Higher Dimensional Field Theories Research Articles

PT -symmetric quantum electrodynamics and unitarity

More than 15 years ago, a new approach to quantum mechanics was suggested, in which Hermiticity of the Hamiltonian was to be replaced by invariance under a discrete symmetry, the product of parity and time-reversal symmetry, . It was shown that, if is unbroken, energies were, in fact, positive, and unitarity was satisfied. Since quantum mechanics is quantum field theory in one dimension—time—it was natural to extend this idea to higher-dimensional field theory, and in fact an apparently viable version of -invariant quantum electrodynamics (QED) was proposed. However, it has proved difficult to establish that the unitarity of the scattering matrix, for example, the Källén spectral representation for the photon propagator, can be maintained in this theory. This has led to questions of whether, in fact, even quantum mechanical systems are consistent with probability conservation when Green’s functions are examined, since the latter have to possess physical requirements of analyticity. The status of QED will be reviewed in this paper, as well as the general issue of unitarity.

Extensions of the conformal representations for orthogonal Lie algebras

The conformal transformations with respect to the metric defining o(n,C) give rise to an inhomogeneous polynomial representation of o(n+2,C). Using Shenʼs technique of mixed product, we generalize the above representation to an inhomogeneous representation of o(n+2,C) on the tensor space of any finite-dimensional irreducible o(n,C)-module with the polynomial space, where a hidden central transformation is involved. Moreover, we find a condition on the constant value taken by the central transformation such that the generalized representation is irreducible. In our approach, Pieriʼs formulas, invariant operators and the idea of Kostantʼs characteristic identities play key roles. The result could be useful in understanding higher-dimensional conformal field theory with the constant value taken by the central transformation as the central charge. Our representations virtually provide natural extensions of the conformal transformations on a Riemannian manifold to its vector bundles.

Noninteraction of Waves in Two-dimensional Conformal Field Theory

In higher dimensional quantum field theory, irreducible representations of the Poincare group are associated with particles. Their counterpart in two-dimensional massless models are "waves" introduced by Buchholz. In this paper we show that waves do not interact in two-dimensional Moebius covariant theories and in- and out-asymptotic fields coincide. We identify the set of the collision states of waves with the subspace generated by the chiral components of the Moebius covariant net from the vacuum. It is also shown that Bisognano-Wichmann property, dilation covariance and asymptotic completeness (with respect to waves) imply Moebius symmetry. Under natural assumptions, we observe that the maps which give asymptotic fields in Poincare covariant theory are conditional expectations between appropriate algebras. We show that a two-dimensional massless theory is asymptotically complete and noninteracting if and only if it is a chiral Moebius covariant theory.

Bifundamental Fuzzy 2-Sphere and Fuzzy Killing Spinors

We review our construction of a bifundamental version of the fuzzy 2-sphere and its relation to fuzzy Killing spinors, first obtained in the context of the ABJM membrane model. This is shown to be completely equivalent to the usual (adjoint) fuzzy sphere. We discuss the mathematical details of the bifundamental fuzzy sphere and its field theory expansion in a model-independent way. We also examine how this new formulation affects the twisting of the fields, when comparing the field theory on the fuzzy sphere background with the compactification of the 'deconstructed' (higher dimensional) field theory.

Casimir energy of the universe and new regularization of higher dimensional quantum field theories

Casimir energy is calculated for the 5D electromagnetism and 5D scalar theory in the warped geometry. It is compared with the flat case. A new regularization, called sphere lattice regularization, is taken. In the integration over the 5D space, we introduce two boundary curves (IR-surface and UV-surface) based on the minimal area principle. It is a direct realization of the geometrical approach to the renormalization group. The regularized configuration is closed-string like. We do not take the KK-expansion approach. Instead, the position/momentum propagator is exploited, combined with the heat-kernel method. All expressions are closed-form (not KK-expanded form). The generalized P/M propagators are introduced. We numerically evaluate Λ(4D UV-cutoff), ω(5D bulk curvature, warp parameter) and T(extra space IR parameter) dependence of the Casimir energy. We present two new ideas in order to define the 5D QFT: 1) the summation (integral) region over the 5D space is restricted by two minimal surfaces (IR-surface, UV-surface); or 2) we introduce a weight function and require the dominant contribution, in the summation, is given by the minimal surface. Based on these, 5D Casimir energy is finitely obtained after the proper renormalization procedure. The warp parameter ω suffers from the renormalization effect. The IR parameter T does not. We examine the meaning of the weight function and finally reach a new definition of the Casimir energy where the 4D momenta (or coordinates) are quantized with the extra coordinate as the Euclidean time (inverse temperature). We examine the cosmological constant problem and present an answer at the end. Dirac's large number naturally appears.

Covering data and higher dimensional global class field theory

For a connected regular scheme X, flat and of finite type over Spec ( Z ) , we construct a reciprocity homomorphism ρ X : C X → π 1 ab ( X ) , which is surjective and whose kernel is the connected component of the identity. The (topological) group C X is explicitly given and built solely out of data attached to points and curves on X. A similar but weaker statement holds for smooth varieties over finite fields. Our results are based on earlier work of G. Wiesend.

Field localization on a brane intersection in anti–de Sitter spacetime

We discuss the localization of scalar, fermion, and gauge field zero modes on a 3-brane that resides at the intersection of two 4-branes in six-dimensional anti-de Sitter space. This setup has been introduced in the context of braneworld models and, higher-dimensional versions of it, in string theory. In both six- and ten-dimensional cases, it has been shown that four-dimensional gravity can be reproduced at the intersection, due to the existence of a massless, localized graviton zero-mode. However, realistic scenarios require also the standard model to be localized on the 3-brane. In this paper, we discuss under which conditions a higher-dimensional field theory, propagating on the above geometry, can have a zero-mode sector localized at the intersection and find that zero modes can be localized only if masses and couplings to the background curvature satisfy certain relations. We also consider the case when other 4-branes cut the bulk at some distance from the intersection and argue that, in the probe brane approximation, there is no significant effect on the localization properties at the 3-brane. The case of bulk fermions is particularly interesting, since the properties of the geometry allow localization of chiral modes independently.

Holographic entanglement entropy for disconnected regions

We present a simple derivation of the entanglement entropy for a region made up of a union of disjoint intervals in 1+1 dimensional quantum field theories using holographic techniques. This generalizes the results for 1+1 dimensional conformal field theories derived previously by exploiting the uniformization map. We further comment on the generalization of our result to higher dimensional field theories.

Extended Supersymmetries in One Dimension

This work covers part of the material presented at the Advanced Summer School in Prague. It is mostly devoted to the structural properties of Extended Supersymmetries in One Dimension. Several results are presented on the classification of linear, irreducible representations realized on a finite number of time-dependent fields. The connections between supersymmetry transformations, Clifford algebras and division algebras are discussed. A manifestly supersymmetric framework for constructing invariants without using the notion of superfields is presented. A few examples of one-dimensional, N-extended, off-shell invariant sigma models are computed. The relation between supersymmetry transformations and graph theory is outlined. The notion of the fusion algebra of irreps tensor products is presented. The relevance of one-dimensional Supersymmetric Quantum Mechanics as a way to extract information on higher dimensional supersymmetric field theories is discussed.

Chiral asymmetry from a 5D Higgs mechanism

An intriguing feature of the Standard Model is that the representations of the unbroken gauge symmetries are vector-like whereas those of the spontaneously broken gauge symmetries are chiral. Here we provide a toy model which shows that a natural explanation of this property could emerge in higher dimensional field theories and discuss the difficulties that arise in the attempt to construct a realistic theory. An interesting aspect of this type of models is that the 4D low energy effective theory is not generically gauge invariant. However, the non-invariant contributions to the observable quantities are very small, of the order of the square of the ratio between the light particle mass scale and the Kaluza-Klein mass scale. Remarkably, when we take the unbroken limit both the chiral asymmetry and the non-invariant terms disappear. © SISSA 2007.

Some consequences of Wiesend’s higher dimensional class field theory

We use a result of G. Wiesend to establish the relation between the integral singular homology in degree zero and the abelianized tame fundamental group of a regular, connected scheme of finite type over Spec\((\mathbb{Z})\) .

Holographic grand unification

We present a framework for grand unification in which the grand unified symmetry is broken spontaneously by strong gauge dynamics, and yet the physics at the unification scale is described by (weakly coupled) effective field theory. These theories are formulated, through the gauge/gravity correspondence, in truncated 5D warped spacetime with the UV and IR branes setting the Planck and unification scales, respectively. In most of these theories, the Higgs doublets arise as composite states of strong gauge dynamics, corresponding to degrees of freedom localized to the IR brane, and the observed hierarchies of quark and lepton masses and mixings are explained by the wavefunction profiles of these fields in the extra dimension. We present several realistic models in this framework. We focus on one in which the doublet-triplet splitting of the Higgs fields is realized within the dynamical sector by the pseudo-Goldstone mechanism, with the associated global symmetry corresponding to a bulk gauge symmetry in the 5D theory. Alternatively, the light Higgs doublets can arise as a result of dynamics on the IR brane, without being accompanied by their triplet partners. Gauge coupling unification and proton decay can be studied in these models using higher dimensional effective field theory. The framework also sets a stage for further studies of, e.g., proton decay, fermion masses, and supersymmetry breaking.

Character formulas and partition functions in higher dimensional conformal field theory

A discussion of character formulas for positive energy, unitary irreducible representations of the conformal group is given, employing Verma modules and Weyl group reflections. Product formulas for various conformal group representations are found. These include generalizations of those found by Flato and Fronsdal for SO(3,2). In even dimensions the products for free representations split into two types depending on whether the dimension is divisible by four or not.

Generalized integrability conditions and target space geometry

In some higher-dimensional nonlinear field theories integrable subsectors with infinitely many conservation laws have been identified by imposing additional integrability conditions. Originally, the complex eikonal equation was chosen as integrability condition, but recently further generalizations have been proposed. Here we show how these new integrability conditions may be derived from the geometry of the target space and, more precisely, from the Noether currents related to a certain class of target space transformations.

Radius stabilization by two-loop Casimir energy

It is well known that the Casimir energy of bulk fields induces a non-trivial potential for the compactification radius of higher-dimensional field theories. On dimensional grounds, the 1-loop potential is ∼ 1 / R 4 . Since the 5d gauge coupling constant g 2 has the dimension of length, the two-loop correction is ∼ g 2 / R 5 . The interplay of these two terms leads, under very general circumstances (including other interacting theories and more compact dimensions), to a stabilization at finite radius. Perturbative control or, equivalently, a parametrically large compact radius is ensured if the 1-loop coefficient is small because of an approximate fermion–boson cancellation. This is similar to the perturbativity argument underlying the Banks–Zaks fixed point proposal. Our analysis includes a scalar toy model, 5d Yang–Mills theory with charged matter, the examination of S 1 and S 1 / Z 2 geometries, as well as a brief discussion of the supersymmetric case with Scherk–Schwarz SUSY breaking. 2-loop calculability in the S 1 / Z 2 case relies on the log-enhancement of boundary kinetic terms at the 1-loop level.

Radiative stabilization of warped space

Higher-dimensional field theory has been applied to explore various issues in recent particle physics such as the gauge hierarchy problem. In order for such approaches to be viable, a crucial ingredient is to fix the sizes of extra dimensions at some finite values, which sizes are generically free parameters in the theory. In this paper, we present several schemes to determine the radius of extra dimension in warped five-dimensional theory. In every case, a non-vanishing Fayet-Iliopoulos term for abelian gauge factor plays a crucial role for the radius stabilization. It is radiatively generated in the presence of charged matter fields and the compactification is therefore spontaneous, not forced by selected operators. The low-energy supersymmetry is broken or unbroken, and the radius can be fixed to give a small or large scale hierarchy without any fine tuning of parameters. We also discuss a model of the radius stabilization correlated with Yukawa hierarchy and supersymmetry breaking.

Gauge unification in extra dimensions: power corrections vs. higher dimension operators

Power-like loop corrections to gauge couplings are a generic feature of higher-dimensional field theories. In supersymmetric grand unified theories in d = 5 dimensions, such corrections arise only in the presence of a vacuum expectation value of the adjoint scalar of the gauge multiplet. We show that, using the analysis of the exact quantum effective action by Intriligator, Morrison and Seiberg, these power corrections can be understood as the effect of higher dimension operators. Such operators, both classical and quantum, are highly constrained by gauge symmetry and supersymmetry. As a result, even non-perturbatively large contributions to gauge coupling unification can be unambiguously determined within 5d low-energy effective field theory. Since no massive hypermultiplet matter exists in 6 dimensions, the predictivity is further enhanced by embedding the 5d model in a 6d gauge theory relevant at smaller distances. Thus, large and quantitatively controlled power-law contributions to gauge couplings arise naturally and can, in the most extreme case, lead to calculable TeV-scale power law unification. We identify a simple 5d SU(5) model with one massless 10 in the bulk where the power-law effect is exactly MSSM-like.

QCD axion from a higher dimensional gauge field theory.

We point out that a QCD axion solving the strong CP problem can arise naturally from a parity-odd gauge field in five-dimensional (5D) orbifold field theory. The required axion coupling to the QCD anomaly comes from the 5D Chern-Simons coupling, and all other unwanted U(1)PQ breaking axion couplings can be avoided naturally by the 5D gauge symmetry and locality. If the fifth dimension is warped, the resulting axion scale is suppressed by a small warp factor compared to the Planck scale, thereby the model can generate naturally an intermediate axion scale fa = 10(10)-10(12) GeV.

Radiative corrections with 5D mixed position-/momentum-space propagators

In higher dimensional field theories with compactified dimensions there are three standard ways to do perturbative calculations: (i) by the summation over Kaluza–Klein towers; (ii) by the summation over winding numbers making use of the Poisson-resummation formula; and (iii) by using mixed propagators, where the coordinates of the four infinite dimensions are Fourier-transformed to momentum space while those of the compactified dimension are kept in configuration space. The third method is broadly used in finite temperature field theory calculations. One of its advantages is that one can easily separate the ultraviolet divergent terms of the uncompactified theory from the non-local finite corrections arising from windings around the compact dimensions. In this note we demonstrate the use of this formalism by calculating one-loop self-energy corrections in a 5D theory formulated on the manifold M 4⊗S 1 and on the orbifold M 4⊗S 1/Z 2 .

On vector Goldstone boson

The possibility that higher dimensional field theories are broken spontaneously, through the usual Nambu-Goldstone mechanism, to 4-dimension is explored. As a consequence, vector Goldstone bosons can arise in this breaking of Lorentzian symmetry from higher dimension to 4-dimension. This can provide a simple mechanism for reduction to 4-dimension in theories with extra dimensions.