String Theory Group at the University of Torino

Type B anomalies (Mis-)Matching

In this talk we analyse several aspects related to type B conformal anomalies associated with Coulomb branch operators in 4d N=2 SCFTs. In particular, when the vacuum preserves the conformal symmetry, these anomalies coincide with the two point function coefficients in the Coulomb branch chiral ring. We analyse the behaviour of these anomalies on the Higgs branch, where conformal symmetry is spontaneously broken. We review the argument developed in arXiv:1911.05827 and, following it, we argue that these anomalies are covariantly constant on conformal manifolds. In some cases this can be used to show that the anomalies match in the broken and unbroken phases. Then, in the second part of the talk, we focus on some specific 4d N=2 SCFTs and we test type B anomaly (Mis-)Matching through an explicit Feynman diagram computation. We finally observe that an implication of Type B anomaly Mismatching is the existence of a second covariantly constant metric on the conformal manifold that imposes restrictions on its holonomy group.

3rd of November 2020, 14:30

Deconfining class S theories

Class S theories are a broad and interesting class of N=2 superconformal field theories arising from wrapping the six dimensional (2,0) theory on Riemann surfaces. Most of these theories have no known Lagrangian description. I will present a method (based on brane engineering) that allows to systematically construct N=1 Lagrangians flowing to some of these N=2 theories. As an illustration of the method, I will construct a Lagrangian description for the simplest non- trivial class-S theory, the \(T_3\) theory with global symmetry \(E_6\), and for some related examples.

20th of October 2020, 14:30

Bootstrapping defects and boundaries for the free scalar field

Is there any room for non-trivial unitary and conformal defects in the theory of a single free massless scalar field? And what about boundaries? We use the free scalar equation of motion and the structure of the bulk-to-defect operator expansion to rule out the existence of such defects in several (co-)dimensions. For boundaries we are led to a non-trivial system of crossing equations that we analyze numerically in four bulk dimensions. We show that large regions of parameter spaces are excluded, but a ‘kink’ in the numerical bounds obeys all our consistency checks and might be an indication of a new conformal boundary condition.

13th of October 2020, 14:30

The fate of discrete 1-form symmetries in 6d

Recently introduced generalized global symmetries have been useful in order to understand non-perturbative aspects of quantum field theories in four and lower dimensions. In this talk I will focus on 1-form symmetries of weakly coupled 6d supersymmetric gauge theories coupled to tensor multiplets and their interplay with large gauge transformations for dynamical tensor fields. In a non-trivial background for the global 1-form symmetry, this leads to an ambiguity of the effective field theory partition function. This anomaly is eliminated by the inclusion of BPS strings. However, the non-trivial 1-form background can induce fractional string charges which are not compatible with Dirac quantization, and hence the symmetry is absent. I will describe how the anomalous term serves as a tool to detect whether the discrete 1-form symmetries are realized in explicit examples originating from string compactifications. I will show how this is corroborated by finding that a non-trivial ambiguity is related to states, which are excitations of the 6d BPS strings and explicitly break the global 1-form symmetry. For 6d theories consistently coupled to gravity, this ambiguity of the partition function hints at the presence of a symmetry breaking tower of states. When the ambiguity is absent, the F-theory realization of the theories points to the gauging of the 1-form symmetries.

6th of October 2020, 14:30

Large charges in QFT

It has been very recently realized that large charge sectors in QFT’s exhibit interesting properties and simplifications. In this talk we will discuss two particular examples, namely the case of N=2 superconformal QCD in d=4 and the case of the Wilson-Fisher fixed point in various dimensions. It turns out that it is possible to find different regimes if the coupling of the theory scales appropriately with the charge, thus finding a variety of interesting behaviors.

29th of September 2020, 15:30

3d S-fold SCFTs

A local SL(2,Z) transformation on the Type IIB brane configuration gives rise to an interesting class of 3d superconformal field theories, known as the S-fold SCFTs. One of the interesting features of such a theory is that, in general, it does not admit a conventional Lagrangian description. Nevertheless, it can be described by a quiver diagram with a link being a superconformal field theory, known as the T(U(N)) theory. In this talk, we discuss various interesting properties of the S-fold theories, including supersymmetry enhancement in the infrared as well as several interesting dualities.

Thursday, 2nd of April 2020, 14:30

Bootstrapping (A)dS exchanges

Gravitational observables are inherently holographic in nature. When the asymptotic boundary of space time admits a canonical definition of time this allows to use standard principles of Quantum Mechanics to describe such observables. On the other hand the situation is not clear otherwise. This is the case in dS space where the boundary is space-like. Nonetheless bulk causality must be imprinted at the boundary in some way.

In this talk I will discuss how to bootstrap tree level amplitudes both in AdS and dS. I will fix from first principles the form of such holographic observables discussing their singularities. In AdS I will also discuss how the knowledge of exchange amplitudes alone allows to derive non-perturbative constraints on CFT correlators writing down extremal functionals for the CFT bootstrap in arbitrary dimensions

Thursday, 26th of March 2020, 14:30

Revisiting the epsilon expansion

There are many interacting CFTs with scalar degrees of freedom, like the O(N) fixed point in 2 < d < 4 dimensions. Surprisingly, little is known about the classification of such fixed points, even in perturbation theory. In this talk I will explore this well-known problem in the framework of the epsilon expansion. I will argue that consistent CFTs can only live in a small region of the high-dimensional space of all possible theories, and I'll discuss bounds on spectra of anomalous dimensions. Finally I’ll address the same question in the case of N gauged complex scalars, which leads to qualitatively different answers.

Tuesday, 18th of February 2019, 14:30, sala Wataghin

Resurgence and non-perturbative physics: from string theory to superconductors

It is often said that perturbation theory is insufficient to understand many physical problems, and that non-perturbative effects are needed. It turns out that making this statement precise requires the mathematical framework of resurgence theory, in which perturbative series are extended to so-called trans-series, and non-perturbative effects can be detected by looking at perturbation theory at large orders.

In this talk I will first review the basics of the theory of resurgence, and then present some of its recent applications to problems ranging from string theory to superconductivity.

Tuesday, 11th of February 2019, 14:30, aula D

Integrable fishnet from γ-deformed N=2 quivers

We introduce bi-fermion fishnet theories, a class of models describing integrable sectors of four-dimensional gauge theories with non-maximal supersymmetry. Bi-fermion theories are characterized by a single complex scalar field and two Weyl fermions interacting only via chiral Yukawa couplings. The latter generate oriented Feynman diagrams forming hexagonal lattices, whose fishnet structure signals an underlying integrability that we exploit to compute anomalous dimensions of BMN-vacuum operators. Furthermore, we investigate Lunin-Maldacena deformations of N=2 superconformal field theories with deformation parameter γ and prove that bi-fermion models emerge in the limit of large imaginary γ and vanishing 't Hooft coupling g , with $ge^{−iγ/2}$ fixed. We explicitly find non-trivial conformal fixed points and compute the scaling dimensions of operators for any γ and in presence of double-trace deformations. Finally we also compute the exact spectrum of the shortest BMN operator.

Tuesday, 28th of January 2019, 15:30, sala Wataghin