12,239 research outputs found
A simple remark on a flat projective morphism with a Calabi-Yau fiber
If a K3 surface is a fiber of a flat projective morphisms over a connected
noetherian scheme over the complex number field, then any smooth connected
fiber is also a K3 surface. Observing this, Professor Nam-Hoon Lee asked if the
same is true for higher dimensional Calabi-Yau fibers. We shall give an
explicit negative answer to his question as well as a proof of his initial
observation.Comment: 8 pages, main theorem is generalized, one more remark is added,
mis-calculation and typos are corrected etc
The New Gas Stripper and Charge State Separator of the Gsi High Current Injector
The GSI Unilac was upgraded as a high current injector for the SIS in 1999.
Therefore, the stripping section [1] at 1.4 MeV/u, where a beam transport under
highest space charge conditions and multi beam operation had to be established,
was completely new designed. Results of the commissioning of the stripper
section will be presented. The beam transport to the new gas stripper, and the
charge state analysis under space charge conditions confirmed the calculations.
A 238U4+ beam - coming from a MEVVA ion source and accelerated by the new
injector linac- was stripped (with the expected stripping efficiency) to the
charge state 28+ and successfully separated by the new spectrometer (15 degree
and 30 degree kicker magnets). The transport and matching to the poststripper
accelerator under highest space charge conditions was investigated with a 15
emA 40Ar10+ beam. Especially space charge and charge state dependent emittance
growth effects in 6d-phase space will be discussed.Comment: Linac 2000, MOD13, 3 pages, 7 figure
Analysing the Security of Google's implementation of OpenID Connect
Many millions of users routinely use their Google accounts to log in to
relying party (RP) websites supporting the Google OpenID Connect service.
OpenID Connect, a newly standardised single-sign-on protocol, builds an
identity layer on top of the OAuth 2.0 protocol, which has itself been widely
adopted to support identity management services. It adds identity management
functionality to the OAuth 2.0 system and allows an RP to obtain assurances
regarding the authenticity of an end user. A number of authors have analysed
the security of the OAuth 2.0 protocol, but whether OpenID Connect is secure in
practice remains an open question. We report on a large-scale practical study
of Google's implementation of OpenID Connect, involving forensic examination of
103 RP websites which support its use for sign-in. Our study reveals serious
vulnerabilities of a number of types, all of which allow an attacker to log in
to an RP website as a victim user. Further examination suggests that these
vulnerabilities are caused by a combination of Google's design of its OpenID
Connect service and RP developers making design decisions which sacrifice
security for simplicity of implementation. We also give practical
recommendations for both RPs and OPs to help improve the security of real world
OpenID Connect systems
Quantum versus classical phase-locking transition in a driven-chirped oscillator
Classical and quantum-mechanical phase locking transition in a nonlinear
oscillator driven by a chirped frequency perturbation is discussed. Different
limits are analyzed in terms of the dimensionless parameters and
( and being the driving amplitude,
the frequency chirp rate, the nonlinearity parameter and the linear frequency
of the oscillator). It is shown that for , the passage
through the linear resonance for above a threshold yields classical
autoresonance (AR) in the system, even when starting in a quantum ground state.
In contrast, for , the transition involves
quantum-mechanical energy ladder climbing (LC). The threshold for the
phase-locking transition and its width in in both AR and LC limits are
calculated. The theoretical results are tested by solving the Schrodinger
equation in the energy basis and illustrated via the Wigner function in phase
space
A Study of the Direct-Fitting Method for Measurement of Galaxy Velocity Dispersions
We have measured the central stellar velocity dispersions of 33 nearby spiral
and elliptical galaxies, using a straightforward template-fitting algorithm
operating in the pixel domain. The spectra, obtained with the Double
Spectrograph at Palomar Observatory, cover both the Ca triplet and the Mg b
region, and we present a comparison of the velocity dispersion measurements
from these two spectral regions. Model fits to the Ca triplet region generally
yield good results with little sensitivity to the choice of template star. In
contrast, the Mg b region is more sensitive to template mismatch and to details
of the fitting procedure such as the order of a polynomial used to match the
continuum shape of the template to the object. As a consequence of the
correlation of the [Mg/Fe] ratio with velocity dispersion, it is difficult to
obtain a satisfactory model fit to the Mg b lines and the surrounding Fe blends
simultaneously, particularly for giant elliptical galaxies with large velocity
dispersions. We demonstrate that if the metallicities of the galaxy and
template star are not well matched, then direct template-fitting results are
improved if the Mg b lines themselves are excluded from the fit and the
velocity dispersion is determined from the surrounding weaker lines.Comment: 14 pages. To appear in A
Schur Q-functions and degeneracy locus formulas for morphisms with symmetries
We give closed-form formulas for the fundamental classes of degeneracy loci
associated with vector bundle maps given locally by (not necessary square)
matrices which are symmetric (resp. skew-symmetric) w.r.t. the main diagonal.
Our description uses essentially Schur Q-polynomials of a bundle, and is based
on a certain push-forward formula for these polynomials in a Grassmann bundle.Comment: 22 pages, AMSTEX, misprints corrected, exposition improved. to appear
in the Proceedings of Intersection Theory Conference in Bologna, "Progress in
Mathematics", Birkhause
On explicit results at the intersection of the Z_2 and Z_4 orbifold subvarieties in K3 moduli space
We examine the recently found point of intersection between the Z_2 and Z_4
orbifold subvarieties in the K3 moduli space more closely. First we give an
explicit identification of the coordinates of the respective Z_2 and Z_4
orbifold theories at this point. Secondly we construct the explicit
identification of conformal field theories at this point and show the
orthogonality of the two subvarieties.Comment: Latex, 23 page
Electronic properties of disordered corner-sharing tetrahedral lattices
We have examined the behaviour of noninteracting electrons moving on a
corner-sharing tetrahedral lattice into which we introduce a uniform (box)
distribution, of width W, of random on-site energies. We have used both the
relative localization length and the spectral rigidity to analyze the nature of
the eigenstates, and have determined both the mobility edge trajectories as a
function of W, and the critical disorder, Wc, beyond which all states are
localized. We find (i) that the mobility edge trajectories (energies Ec vs.
disorder W) are qualitatively different from those found for a simple cubic
lattice, and (ii) that the spectral rigidity is scale invariant at Wc and thus
provides a reliable method of estimating this quantity -- we find Wc/t=14.5. We
discuss our results in the context of the metal-to-insulator transition
undergone by LiAlyTi{2-y}O4 in a quantum site percolation model that also
includes the above-mentioned Anderson disorder, and show that the effects
produced by Anderson disorder are far less important than those produced by
quantum site percolation, at least in the determination of the doping
concentration at which the metal-to-insulator transition is predicted to occur
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