and the K*₀-mesons. It is shown that in the 1500-MeV region, there are two states, wide (interpreted as a glueball) and narrow (q $ \bar q $ \bar q ). In the Kπ-scattering data analysis, the proof for the K*₀(900) is given.     

Electronic transport through a contact of a correlated quantum wire with leads of higher dimension

We study theoretically electronic transport through a contact of a quantum wire with 2D or 3D leads and find that if the contact is not smooth and adiabatic then the conduction is strongly suppressed below a threshold voltage V _T , while above V _T the dc current $ \bar I $ \bar I is accompanied by coherent oscillations of frequency f = $ \bar I $ \bar I /e. The effect is related to interelectronic repulsion and interaction of dc current with the Friedel oscillations near a sharp contact. In short conducting channels of length L < L ₀ ≅ ℏV _F /eV _T and at high temperatures T > T ₀ ≅ eV _T /k _B the effect is destroyed by fluctuations.     

Properties of neutral charmed mesons in <Emphasis Type="Italic">pA</Emphasis> interactions at 70 GeV

The results of data handling for the E-184 experiment involving the irradiation of the active target, consisting of carbon, silicon, and lead plates by a 70-GeV proton beam are presented. When the two-prong neutral charmed meson decay signal was selected and the cross section for charm production at a near-threshold energy was estimated (σ(c $ \bar c $ \bar c ) = 7.1 ± 2.4(stat.) ± 1.4(syst.) μb/nucleon), some properties of D ⁰ and $ \bar D^0 $ \bar D^0 were investigated. These include the atomic-weight dependence of the cross section on the target mass number (its A dependence); the differential cross sections dσ/dp _t ² and dσ/dx _F; and the dependence of the parameter α on x _F, p _t ², and p _lab. The experimental results are compared with the predictions of the FRITIOF7.02 program.     

Neutron diffraction studies of the negative thermal expansion in a layered indium selenide crystal

The neutron diffraction patterns have been analyzed for a layered single crystal and a powder of the γ-polytype of indium selenide in the temperature range 10–300 K. In the temperature range 10–50 K, the excitation of bending vibrations due to the charge density waves changes the phonon spectrum and gives rise to a negative thermal expansion in the plane of layers, i.e., α_‖c = −2.2 × 10⁻⁶ K⁻¹, which is characteristic of two-dimensional structures. The average (over the range T = 50–300 K) coefficients of thermal expansion along the principal crystallographic directions have been calculated: $ \bar \alpha _{ \bot c} $ \bar \alpha _{ \bot c} = 10.48 × 10⁻⁶ K⁻¹ and $ \bar \alpha _{\parallel c} $ \bar \alpha _{\parallel c} = 12.97 × 10⁻⁶ K⁻¹, which agree with the X-ray diffraction data previously obtained by the authors at T = 290 K.     

Diquark and triquark correlations in the deconfined phase of QCD

We use the non-perturbative Q$ \bar Q $ \bar Q potential at finite temperatures derived in the Field Correlator Method to obtain binding energies for the lowest eigenstates in the Q$ \bar Q $ \bar Q and QQQ systems (Q = c, b). The three-quark problem is solved by the hyperspherical method. The solution provides an estimate of the melting temperature and the radii for the different diquark and triquark bound states. In particular we find that J/ψ and ccc ground states survive up to T − 1.3T _c , where T _c is the critical temperature, while the corresponding bottomonium states survive even up to higher temperature, T − 2.3T _c .     

Production of strange secondaries in high-energy Σ<Superscript>−</Superscript><Emphasis Type="Italic">A</Emphasis> collisions

The WA89 Collaboration experimental data on production of Λ, Σ⁻, Σ⁺, Ξ⁻, Ω⁻ baryons, $ \bar \Lambda $ \bar \Lambda and $ \bar \Xi ^ + $ \bar \Xi ^ + antibaryons in Σ⁻ collisions with C and Cu targets at 345 GeV/c ($ \sqrt {s_{\Sigma N} } $ \sqrt {s_{\Sigma N} } ≈ 25.5 GeV) in the frame of the Quark-Gluon String Model is described. The comparison of the theoretical results with the experimental data is discussed. Finally, some relations among the values of the model parameters obtained with the help of quark combinatorics are presented.     

Two-photon decays of pseudotensor mesons <Emphasis Type="Italic">π</Emphasis><Subscript>2</Subscript>(1670), <Emphasis Type="Italic">π</Emphasis><Subscript>2</Subscript>(2250), and <Emphasis Type="Italic">π</Emphasis><Subscript>2</Subscript>(2510)

In terms of relativistic and gauge-invariant spectral-integral technique, we calculate amplitude for the π ₂ → γγ decay as a three-step transition π ₂ → q $ \bar q $ \bar q (2⁻⁺) → γ + q $ \bar q $ \bar q (1⁻⁻) → γγ. Using wave functions of the q $ \bar q $ \bar q components of photon (hep-ph/0406320) and those of pseudotensor mesons π ₂(1670), π ₂(2000), π ₂(2250), and π ₂(2510) (hep-ph/0511109), we obtain the following values for partial widths: Γ _{π2(1670)→γγ} = 0.072 ± 0.007 keV, Γ _{π2(2000)→γγ} = 0.037 ± 0.004 keV, Γ _{π2(2250)→γγ} = 0.027 ± 0.003 keV, and Γ _{π2(2510)→γγ} = 0.022 ± 0.002 keV. In the calculations, we use the momentum operator expansion, and, in this connection, discuss the problem of nilpotent operators inherent in the considered amplitude. It should be emphasized that measurement of two-photon decays of the π ₂ states can give valuable information about the long-range quark-antiquark forces which, as one may guess, are related to the quark confinement.     

Mixing parameters of η and η′ mesons

Mixing angles of η and η′ mesons are obtained in the scheme of octet-singlet mixing θ_η = −15.4°, θ′_η = −17.9°, and in the scheme connected with expansion in the quark basis $ \bar q $ \bar q q − $ \bar s $ \bar s s φ = 39.3°. The constants gV _ηγ, g _η′ V _γ of radiative decays η′ → V _γ and V → ηγ (V ≡ ρ, ω, φ) and the ratios of constants of weak decays of η and η′ mesons are calculated. The numerical values that were found are in agreement with the experimental data.