160 research outputs found
Physical Fock space of tensionless strings
We study the physical Fock space of the tensionless string theory with
perimeter action which has pure massless spectrum. The states are classified by
the Wigner's little group for massless particles. The ground state contains
infinite many massless fields of fixed helicity, the excitation levels realize
CSR representations. We demonstrate that the first and the second excitation
levels are physical null states.Comment: Latex file, 12 page
Modeling and Control of Rare Segments in BitTorrent with Epidemic Dynamics
Despite its existing incentives for leecher cooperation, BitTorrent file
sharing fundamentally relies on the presence of seeder peers. Seeder peers
essentially operate outside the BitTorrent incentives, with two caveats: slow
downlinks lead to increased numbers of "temporary" seeders (who left their
console, but will terminate their seeder role when they return), and the
copyright liability boon that file segmentation offers for permanent seeders.
Using a simple epidemic model for a two-segment BitTorrent swarm, we focus on
the BitTorrent rule to disseminate the (locally) rarest segments first. With
our model, we show that the rarest-segment first rule minimizes transition time
to seeder (complete file acquisition) and equalizes the segment populations in
steady-state. We discuss how alternative dissemination rules may {\em
beneficially increase} file acquisition times causing leechers to remain in the
system longer (particularly as temporary seeders). The result is that leechers
are further enticed to cooperate. This eliminates the threat of extinction of
rare segments which is prevented by the needed presence of permanent seeders.
Our model allows us to study the corresponding trade-offs between performance
improvement, load on permanent seeders, and content availability, which we
leave for future work. Finally, interpreting the two-segment model as one
involving a rare segment and a "lumped" segment representing the rest, we study
a model that jointly considers control of rare segments and different uplinks
causing "choking," where high-uplink peers will not engage in certain
transactions with low-uplink peers.Comment: 18 pages, 6 figures, A shorter version of this paper that did not
include the N-segment lumped model was presented in May 2011 at IEEE ICC,
Kyot
Extension of Chern-Simons forms and new gauge anomalies
We present a general analysis of gauge invariant, exact and metric
independent forms which can be constructed using higher rank field-strength
tensors. The integrals of these forms over the corresponding space-time
coordinates provides new topological Lagrangians. With these Lagrangians one
can define gauge field theories which generalize the Chern-Simons quantum field
theory. We also present explicit expressions for the potential gauge anomalies
associated with the tensor gauge fields and classify all possible anomalies
that can appear in lower dimensions. Of special interest are those which can be
constructed in four, six, eight and ten dimensions.Comment: LaTeX file, 15 page
CSMA Local Area Networking under Dynamic Altruism
In this paper, we consider medium access control of local area networks
(LANs) under limited-information conditions as befits a distributed system.
Rather than assuming "by rule" conformance to a protocol designed to regulate
packet-flow rates (e.g., CSMA windowing), we begin with a non-cooperative game
framework and build a dynamic altruism term into the net utility. The effects
of altruism are analyzed at Nash equilibrium for both the ALOHA and CSMA
frameworks in the quasistationary (fictitious play) regime. We consider either
power or throughput based costs of networking, and the cases of identical or
heterogeneous (independent) users/players. In a numerical study we consider
diverse players, and we see that the effects of altruism for similar players
can be beneficial in the presence of significant congestion, but excessive
altruism may lead to underuse of the channel when demand is low
Logarithmic loop corrections, moduli stabilisation and de Sitter vacua in string theory
We study string loop corrections to the gravity kinetic terms in type IIB com- pactifications on Calabi-Yau threefolds or their orbifold limits, in the presence of D7-branes and orientifold planes. We show that they exhibit in general a logarithmic behaviour in the large volume limit transverse to the D7-branes, induced by a localised four-dimensional Einstein-Hilbert action that appears at a lower order in the closed string sector, found in the past. Here, we compute the coefficient of the logarithmic corrections and use them to provide an explicit realisation of a mechanism for K ̈ahler moduli stabilisation that we have proposed recently, which does not rely on non-perturbative effects and lead to de Sit- ter vacua. Our result avoids no-go theorems of perturbative stabilisation due to runaway potentials, in a way similar to the Coleman-Weinberg mechanism, and provides a counter example to one of the swampland conjectures concerning de Sitter vacua in quantum grav- ity, once string loop effects are taken into account; it thus paves the way for embedding the Standard Model of particle physics and cosmology in string theory
Inflation near a metastable de Sitter vacuum from moduli stabilisation
We study the cosmological properties of a metastable de Sitter vacuum
obtained recently in the framework of type IIB flux compactifications in the
presence of three D7-brane stacks, based on perturbative quantum corrections at
both world-sheet and string loop level that are dominant at large volume and
weak coupling. In the simplest case, the model has one effective parameter
controlling the shape of the potential of the inflaton which is identified with
the volume modulus. The model provides a phenomenological successful
small-field inflation for a value of the parameter that makes the minimum very
shallow and near the maximum. The horizon exit is close to the inflection point
while most of the required e-folds of the Universe expansion are generated near
the minimum, with a prediction for the ratio of tensor-to-scalar primordial
fluctuations . Despite its shallowness, the minimum
turns out to be practically stable. We show that it can decay only through the
Hawking-Moss instanton leading to an extremely long decay rate. Obviously, in
order to end inflation and obtain a realistic model, new low-energy physics is
needed around the minimum, at intermediate energy scales of order
GeV. An attractive possibility is by introducing a "waterfall"' field within
the framework of hybrid inflation.Comment: 25 pages, 9 figures; v2: published version, minor correction
Extensions of the Poincare group
We construct an extension of the Poincare group which involves a mixture of
internal and space-time supersymmetries. The resulting group is an extension of
the superPoincare group with infinitely many generators which carry internal
and space-time indices. It is a closed algebra since all Jacobi identities are
satisfied and it has therefore explicit matrix representations. We investigate
the massless case and construct the irreducible representations of the extended
symmetry. They are divided into two sets, longitudinal and transversal
representations. The transversal representations involve an infinite series of
integer and half-integer helicities. Finally we suggest an extension of the
conformal group along the same line.Comment: 13 pages, LaTex fil
Kaluza-Klein Thresholds and Regularization (In)dependence
We present a method to control the regularization scheme dependence in the
running of couplings in Kaluza-Klein theories. Specifically we consider the
scalar theory in five dimensions, assuming that one dimension is compactified
and we study various regularization schemes in order to analyze concretely the
regularization scheme dependence of the Kaluza-Klein threshold effects. We find
that in one-loop order, although the -functions are different for the
different schemes, the net difference in the running of the coupling among the
different schemes is very small for the entire range of energies. Our results
have been extended to include more than one radii, and the gauge coupling
unification is re-examined. Strings are also used as a regulator. We obtain a
particular regularization of effective field theory which can accurately
describe the string Kaluza-Klein threshold effects.Comment: 39 pages, 12 figures uses epsf.st
Resource Management for the Construction Industry
The management of resources is an essential task in each construction company. Today, ERP systems and e-Business systems are available to assist construction companies to efficiently organise the allocation of their personnel and equipment within the company, but they cannot provide the company with the idle resources for every single task that has to be performed during a construction project. Therefore, companies should have an alternative solution to better exploit expensive resources and compensate their fixed costs, but also have them available at the right time for their own business activities. This paper outlines the approach taken by the EU funded project “e-Sharing” (IST-2001-33325) to support resource management between construction companies. It will describe requirements for the management of construction resources, its core features, and the integration approach. Therefore, we will outline the approach of an integrated resource type model supporting the management and classification of construction equipment, construction tasks and qualification profiles. The development is based on a cross-domain analysis and evaluation of existing models. ..
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