684 research outputs found
Alternative Signature of TeV Strings
In string theory, it is well known that any hard scattering amplitude
inevitably suffers exponential suppression. We demonstrate that, if the string
scale is M_s < 2TeV, this intrinsically stringy behavior leads to a dramatic
reduction in the QCD jet production rate with very high transverse momenta p_T
> 2TeV at LHC. This suppression is sufficient to be observed in the first year
of low-luminosity running. Our prediction is based on the universal behavior of
string theory, and therefore is qualitatively model-independent. This signature
is alternative and complementary to conventional ones such as Regge resonance
(or string ball/black hole) production.Comment: a note added; version to appear in Phys. Rev. D; 11 pages, 1 eps
figure, LaTeX2e; BibTeX with utphys style use
Piecewise continuous partition function method in the theory of wave perturbations of inhomogeneous gas
The problem of wave disturbance propagation in rarefied gas in gravity field
is explored. The system of hydrodynamic-type equations for a stratified gas in
gravity field is derived from BGK equation by method of piecewise continuous
partition function. The obtained system of the equations generalizes the
Navier-Stokes at arbitrary density (Knudsen numbers). The verification of the
model is made for a limiting case of a homogeneous medium. Results are in the
good agreement with experiment and former theories at arbitrary Knudsen
numbers.Comment: 12 pages, 5 figure
Piecewise continuous distribution function method: Fluid equations and wave disturbances at stratified gas
Wave disturbances of a stratified gas are studied. The description is built
on a basis of the Bhatnagar -- Gross -- Krook (BGK) kinetic equation which is
reduced down the level of fluid mechanics. The double momenta set is introduced
inside a scheme of iterations of the equations operators, dividing the velocity
space along and opposite gravity field direction. At both half-spaces the local
equilibrium is supposed. As the result, the momenta system is derived. It
reproduce Navier-Stokes and Barnett equations at the first and second order in
high collision frequencies. The homogeneous background limit gives the known
results obtained by direct kinetics applications by Loyalka and Cheng as the
recent higher momentum fluid mechanics results of Chen, Rao and Spiegel. The
ground state declines from exponential at the Knudsen regime. The WKB solutions
for ultrasound in exponentially stratified medium are constructed in explicit
form, evaluated and plotted.Comment: 20 pages, 7 figures, 14 ISNA conference, 199
Positron emission tomography detects evidence of viability in rest technetium-99m sestamibi defects
AbstractObjectives. The purpose of this study was to determine the relative value of single-photon emission computed tomographic (SPECT) imaging at rest using technetium-99m methoxyisobutyl isonitrile (technetium-99m sestamibi) with positron emission tomography for detection of viable myocardium.Background. Recent studies comparing positron emission tomography and thallium-201 reinjection with rest technetium-99m sestamibi imaging have suggested that the latter technique underestimates myocardial viability.Methods. Twenty patients with a previous myocardial infarction underwent rest technetium-99m sestamibi imaging and positron emission tomography using fluorine (F)-18 deoxyglucose and nitrogen (N)-13 ammonia. In each patient, circumferential profile analysis was used to determine technetium-99m sestamibi, F-18 deoxyglucose and N-13 ammonia activity (expressed as percent of peak activity) in nine cardiac segments and in the perfusion defect defined by the area having technetium-99m sestamibi activity <60%. Technetium-99m sestamibi defects were graded as moderate (50% to 59% of peak activity) and severe (<50% of peak activity). Estimates of perfusion defect size were compared between technetium-99m sestamibi and N-13 ammonia.Results. Sixteen (53%) of 30 segments with moderate defects and 16 (47%) of 34 segments with severe defects had â„60% F-18 deoxyglocose activity considered indicative of viability. Fluorine-18 deoxyglucose evidence of viability was still present in 50% of segments with technetium-99m sestamibi activity <40%. There was no significant difference in the mean (± SD) technetium-99m sestamibi activity in segments with viable (40 ± 7%) and nonviable segments (49 ± 7%, p = 0.84). Of the 18 patients who had adequate F-18 deoxyglucose studies, the area of the technetium-99m sestamibi defect was viable in 5 (28%). In 16 patients (80%), perfusion defect size determined by technetium-99m sestamibi exceeded that measured by N-13 ammonia. The difference in defect size between technetium-99m sestamibi and N-13 ammonia was significantly greater in patients with viable (21 ± 9%) versus nonviable segments (7 ± 9%, p = 0.007).Conclusions. Moderate and severe rest technetium-99m sestamibi defects frequently have metabolic evidence of viability. Technetium-99m sestamibi SPECT yields larger perfusion defects than does N-13 ammonia positron emission tomography when the same threshold values are used
Thermal history of the string universe
Thermal history of the string universe based on the Brandenberger and Vafa's
scenario is examined. The analysis thereby provides a theoretical foundation of
the string universe scenario. Especially the picture of the initial oscillating
phase is shown to be natural from the thermodynamical point of view. A new tool
is employed to evaluate the multi state density of the string gas. This
analysis points out that the well-known functional form of the multi state
density is not applicable for the important region , and derives a
correct form of it.Comment: 39 pages, no figures, use revtex.sty, aps.sty, aps10.sty &
preprint.st
Preparation of decoherence-free, subradiant states in a cavity
The cause of decoherence in a quantum system can be traced back to the
interaction with the environment. As it has been pointed out first by Dicke, in
a system of N two-level atoms where each of the atoms is individually dipole
coupled to the environment, there are collective, subradiant states, that have
no dipole coupling to photon modes, and therefore they are expected to decay
slower. This property also implies that these type of states, which form an N-1
dimensional subspace of the atomic subsytem, also decohere slower. We propose a
scheme which will create such states. First the two-level atoms are placed in a
strongly detuned cavity and one of the atoms, called the control atom is
excited. The time evolution of the coupled atom-cavity system leads to an
appropriately entangled state of the atoms. By applying subsequent laser pulses
at a well defined time instant, it is possible to drive the atomic state into
the subradiant, i. e., decoherence free subspace. Up to a certain average
number of the photons, the result is independent of the state of the cavity.
The analysis of the conditions shows that this scheme is feasible with present
day techniques achieved in atom cavity interaction experiments.Comment: 5 page
Quasiparticle Density-Matrix Representation of Nonlinear TDDFT Response Functions
The time-dependent density functional (TDDFT) equations may be written either
for the Kohn-Sham orbitals in Hilbert space or for the single electron density
matrix in Liouville space. A collective-oscillator, quasiparticle,
representation of the density response of many-electron systems which
explicitly reveals the relevant electronic coherence sizes is developed using
the Liouville space representation of adiabatic TDDFT. Closed expressions for
the nonlinear density-density response are derived, eliminating the need to
solve nonlinear integral equations, as required in the Hilbert space
formulation of the response.Comment: 24 page
Differential geometry construction of anomalies and topological invariants in various dimensions
In the model of extended non-Abelian tensor gauge fields we have found new
metric-independent densities: the exact (2n+3)-forms and their secondary
characteristics, the (2n+2)-forms as well as the exact 6n-forms and the
corresponding secondary (6n-1)-forms. These forms are the analogs of the
Pontryagin densities: the exact 2n-forms and Chern-Simons secondary
characteristics, the (2n-1)-forms. The (2n+3)- and 6n-forms are gauge invariant
densities, while the (2n+2)- and (6n-1)-forms transform non-trivially under
gauge transformations, that we compare with the corresponding transformations
of the Chern-Simons secondary characteristics. This construction allows to
identify new potential gauge anomalies in various dimensions.Comment: 27 pages, references added, matches published versio
Critical points and supersymmetric vacua, III: String/M models
A fundamental problem in contemporary string/M theory is to count the number
of inequivalent vacua satisfying constraints in a string theory model. This
article contains the first rigorous results on the number and distribution of
supersymmetric vacua of type IIb string theories compactified on a Calabi-Yau
3-fold with flux. In particular, complete proofs of the counting formulas
in Ashok-Douglas and Denef-Douglas are given, together with van der Corput
style remainder estimates. We also give evidence that the number of vacua
satisfying the tadpole constraint in regions of bounded curvature in moduli
space is of exponential growth in .Comment: Final revision for publication in Commun. Math. Phys. Minor
corrections and editorial change
Abelian and nonabelian vector field effective actions from string field theory
The leading terms in the tree-level effective action for the massless fields
of the bosonic open string are calculated by integrating out all massive fields
in Witten's cubic string field theory. In both the abelian and nonabelian
theories, field redefinitions make it possible to express the effective action
in terms of the conventional field strength. The resulting actions reproduce
the leading terms in the abelian and nonabelian Born-Infeld theories, and
include (covariant) derivative corrections.Comment: 49 pages, 1 eps figur
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