539 research outputs found
Fluidized-bed combustion reduces atmospheric pollutants
Method of reducing sulfur and nitrogen oxides released during combustion of fossil fuels is described. Fuel is burned in fluidized bed of solids with simultaneous feeding of crushed or pulverized limestone to control emission. Process also offers high heat transfer rates and efficient contacting for gas-solid reactions
-algebras and membrane sigma models
Membrane sigma-models have been used for the systematic description of closed
strings in non-geometric flux backgrounds. In particular, the conditions for
gauge invariance of the corresponding action functionals were related to the
Bianchi identities for the fluxes. In this contribution we demonstrate how to
express these Bianchi identities in terms of homotopy relations of the
underlying -algebra for the case of the Courant sigma-model. We argue
that this result can be utilized in understanding the constraint structure of
Double Field Theory and the corresponding membrane sigma-model.Comment: 13 pages; contribution to the proceedings of the Humboldt Kolleg
Frontiers in Physics: From the Electroweak to the Planck Scales, 15 - 19
September 2019, Corfu, Greec
Gauge Theory on Twisted -Minkowski: Old Problems and Possible Solutions
We review the application of twist deformation formalism and the construction
of noncommutative gauge theory on -Minkowski space-time. We compare two
different types of twists: the Abelian and the Jordanian one. In each case we
provide the twisted differential calculus and consider gauge theory.
Different methods of obtaining a gauge invariant action and related
problems are thoroughly discussed
Dirac structures on nilmanifolds and coexistence of fluxes
We study some aspects of the generalized geometry of nilmanifolds and examine
to which extent different types of fluxes can coexist on them. Nilmanifolds
constitute a class of homogeneous spaces which are interesting in string
compactifications with fluxes since they carry geometric flux by construction.
They are generalized Calabi-Yau spaces and therefore simple examples of
generalized geometry at work. We identify and classify Dirac structures on
nilmanifolds, which are maximally isotropic subbundles closed under the Courant
bracket. In the presence of non-vanishing fluxes, these structures are twisted
and closed under appropriate extensions of the Courant bracket. Twisted Dirac
structures on a nilmanifold may carry multiple coexistent fluxes of any type.
We also show how dual Dirac structures combine to Courant algebroids and work
out an explicit example where all types of generalized fluxes coexist. These
results may be useful in the context of general flux compactifications in
string theory.Comment: 1+25 pages; v2: clarifying comments and 6 references added, published
versio
T-duality without isometry via extended gauge symmetries of 2D sigma models
Target space duality is one of the most profound properties of string theory.
However it customarily requires that the background fields satisfy certain
invariance conditions in order to perform it consistently; for instance the
vector fields along the directions that T-duality is performed have to generate
isometries. In the present paper we examine in detail the possibility to
perform T-duality along non-isometric directions. In particular, based on a
recent work of Kotov and Strobl, we study gauged 2D sigma models where gauge
invariance for an extended set of gauge transformations imposes weaker
constraints than in the standard case, notably the corresponding vector fields
are not Killing. This formulation enables us to follow a procedure analogous to
the derivation of the Buscher rules and obtain two dual models, by integrating
out once the Lagrange multipliers and once the gauge fields. We show that this
construction indeed works in non-trivial cases by examining an explicit class
of examples based on step 2 nilmanifolds.Comment: 1+18 pages; version 2: corrections and improvements, more complete
version than the published on
Sigma models for genuinely non-geometric backgrounds
The existence of genuinely non-geometric backgrounds, i.e. ones without
geometric dual, is an important question in string theory. In this paper we
examine this question from a sigma model perspective. First we construct a
particular class of Courant algebroids as protobialgebroids with all types of
geometric and non-geometric fluxes. For such structures we apply the
mathematical result that any Courant algebroid gives rise to a 3D topological
sigma model of the AKSZ type and we discuss the corresponding 2D field
theories. It is found that these models are always geometric, even when both
2-form and 2-vector fields are neither vanishing nor inverse of one another.
Taking a further step, we suggest an extended class of 3D sigma models, whose
world volume is embedded in phase space, which allow for genuinely
non-geometric backgrounds. Adopting the doubled formalism such models can be
related to double field theory, albeit from a world sheet perspective.Comment: 1+34 pages, v2. added references and additional comments; published
versio
Matrix theory compactifications on twisted tori
We study compactifications of Matrix theory on twisted tori and
non-commutative versions of them. As a first step, we review the construction
of multidimensional twisted tori realized as nilmanifolds based on certain
nilpotent Lie algebras. Subsequently, matrix compactifications on tori are
revisited and the previously known results are supplemented with a background
of a non-commutative torus with non-constant non-commutativity and an
underlying non-associative structure on its phase space. Next we turn our
attention to 3- and 6-dimensional twisted tori and we describe consistent
backgrounds of Matrix theory on them by stating and solving the conditions
which describe the corresponding compactification. Both commutative and
non-commutative solutions are found in all cases. Finally, we comment on the
correspondence among the obtained solutions and flux compactifications of
11-dimensional supergravity, as well as on relations among themselves, such as
Seiberg-Witten maps and T-duality.Comment: 1+31 pages, v2: some comments and clarifications added, accepted for
publication in Physical Review
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