12,366 research outputs found

### Detecting Cohomology for Lie Superalgebras

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

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

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 $R_{\rm MRI} \equiv v_A^2/\nu\Omega$, where $v_A$ is the Alfv{\'e}n
velocity, $\nu$ is the kinematic viscosity, and $\Omega$ 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 $R_{\rm MRI}$. 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 $S_{\rm MRI} \equiv v_A^2/\eta\Omega$ for active turbulence
depends on the magnetic Prandtl number $S_{{\rm MRI},c} \propto Pm^{1/2}$ in
the regime of $Pm \gg 1$ and remains constant when $Pm \ll 1$, where $Pm \equiv
S_{\rm MRI}/R_{\rm MRI} = \nu/\eta$ and $\eta$ 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

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
C$^{18}$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.

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 $\eta$ Photoproduction on Protons Found Using Dispersion Relations and an Isobar Model

The contributions of the resonances $D_{13}(1520)$, $S_{11}(1535)$,
$S_{11}(1650)$, $D_{15}(1675)$, $F_{15}(1680)$, $D_{13}(1700)$, $P_{11}(1710)$,
$P_{13}(1720)$ to $\gamma p\to \eta p$ 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 $D_{13}(1520)$, $S_{11}(1535)$, $F_{15}(1680)$ to corresponding
multipole amplitudes are stable. With this the results for $D_{13}(1520)$ and
$F_{15}(1680)$, combined with their PDG photoexcitation helicity amplitudes,
allowed us to find the branching ratios $Br (D_{13}(1520)\to \eta N)=0.05\pm
0.02%$, $Br (F_{15}(1680)\to \eta N)=0.16\pm0.04%$ which have significantly
better accuracy than the PDG data. The total Breit-Wigner width of the
$S_{11}(1535)$ is model-dependent, we have obtained $\Gamma (S_{11}(1520))=142
MeV$ and $195 MeV$ using dispersion relations and the isobar model,
respectively. The results for the $S_{11}(1650)$, $D_{15}(1675)$,
$P_{11}(1710)$, $P_{13}(1720)$ are model dependent, only the signs and orders
of magnitude of their contributions to multipole amplitudes are determined. The
results for the $D_{13}(1700)$ 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

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

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

- …