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arXiv:1208.4763 · Journal of Physics A Mathematical and Theoretical

An operator expansion for integrable quantum field theories

Henning Bostelmann, D Cadamuro

Published Feb 15, 2013• 26 citations
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Abstract

A large class of quantum field theories on 1+1 dimensional Minkowski space, namely, certain integrable models, has recently been constructed rigorously by Lechner. However, the construction is very abstract and the concrete form of local observables in these models remains largely unknown. Aiming for more insight into their structure, we establish a series expansion for observables, similar but not identical to the well-known form factor expansion. This expansion will be the basis for a characterization and explicit construction of local observables, to be discussed elsewhere. Here, we establish the expansion independent of the localization aspect, and analyze its behavior under space-time symmetries. We also clarify relations with deformation methods in quantum field theory, specifically, with the warped convolution in the sense of Buchholz and Summers.

DOI
10.1088/1751-8113/46/9/095401

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A large class of quantum field theories on 1+1 dimensional Minkowski space, namely, certain integrable models, has recently been constructed rigorously by Lechner. However, the construction is very abstract and the concrete form of local observables in these models remains largely unknown. Aiming for more insight into their structure, we establish a series expansion for observables, similar but not identical to the well-known form factor expansion. This expansion will be the basis for a characterization and explicit construction of local observables, to be discussed elsewhere. Here, we establish the expansion independent of the localization aspect, and analyze its behavior under space-time symmetries. We also clarify relations with deformation methods in quantum field theory, specifically, with the warped convolution in the sense of Buchholz and Summers.
In quantum field theory, the rigorous construction of local observables in the presence of nontrivial interaction is a crucial problem. In a class of integrable quantum field theories, a very abstract existence proof has recently been given by Lechner. We give an explicit characterization of these local observables in terms of the properties of the coefficient functions in an expansion by interacting creators and annihilators. Some results on the operator domains of these local observables are given. Using these, we constructed explicit examples of local observables in the quantum Ising model.
We present a new viewpoint on the construction of pointlike local fields in integrable models of quantum field theory. As usual, we define these local observables by their form factors; but rather than exhibiting their n-point functions and verifying the Wightman axioms, we aim to establish them as closed operators affiliated with a net of local von Neumann algebras, which is defined indirectly via wedge-local quantities. We also investigate whether these fields have the Reeh-Schlieder property, and in which sense they generate the net of algebras. Our investigation focuses on scalar models without bound states. We establish sufficient criteria for the existence of averaged fields as closable operators, and complete the construction in the specific case of the massive Ising model.
Abstract We present a new viewpoint on the construction of pointlike local fields in integrable models of quantum field theory. As usual, we define these local observables by their form factors; but rather than exhibiting their n-point functions and verifying the Wightman axioms, we aim to establish them as closed operators affiliated with a net of local von Neumann algebras, which is defined indirectly via wedge-local quantities. We also investigate whether these fields have the Reeh–Schlieder property, and in which sense they generate the net of algebras. Our investigation focuses on scalar models without bound states. We establish sufficient criteria for the existence of averaged fields as closable operators, and complete the construction in the specific case of the massive Ising model.
TT‾ T\overline{T} TT-deformation and long range spin chains
Abstract We point out that two classes of deformations of integrable models, developed completely independently, have deep connections and share the same algebraic origin. One class includes the TT‾ T\overline{T} TT T T ¯ -deformation of 1+1 dimensional integrable quantum field theory and related solvable irrelevant deformations proposed recently. The other class is a specific type of long range integrable deformation of quantum spin chains introduced a decade ago, in the context of N \mathcal{N} N N = 4 super-Yang-Mills theory. We show that the detailed structures of the two deformations are formally identical and therefore share many features. Both deformations preserve integrability and lead to non-local deformed theories, resulting in a change of the corresponding factorized S-matrices. We also prove a factorisation formula for the expectation value of the operators which trigger the deformation on the lattice; similar results in quantum field theory play an essential role in the solvability of such deformations. We point out that the long range deformation is a natural counterpart of the TT‾ T\overline{T} TT T T ¯ -deformation for integrable spin chains, and argue that this observation leads to interesting new avenues to explore.
Abstract In integrable models of quantum field theory, local fields are normally constructed by means of the bootstrap-formfactor program. However, the convergence of their n-point functions is unclear in this setting. An alternative approach uses fully convergent expressions for fields with weaker localization properties in spacelike wedges, and deduces existence of observables in bounded regions from there, but yields little information about their explicit form. We propose a new, hybrid construction: We aim to describe pointlike local quantum fields; but rather than exhibiting their n-point functions and verifying the Wightman axioms, we establish them as closed operators affiliated with a net of local von Neumann algebras that is known from the wedge-local approach. This is shown to work at least in the Ising model.
Integrable quantum models (IQMs) are a class of interacting quantum field theories in 1+1 dimensions in which the Hilbert space can be described as a Fock space with generalized statistics. Because of this Fock space description, any observables can be expanded in the creation- and annihilation operators with the coefficients of the expansions being the connected matrix elements. One example of such an observable in IQMs without bound states is the so-called wedge-local field (a free field analog with the interacting creation- and annihilation operators) which is wedge-local in the sense that it strongly commutes with its reflection when smeared with a function localized in the left wedge. In this thesis, we study wedge-local observables in integrable quantum models and the effect of bound states on the properties of the connected matrix elements of these observables. This allows us to shine light on the connection between the localization of an observable and the functional analytic properties of its connected matrix elements in models with critical phenomena, in this case models which exhibit bound states. In scalar integrable quantum models with bound states (e.g. the Bullough-Dodd model), we proof that the connected matrix elements of quadratic forms which satisfy a wedge-locality condition are boundary values of meromorphic functions, so-called form factors, which satisfy a number of properties. Conversely, we show that any expansion in such form factors defines a quadratic form which satisfies the wedge-locality condition. In contrast to models without bound states, such an expansion does not terminate due to a recursion relation between the form factors and the wedge-local field, a wedge-local operator which creates one-particle states from the vacuum, does not fall into the class of the characterization theorem. Further, we conjecture a candidate for the wedge-local field in the Z(4)-Ising model. This model is interesting because it has a ”higher-order” bound state structure and no established wedge-local field is known in such models. However, the conjectured candidate comes with some difficulties. Most notably, it is ill-defined as an unbounded operator.