4,690 research outputs found
New Examples of Flux Vacua
Type IIB toroidal orientifolds are among the earliest examples of flux vacua.
By applying T-duality, we construct the first examples of massive IIA flux
vacua with Minkowski space-times, along with new examples of type IIA flux
vacua. The backgrounds are surprisingly simple with no four-form flux at all.
They serve as illustrations of the ingredients needed to build type IIA and
massive IIA solutions with scale separation. To check that these backgrounds
are actually solutions, we formulate the complete set of type II supergravity
equations of motion in a very useful form that treats the R-R fields
democratically.Comment: 38 pages, LaTeX; references updated; additional minor comments added;
published versio
On Feedback Vertex Set: New Measure and New Structures
We present a new parameterized algorithm for the {feedback vertex set}
problem ({\sc fvs}) on undirected graphs. We approach the problem by
considering a variation of it, the {disjoint feedback vertex set} problem ({\sc
disjoint-fvs}), which finds a feedback vertex set of size that has no
overlap with a given feedback vertex set of the graph . We develop an
improved kernelization algorithm for {\sc disjoint-fvs} and show that {\sc
disjoint-fvs} can be solved in polynomial time when all vertices in have degrees upper bounded by three. We then propose a new
branch-and-search process on {\sc disjoint-fvs}, and introduce a new
branch-and-search measure. The process effectively reduces a given graph to a
graph on which {\sc disjoint-fvs} becomes polynomial-time solvable, and the new
measure more accurately evaluates the efficiency of the process. These
algorithmic and combinatorial studies enable us to develop an
-time parameterized algorithm for the general {\sc fvs} problem,
improving all previous algorithms for the problem.Comment: Final version, to appear in Algorithmic
M-Theory and Maximally Supersymmetric Gauge Theories
In this informal review for non-specalists we discuss the construction of
maximally supersymmetric gauge theories that arise on the worldvolumes branes
in String Theory and M-Theory. Particular focus is made on the relatively
recent construction of M2-brane worldvolume theories. In a formal sense, the
existence of these quantum field theories can be viewed as predictions of
M-Theory. Their construction is therefore a reinforcement of the ideas
underlying String Theory and M-Theory. We also briefly discuss the
six-dimensional conformal field theory that is expected to arise on M5-branes.
The construction of this theory is not only an important open problem for
M-Theory but also a significant challenge to our current understanding of
quantum field theory more generally.Comment: Minor corrections and an added acknowledgement. Accepted for
publication in Annual Review
Moduli Stabilisation in Heterotic Models with Standard Embedding
In this note we analyse the issue of moduli stabilisation in 4d models
obtained from heterotic string compactifications on manifolds with SU(3)
structure with standard embedding. In order to deal with tractable models we
first integrate out the massive fields. We argue that one can not only
integrate out the moduli fields, but along the way one has to truncate also the
corresponding matter fields. We show that the effective models obtained in this
way do not have satisfactory solutions. We also look for stabilised vacua which
take into account the presence of the matter fields. We argue that this also
fails due to a no-go theorem for Minkowski vacua in the moduli sector which we
prove in the end. The main ingredient for this no-go theorem is the constraint
on the fluxes which comes from the Bianchi identity.Comment: 20 pages, LaTeX; references adde
D-SPACE4Cloud: A Design Tool for Big Data Applications
The last years have seen a steep rise in data generation worldwide, with the
development and widespread adoption of several software projects targeting the
Big Data paradigm. Many companies currently engage in Big Data analytics as
part of their core business activities, nonetheless there are no tools and
techniques to support the design of the underlying hardware configuration
backing such systems. In particular, the focus in this report is set on Cloud
deployed clusters, which represent a cost-effective alternative to on premises
installations. We propose a novel tool implementing a battery of optimization
and prediction techniques integrated so as to efficiently assess several
alternative resource configurations, in order to determine the minimum cost
cluster deployment satisfying QoS constraints. Further, the experimental
campaign conducted on real systems shows the validity and relevance of the
proposed method
M-theory and Type IIA Flux Compactifications
We consider compactifications of M-theory and type IIA string theory to four
dimensions. For Minkowski space-time, a supergravity no-go theorem forbids flux
supported in the internal space. We show how to evade this no-go theorem by
exhibiting new sources of brane charge: in string theory, the basic physical
phenomenon is the generation of new brane charges from D-branes in transverse
fluxes. In M-theory, there is a new source of M5-brane charge from novel higher
derivative couplings that involve fluxes as well as curvatures. We present some
explicit orientifold examples with both N=1 and N=2 space-time supersymmetry.
Finally, we explain the status of massive type IIA flux compactifications.Comment: 36 pages, LaTeX; references adde
On RR couplings on D-branes at order
Recently, it has been found that there are couplings of the RR field strength
and the B-field strength on the world volume of D-branes at
order . These couplings which have both world-volume and
transverse indices, are invariant under the linear T-duality transformations.
Consistency with the nonlinear T-duality indicates that the RR field strength
in these couplings should be replaced by where . This replacement, however, reproduces some
non-gauge invariant terms. On the other hand, the nonlinear terms are invariant
under the linear T-duality transformations at the level of two B-fields. This
allows one to remove some of the nonlinear terms in . We fix
this by comparing the nonlinear couplings with the S-matrix element of one RR
and two NSNS vertex operators. Our results indicate that in the expansion of
one should keep only the B-field gauge invariant terms, e.g.
where both indices of B-field lie along the brane.
Moreover, in this case one should replace with to have the
-field gauge invariance.Comment: 23 pages, Latex file, 1 figure; v2:typos corrected, to appear in JHE
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