183 research outputs found
Yang--Mills sphalerons in all even spacetime dimensions , : =3,4
The classical solutions to higher dimensional Yang--Mills (YM) systems, which
are integral parts of higher dimensional Einstein--YM (EYM) systems, are
studied. These are the gravity decoupling limits of the fully gravitating EYM
solutions. In odd spacetime dimensions, depending on the choice of gauge group,
these are either topologically stable or unstable. Both cases are analysed, the
latter numerically only. In even spacetime dimensions they are always unstable,
describing saddle points of the energy, and can be described as {\it
sphalerons}. This instability is analysed by constructing the noncontractible
loops and calculating the Chern--Simons (CS) charges, and also perturbatively
by numerically constructing the negative modes. This study is restricted to the
simplest YM system in spacetime dimensions , which is amply
illustrative of the generic case.Comment: 16 pages, 3 figures ; comments added, to appear in J. Phys.
Gauge-Invariant Resummation Formalism and Unitarity in Non-Commutative QED
We re-examine the perturbative properties of four-dimensional non-commutative
QED by extending the pinch techniques to the theta-deformed case. The explicit
independence of the pinched gluon self-energy from gauge-fixing parameters, and
the absence of unphysical thresholds in the resummed propagators permits a
complete check of the optical theorem for the off-shell two-point function. The
known anomalous (tachyonic) dispersion relations are recovered within this
framework, as well as their improved version in the (softly broken) SUSY case.
These applications should be considered as a first step in constructing
gauge-invariant truncations of the Schwinger-Dyson equations in the
non-commutative case. An interesting result of our formalism appears when
considering the theory in two dimensions: we observe a finite gauge-invariant
contribution to the photon mass because of a novel incarnation of IR/UV mixing,
which survives the commutative limit when matter is present.Comment: 30 pages, 2 eps figure, uses axodraw. Citations adde
Moduli Stabilization from Fluxes in a Simple IIB Orientifold
We study novel type IIB compactifications on the T^6/Z_2 orientifold. This
geometry arises in the T-dual description of Type I theory on T^6, and one
normally introduces 16 space-filling D3-branes to cancel the RR tadpoles. Here,
we cancel the RR tadpoles either partially or fully by turning on three-form
flux in the compact geometry. The resulting (super)potential for moduli is
calculable. We demonstrate that one can find many examples of N=1
supersymmetric vacua with greatly reduced numbers of moduli in this system. A
few examples with N>1 supersymmetry or complete supersymmetry breaking are also
discussed.Comment: 49 pages, harvmac big; v2, corrected some typo
Brane Decay and Death of Open Strings
We show how open strings cease to propagate when unstable D-branes decay. The
information on the propagation is encoded in BSFT two-point functions for
arbitrary profiles of open string excitations. We evaluate them in tachyon
condensation backgrounds corresponding to (i) static spatial tachyon kink (=
lower dimensional BPS D-brane) and (ii) homogeneous rolling tachyon. For (i)
the propagation is restricted to the directions along the tachyon kink, while
for (ii) all the open string excitations cease to propagate at late time and
are subject to a collapsed light cone characterized by Carrollian contraction
of Lorentz group.Comment: 19 pages, published version (typos corrected, a reference added
Mineralogy and petrology of comet 81P/wild 2 nucleus samples
The bulk of the comet 81P/Wild 2 (hereafter Wild 2) samples returned to Earth by the Stardust spacecraft appear to be weakly constructed mixtures of nanometer-scale grains, with occasional much larger (over 1 micrometer) ferromagnesian silicates, Fe-Ni sulfides, Fe-Ni metal, and accessory phases. The very wide range of olivine and low-Ca pyroxene compositions in comet Wild 2 requires a wide range of formation conditions, probably reflecting very different formation locations in the protoplanetary disk. The restricted compositional ranges of Fe-Ni sulfides, the wide range for silicates, and the absence of hydrous phases indicate that comet Wild 2 experienced little or no aqueous alteration. Less abundant Wild 2 materials include a refractory particle, whose presence appears to require radial transport in the early protoplanetary disk
Rise and Demise of Bioinformatics? Promise and Progress
The field of bioinformatics and computational biology has gone through a number of transformations during the past 15 years, establishing itself as a key component of new biology. This spectacular growth has been challenged by a number of disruptive changes in science and technology. Despite the apparent fatigue of the linguistic use of the term itself, bioinformatics has grown perhaps to a point beyond recognition. We explore both historical aspects and future trends and argue that as the field expands, key questions remain unanswered and acquire new meaning while at the same time the range of applications is widening to cover an ever increasing number of biological disciplines. These trends appear to be pointing to a redefinition of certain objectives, milestones, and possibly the field itself
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