arXiv subjects

Thomas Strobl

Publications and source records attributed to Thomas Strobl.

At least 19 recordsLinked to original sources

arXiv2025

Principaloid bundles

We present a novel generalisation of principal bundles -- principaloid bundles: These are fibre bundles $\pi:P\to B$ where the typical fibre is the arrow manifold $G$ of a Lie groupoid $G\rightrightarrows M$ and the structure group is reduced to the latter's group of bisections. Each such bundle canonically comes with a bundle map $D:P\to F$ to another fibre bundle $F$ over the base $B$, with typical fibre $M$. Examples of principaloid bundles include ordinary principal $\underline G$-bundles, obtained for $G:=\underline G\rightrightarrows\bullet$, bundles associated to them, obtained for action groupoids $G:=\underline G\ltimes M$, and general fibre bundles if $G$ is a pair groupoid. While $\pi$ is far from being a principal $G$-bundle, we prove that $D$ is one. Connections on the principaloid bundle $\pi$ are thus required to be $G$-invariant Ehresmann connections. In the three examples mentioned above, this reproduces the usual types of connection for each of them. In a local description over a trivialising cover $\{O_i\}$ of $B$, the connection gives rise to Lie algebroid-valued objects living over bundle trivialisations $\{O_i\times M\}$ of $F$. Their behaviour under bundle automorphisms, including gauge transformations, is studied in detail. Finally, we construct the Atiyah-Ehresmann groupoid ${\rm At}(P)\rightrightarrows F$ which governs symmetries of $P$, this time mapping distinct $D$-fibres to one another in general. It is a fibre-bundle object in the category of Lie groupoids, with typical fibre $G\rightrightarrows M$ and base $B\times B\rightrightarrows B$. We show that those of its bisections which project to bisections of its base are in a one-to-one correspondence with automorphisms of $\pi$.

math.DG↗

arXiv2024

The minimal Lie groupoid and infinity algebroid of the singular octonionic Hopf foliation

The famous singular leaf decomposition $\mathcal{L}_{OH}$ of $\mathbb{R}^{16}\cong \mathbb{O}^2$ induced by the Hopf construction for octonions $\mathbb{O}$ has no known Lie group action generating it. In this article we construct a $\mathrm{G}_2$-equivariant Lie groupoid $\mathcal{G} \Rightarrow \mathbb{O}^{2}$ whose orbits coincide with $\mathcal{L}_{OH}$. Its Lie algebroid $E=\mathrm{Lie}(\mathcal{G})$ is of the form $\mathbb{O}^4 \to \mathbb{O}^2$ with polynomial structure functions. Its sheaf of sections induces a singular foliation $\mathcal{F}_{OH} := \rho(\Gamma(E))$ on $\mathbb{O}^{2}$, which we call the singular octonionic Hopf foliation (SOHF). $\mathcal{F}_{OH}$ is shown to be maximal among all singular foliations $\mathcal{F}$ generating $\mathcal{L}_{OH}$ -- in the polynomial, the real analytic, as well as in the smooth setting. We extend $E$ to a Lie $3$-algebroid, which is a minimal length representative of the universal Lie $\infty-$algebroid of the SOHF. This permits to prove that $E$ is the minimal rank Lie algebroid and that $\mathcal{G}$ the lowest dimensional Lie groupoid which generate the SOHF. The leaf decomposition $\mathcal{L}_{OH}$ is one of the few known examples of a singular Riemannian foliation in the sense of Molino which cannot be generated by local isometries (local non-homogeneity). We improve this result by showing that any smooth singular foliation $\mathcal{F}$ inducing $\mathcal{L}_{OH}$ cannot be even Hausdorff Morita equivalent to a singular foliation $\mathcal{F}_M$ on a Riemannian manifold $(M,g)$ generated by local isometries. Furthermore, we show that there is no real analytic singular foliation $\mathcal{F}$ generating $\mathcal{L}_{OH}$ which turns $(\mathbb{R}^{16}, g_{st}, \mathcal{F})$ into a module singular Riemannian foliation as defined in \cite{NS24}.

math.DG↗

arXiv2024

Koszul-Tate resolutions and decorated trees

Given a commutative algebra $\mathcal O$, a proper ideal $\mathcal I$, and a resolution of $\mathcal O/ \mathcal I$ by projective $\mathcal O $-modules, we construct an explicit Koszul-Tate resolution. We call it the arborescent Koszul-Tate resolution since it is indexed by decorated trees. When the $ \mathcal O$-module resolution has finite length, only finitely many operations are needed in our constructions -- this is to be compared with the classical Tate algorithm, which requires infinitely many such computations if $ \mathcal I$ is not a complete intersection. As a by-product of our construction, the initial projective $\mathcal O $-module resolution becomes equipped with an explicit $A_\infty$-algebra.

math.AC↗

arXiv2022

Singular Riemannian foliations and $\mathcal{I}$-Poisson manifolds

We recall the notion of a singular foliation (SF) on a manifold $M$, viewed as an appropriate submodule of $\mathfrak{X}(M)$, and adapt it to the presence of a Riemannian metric $g$, yielding a module version of a singular Riemannian foliation (SRF). Following Garmendia-Zambon on Hausdorff Morita equivalence of SFs, we define the Morita equivalence of SRFs (both in the module sense as well as in the more traditional geometric one of Molino) and show that the leaf spaces of Morita equivalent SRFs are isomrophic as pseudo-metric spaces. In a second part, we introduce the category of $\mathcal{I}$-Poisson manifolds. Its objects and morphisms generalize Poisson manifolds and morphisms in the presence of appropriate ideals $\mathcal{I}$ of the smooth functions on the manifold such that two conditions are satisfied: $(i)$ The category of Poisson manifolds becomes a full subcategory when choosing $\mathcal{I}=0$ and $(ii)$ there is a reduction functor from this new category to the category of Poisson algebras, which generalizes coistropic reduction to the singular setting. Every SF on $M$ gives rise to an $\mathcal{I}$-Poisson manifold on $T^*M$ and $g$ enhances this to an SRF if and only if the induced Hamiltonian lies in the normalizer of $\mathcal{I}$. This perspective provides, on the one hand, a simple proof of the fact that every module SRF is a geometric SRF and, on the other hand, a construction of an algebraic invariant of singular foliations: Hausdorff Morita equivalent SFs have isomorphic reduced Poisson algebras.

math.DG↗

arXiv2022

Topological Dirac Sigma Models and the Classical Master Equation

We present the construction of the classical Batalin-Vilkovisky action for topological Dirac sigma models. The latter are two-dimensional topological field theories that simultaneously generalise the completely gauged Wess-Zumino-Novikov-Witten model and the Poisson sigma model. Their underlying structure is that of Dirac manifolds associated to maximal isotropic and integrable subbundles of an exact Courant algebroid twisted by a 3-form. In contrast to the Poisson sigma model, the AKSZ construction is not applicable for the general Dirac sigma model. We therefore follow a direct approach for determining a suitable BV extension of the classical action functional with ghosts and antifields satisfying the classical master equation. Special attention is paid on target space covariance, which requires the introduction of two connections with torsion on the Dirac structure.

hep-th↗

arXiv2021

BFV extensions for mechanical systems with Lie-2 symmetry

We consider mechanical systems on $T^*M$ with possibly irregular and reducible first class contraints linear in the momenta, which thus correspond to singular foliations on $M$. According to a recent result, the latter ones have a Lie-infinity algebroid $(\cal M,Q)$ covering them, where we restrict to the case of Lie-2 algebroids. We propose to consider $T^*\cal M$ as a potential BFV extended phase space of the constrained system, such that the canonical lift of the nilpotent vector field $Q$ yields automatically a solution to the BFV master equation. We show that in this case, the BFV extension of the Hamiltonian, providing a second corner stone of the BFV formalism, may be obstructed. We identify the corresponding complex governing this second extension problem explicitly (the first extension problem was circumvented by means of the lift of the Lie-2 algebroid structure). We repeatedly come back to the example of angular momenta on $T^*\mathbb R^3$: in this procedure, the standard free Hamiltonian does not have a BFV extension -- while it does so on $T^*(\mathbb R^3 \backslash \{0 \})$, with a relatively involved ghost contribution singular at the origin.

hep-th↗

arXiv2020

From BFV to BV and spacetime covariance

The BFV formulation of a given gauge theory is usually significantly easier to obtain than its BV formulation. Grigoriev and Damgaard introduced simple formulas for obtaining the latter from the former. Since BFV relies on the Hamiltonian version of the gauge theory, however, it does not come as a surprise that in general the resulting BV theory does not exhibit space-time covariance. We provide an explicit example of this phenomenon in two spacetime dimensions and show how to restore covariance of the BV data by improving the Grigoriev--Damgaard procedure with appropriate adaptations of its original formulas.

hep-th↗

arXiv2019

BV and BFV for the H-twisted Poisson sigma model

We present the BFV and the BV extension of the Poisson sigma model (PSM) twisted by a closed 3-form H. There exist superfield versions of these functionals such as for the PSM and, more generally, for the AKSZ sigma models. However, in contrast to those theories, here they depend on the Euler vector field of the source manifold and contain terms mixing data from the source and the target manifold. Using an auxiliary connection $\nabla$ on the target manifold M, we obtain alternative, purely geometrical expressions without the use of superfields, which are new also for the ordinary PSM and promise straightforward adaptations to other Lie algebroid based gauge theories: The BV functional, in particular, is the sum of the classical action, the Hamiltonian lift of the (only on-shell-nilpotent) BRST differential, and a term quadratic in the antifields which is essentially the basic curvature and measures the compatibility of $\nabla$ with the Lie algebroid structure on T*M. We finally construct a Diff(M)-equivariant isomorphism between the two BV formulations.

hep-th↗

arXiv2019

Universal Cartan-Lie algebroid of an anchored bundle with connection and compatible geometries

Consider an anchored bundle $(E,\rho)$, i.e. a vector bundle $E\to M$ equipped with a bundle map $\rho \colon E \to TM$ covering the identity. M.~Kapranov showed in the context of Lie-Rinehard algebras that there exists an extension of this anchored bundle to an infinite rank universal free Lie algebroid $FR(E)\supset E$. We adapt his construction to the case of an anchored bundle equipped with an arbitrary connection, $(E,\nabla)$, and show that it gives rise to a unique connection $\tilde \nabla$ on $FR(E)$ which is compatible with its Lie algebroid structure, thus turning $(FR(E), \tilde \nabla)$ into a Cartan-Lie algebroid. Moreover, this construction is universal: any connection-preserving vector bundle morphism from $(E,\nabla)$ to a Cartan-Lie Algebroid $(A,\bar \nabla)$ factors through a unique Cartan-Lie algebroid morphism from $(FR(E), \tilde \nabla)$ to $(A,\bar \nabla)$. Suppose that, in addition, $M$ is equipped with a geometrical structure defined by some tensor field $t$ which is compatible with $(E,\rho,\nabla)$ in the sense of being annihilated by a natural $E$-connection that one can associate to these data. For example, for a Riemannian base $(M,g)$ of an involutive anchored bundle $(E,\rho)$, this condition implies that $M$ carries a Riemannian foliation. %In general, the compatibility of a tensor $t$ with $(E,\rho,\nabla)$ implies its adequate invariance transversal to $\rho(E)$. It is shown that every $E$-compatible tensor field $t$ becomes invariant with respect to the Lie algebroid representation associated canonically to the Cartan-Lie algebroid $(FR(E), \tilde \nabla)$.

math.DG↗

arXiv2019

Leibniz-Yang-Mills Gauge Theories and the 2-Higgs Mechanism

A quadratic Leibniz algebra $(\mathbb{V},[ \cdot, \cdot ],\kappa)$ gives rise to a canonical Yang-Mills type functional $S$ over every space-time manifold. The gauge fields consist of 1-forms $A$ taking values in $\mathbb{V}$ and 2-forms $B$ with values in the subspace $\mathbb{W} \subset \mathbb{V}$ generated by the symmetric part of the bracket. If the Leibniz bracket is anti-symmetric, the quadratic Leibniz algebra reduces to a quadratic Lie algebra, $B\equiv 0$, and $S$ becomes identical to the usual Yang-Mills action functional. We describe this gauge theory for a general quadratic Leibniz algebra. We then prove its (classical and quantum) equivalence to a Yang-Mills theory for the Lie algebra ${\mathfrak{g}} = \mathbb{V}/\mathbb{W}$ to which one couples massive 2-form fields living in a ${\mathfrak{g}}$-representation. Since in the original formulation the B-fields have their own gauge symmetry, this equivalence can be used as an elegant mass-generating mechanism for 2-form gauge fields, thus providing a 'higher Higgs mechanism' for those fields.

hep-th↗

arXiv2019

Transverse generalized metrics and 2d sigma models

We reformulate the compatibility condition between a generalized metric and a small (non-maximal rank) Dirac structure in an exact Courant algebroid found in the context of the gauging of strings and formulated by means of two connections in purely Dirac-geometric terms. The resulting notion, a transverse generalized metric, is also what is needed for the dynamics on the reduced phase space of a string theory.

math.DG↗

arXiv2019

Enhanced Leibniz Algebras: Structure Theorem and Induced Lie 2-Algebra

An enhanced Leibniz algebra is an algebraic struture that arises in the context of particular higher gauge theories describing self-interacting gerbes. It consists of a Leibniz algebra $(\mathbb{V},[ \cdot, \cdot ])$, a bilinear form on $\mathbb{V}$ with values in another vector space $\mathbb{W}$, and a map $t \colon \mathbb{W} \to \mathbb{V}$, satisfying altogether four compatibility relations. Our structure theorem asserts that an enhanced Leibniz algebra is uniquely determined by the underlying Leibniz algebra $(\mathbb{V},[ \cdot, \cdot ])$, an appropriate abelian ideal ${\mathfrak i}$ inside it, as well as a cohomology 2-class $[\Delta]$ which only effects the $\mathbb{W}$-valued product. Positive quadratic enhanced Leibniz algebras, as needed for the definition of a Yang-Mills type action functional, turn out to be rather restrictive on the underlying Leibniz algebra $(\mathbb{V},[ \cdot, \dot ])$: $\mathbb{V}$ has to be the hemisemidirect product of a positive quadratic Lie algebra ${\mathfrak g}$ with a ${\mathfrak g}$-module ${\mathfrak i}$, $\mathbb{V} \cong {\mathfrak g}\ltimes{\mathfrak i}$, with ${\mathfrak i}$ the above-mentioned ideal in this case. The second main result of this article is the construction of a functor from the category of such enhanced Leibniz algebras to the category of (semi-strict) Lie 2-algebras or, equivalentely, of two-term $L_\infty$-algebras.

math.AT↗

arXiv2018

The Embedding Tensor, Leibniz-Loday Algebras, and Their Higher Gauge Theories

We show that the data needed for the method of the embedding tensor employed in gauging supergravity theories are precisely those of a Leibniz algebra (with one of its induced quotient Lie algebras embedded into a rigid symmetry Lie algebra that provides an additional "represtentation constraint"). Every Leibniz algebra gives rise to a Lie n-algebra in a canonical way (for every $n\in\mathbb{N}\cup \{ \infty \}$). It is the gauging of this $L_\infty$-algebra that explains the tensor hierarchy of the bosonic sector of gauged supergravity theories. The tower of p-from gauge fields corresponds to Lyndon words of the universal enveloping algebra of the free Lie algebra of an odd vector space in this construction. Truncation to some $n$ yields the reduced field content needed in a concrete spacetime dimension.

hep-th↗

arXiv2018

The universal Lie $\infty$-algebroid of a singular foliation

We associate a Lie $\infty$-algebroid to every resolution of a singular foliation, where we consider a singular foliation as a locally generated $\mathscr{O}$-submodule of vector fields on the underlying manifold closed under Lie bracket. Here $\mathscr{O}$ can be the ring of smooth, holomorphic, or real analytic functions. The choices entering the construction of this Lie $\infty$-algebroid, including the chosen underlying resolution, are unique up to homotopy and, moreover, every other Lie $\infty$-algebroid inducing the same foliation or any of its sub-foliations factorizes through it in an up-to-homotopy unique manner. We thus call it the universal Lie $\infty$-algebroid of the singular foliation. For real analytic or holomorphic singular foliations, it can be chosen, locally, to be a Lie $n$-algebroid for some finite $n$. We show that this universal structure encodes several aspects of the geometry of the leaves of a singular foliation. In particular, it contains the holonomy algebroid and groupoid of a leaf in the sense of Androulidakis and Skandalis. But even more, each leaf carries an isotropy $L_\infty$-algebra structure that is unique up to isomorphism. It extends a minimal isotropy Lie algebra, that can be associated to each leaf, by higher brackets, which give rise to additional invariants of the foliation. As a byproduct, we construct an example of a foliation generated by $r$ vector fields for which we show by these techniques that it cannot be generated by the image through the anchor map of a Lie algebroid of the minimal rank $r$.

math.DG↗

arXiv2018

On the relation of Lie algebroids to constrained systems and their BV/BFV formulation

We observe that a system of irreducible, fiber-linear, first class constraints on T*M is equivalent to the definition of a foliation Lie algebroid over M. The BFV formulation of the constrained system is given by the Hamiltonian lift of the Vaintrob description (E[1],Q) of the Lie algebroid to its cotangent bundle T*E[1]. Affine deformations of the constraints are parametrized by the first Lie algebroid cohomology H^1_Q and lead to irreducible constraints also for much more general Lie algebroids such as Dirac structures; the modified BFV function follows by the addition of a representative of the deformation charge. Adding a Hamiltonian to the system corresponds to a metric g on M. Evolution invariance of the constraint surface introduces a connection nabla on E and one reobtains the compatibility of g with (E,rho,nabla) found previously in the literature. The covariantization of the Hamiltonian to a function on T*E[1] serves as a BFV-Hamiltonian, iff, in addition, this connection is compatible with the Lie algebroid structure, turning (E, rho, [ , ], nabla) into a Cartan-Lie algebroid. The BV formulation of the system is obtained from BFV by a (time-dependent) AKSZ procedure.

math-ph↗

arXiv2018

Integration of quadratic Lie algebroids to Riemannian Cartan-Lie groupoids

Cartan-Lie algebroids, i.e. Lie algebroids equipped with a compatible connection, permit the definition of an adjoint representation, on the fiber as well as on the tangent of the base. We call (positive) quadratic Lie algebroids, Cartan-Lie algebroids with ad-invariant (Riemannian) metrics on their fibers and base $\kappa$ and $g$, respectively. We determine the necessary and sufficient conditions for a positive quadratic Lie algebroid to integrate to a Riemmanian Cartan-Lie groupoid. Here we mean a Cartan-Lie groupoid $\mathcal{G}$ equipped with a bi-invariant and inversion invariant metric $\eta$ on $T\mathcal{G}$ such that it induces by submersion the metric $g$ on its base and its restriction to the $t$-fibers coincides with $\kappa$.

math.DG↗

arXiv2017

Gauging as constraining: the universal generalised geometry action in two dimensions

One of the central concepts in modern theoretical physics, gauge symmetry, is typically realised by lifting a finite-dimensional global symmetry group of a given functional to an infinite-dimensional local one by extending the functional to include gauge fields. In this contribution we review the construction of gauged actions for two-dimensional sigma models, considering a more general notion to be gauged, namely that of a (possibly singular) foliation. In particular, the original action does not need to have any global symmetry for this purpose. Moreover, reformulating the ungauged theory by means of auxiliary 1-form fields taking values in the generalised tangent bundle over the target, all possible such gauge theories result from restriction of these fields to take values in (possibly small) Dirac structures. This turns all the remaining 1-form fields into gauge fields and leads to the presence of a local symmetry. We recall all needed mathematical notions, those of (higher) Lie algebroids, Courant algebroids, and Dirac structures.

hep-th↗

arXiv2016

Monopole star products are non-alternative

Non-associative algebras appear in some quantum-mechanical systems, for instance if a charged particle in a distribution of magnetic monopoles is considered. Using methods of deformation quantization it is shown here, that algebras for such systems cannot be alternative, i.e. their associator cannot be completely anti-symmetric.

math-ph↗