12,375 research outputs found

    Detecting Cohomology for Lie Superalgebras

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    In this paper we use invariant theory to develop the notion of cohomological detection for Type I classical Lie superalgebras. In particular we show that the cohomology with coefficients in an arbitrary module can be detected on smaller subalgebras. These results are used later to affirmatively answer questions, which were originally posed in \cite{BKN1} and \cite{BaKN}, about realizing support varieties for Lie superalgebras via rank varieties constructed for the smaller detecting subalgebras

    Comments on differential cross section of phi-meson photoproduction at threshold

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    We show that the differential cross section d_sigma/d_t of gamma p --> \phi p reaction at the threshold is finite and its value is crucial to the mechanism of the phi meson photoproduction and for the models of phi-N interaction.Comment: 8 pages, 2 figure

    Axisymmetric Magnetorotational Instability in Viscous Accretion Disks

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    Axisymmetric magnetorotational instability (MRI) in viscous accretion disks is investigated by linear analysis and two-dimensional nonlinear simulations. The linear growth of the viscous MRI is characterized by the Reynolds number defined as RMRIvA2/νΩR_{\rm MRI} \equiv v_A^2/\nu\Omega , where vAv_A is the Alfv{\'e}n velocity, ν\nu is the kinematic viscosity, and Ω\Omega is the angular velocity of the disk. Although the linear growth rate is suppressed considerably as the Reynolds number decreases, the nonlinear behavior is found to be almost independent of RMRIR_{\rm MRI}. At the nonlinear evolutionary stage, a two-channel flow continues growing and the Maxwell stress increases until the end of calculations even though the Reynolds number is much smaller than unity. A large portion of the injected energy to the system is converted to the magnetic energy. The gain rate of the thermal energy, on the other hand, is found to be much larger than the viscous heating rate. Nonlinear behavior of the MRI in the viscous regime and its difference from that in the highly resistive regime can be explained schematically by using the characteristics of the linear dispersion relation. Applying our results to the case with both the viscosity and resistivity, it is anticipated that the critical value of the Lundquist number SMRIvA2/ηΩS_{\rm MRI} \equiv v_A^2/\eta\Omega for active turbulence depends on the magnetic Prandtl number SMRI,cPm1/2S_{{\rm MRI},c} \propto Pm^{1/2} in the regime of Pm1Pm \gg 1 and remains constant when Pm1Pm \ll 1, where PmSMRI/RMRI=ν/ηPm \equiv S_{\rm MRI}/R_{\rm MRI} = \nu/\eta and η\eta is the magnetic diffusivity.Comment: Accepted for publication in ApJ -- 18 pages, 9 figures, 1 tabl

    Magnetically Regulated Star Formation in 3D: The Case of Taurus Molecular Cloud Complex

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    We carry out three-dimensional MHD simulations of star formation in turbulent, magnetized clouds, including ambipolar diffusion and feedback from protostellar outflows. The calculations focus on relatively diffuse clouds threaded by a strong magnetic field capable of resisting severe tangling by turbulent motions and retarding global gravitational contraction in the cross-field direction. They are motivated by observations of the Taurus molecular cloud complex (and, to a lesser extent, Pipe Nebula), which shows an ordered large-scale magnetic field, as well as elongated condensations that are generally perpendicular to the large-scale field. We find that stars form in earnest in such clouds when enough material has settled gravitationally along the field lines that the mass-to-flux ratios of the condensations approach the critical value. Only a small fraction (of order 1% or less) of the nearly magnetically-critical, condensed material is turned into stars per local free-fall time, however. The slow star formation takes place in condensations that are moderately supersonic; it is regulated primarily by magnetic fields, rather than turbulence. The quiescent condensations are surrounded by diffuse halos that are much more turbulent, as observed in the Taurus complex. Strong support for magnetic regulation of star formation in this complex comes from the extremely slow conversion of the already condensed, relatively quiescent C18^{18}O gas into stars, at a rate two orders of magnitude below the maximum, free-fall value. We analyze the properties of dense cores, including their mass spectrum, which resembles the stellar initial mass function.Comment: submitted to Ap

    MELK-a conserved kinase: functions, signaling, cancer, and controversy.

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    Maternal embryonic leucine zipper kinase (MELK) is a highly conserved serine/threonine kinase initially found to be expressed in a wide range of early embryonic cellular stages, and as a result has been implicated in embryogenesis and cell cycle control. Recent evidence has identified a broader spectrum of tissue expression pattern for this kinase than previously appreciated. MELK is expressed in several human cancers and stem cell populations. Unique spatial and temporal patterns of expression within these tissues suggest that MELK plays a prominent role in cell cycle control, cell proliferation, apoptosis, cell migration, cell renewal, embryogenesis, oncogenesis, and cancer treatment resistance and recurrence. These findings have important implications for our understanding of development, disease, and cancer therapeutics. Furthermore understanding MELK signaling may elucidate an added dimension of stem cell control

    Resonance Contributions to η\eta Photoproduction on Protons Found Using Dispersion Relations and an Isobar Model

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    The contributions of the resonances D13(1520)D_{13}(1520), S11(1535)S_{11}(1535), S11(1650)S_{11}(1650), D15(1675)D_{15}(1675), F15(1680)F_{15}(1680), D13(1700)D_{13}(1700), P11(1710)P_{11}(1710), P13(1720)P_{13}(1720) to γpηp\gamma p\to \eta p are found from the data on cross sections, beam and target asymmetries using two approaches: fixed-t dispersion relations and an isobar model. Utilization of the two approaches and comparison of the results obtained with different parametrizations of the resonance contributions allowed us to make conclusions about the model-dependence of these contributions. We conclude that the results for the contributions of the resonances D13(1520)D_{13}(1520), S11(1535)S_{11}(1535), F15(1680)F_{15}(1680) to corresponding multipole amplitudes are stable. With this the results for D13(1520)D_{13}(1520) and F15(1680)F_{15}(1680), combined with their PDG photoexcitation helicity amplitudes, allowed us to find the branching ratios Br(D13(1520)ηN)=0.05±0.02Br (D_{13}(1520)\to \eta N)=0.05\pm 0.02%, Br(F15(1680)ηN)=0.16±0.04Br (F_{15}(1680)\to \eta N)=0.16\pm0.04% which have significantly better accuracy than the PDG data. The total Breit-Wigner width of the S11(1535)S_{11}(1535) is model-dependent, we have obtained Γ(S11(1520))=142MeV\Gamma (S_{11}(1520))=142 MeV and 195MeV195 MeV using dispersion relations and the isobar model, respectively. The results for the S11(1650)S_{11}(1650), D15(1675)D_{15}(1675), P11(1710)P_{11}(1710), P13(1720)P_{13}(1720) are model dependent, only the signs and orders of magnitude of their contributions to multipole amplitudes are determined. The results for the D13(1700)D_{13}(1700) are strongly model-dependent.Comment: 26 pages, 6 figure

    Optical characteristics of single wavelength-tunable InAs/InGaAsP/InP(100) quantum dots emitting at 1.55 um

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    We have studied the emission properties of individual InAs quantum dots (QDs) grown in an InGaAsP matrix on InP(100) by metal-organic vapor-phase epitaxy. Low-temperature microphotoluminescence spectroscopy shows emission from single QDs around 1550 nm with characteristic exciton-biexciton behavior, and a biexciton antibinding energy of more than 2 meV. Temperature-dependent measurements reveal negligible optical-phonon induced broadening of the exciton line up to 50 K, and emission from the exciton state clearly persists above 70 K. Furthermore, we find no measurable polarized fine structure splitting of the exciton state within the experimental precision. These results are encouraging for the development of a controllable photon source for fiber-based quantum information and cryptography systems.Comment: 3 pages, 4 figures, submitted AP

    Search for Anomalous Couplings in Top Decay at Hadron Colliders

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    We present a quantitative study on sensitivities to the top-decay anomalous couplings, taking into account realistic experimental conditions expected at Tevatron and LHC. A double angular distribution of W and charged lepton in the top decay is analyzed, using ttbar events in the lepton+jets channel. In order to improve sensitivities to the anomalous couplings, we apply two techniques: (1) We use a likelihood fitting method for full kinematical reconstruction of each top event. (2) We develop a new effective spin reconstruction method for leptonically-decayed top quarks; this method does not require spin information of the antitop side. For simplicity, we neglect couplings of right-handed bottom quark as well as CP violating couplings. The 95% C.L. estimated bound on a ratio of anomalous couplings reads -0.81 < f_2/f_1 < -0.70, -0.12<f_2/f_1<0.14 using 1000 reconstructed top events at Tevatron, while -0.74<f_2/f_1<-0.72, -0.01<f_2/f_1<0.01 is expected with 100k reconstructed top events at LHC, where only statistical errors are taken into account. A two-fold ambiguity in the allowed range remains when the number of events exceeds a few hundred.Comment: 21 pages, 15 figure
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