Please note that the list below only shows forthcoming events, which may not include regular events that have not yet been entered for the forthcoming term. Please see the past events page for a list of all seminar series that the department has on offer.

 

Past events in this series


Tue, 20 Jan 2026
13:00
L2

How to get an interacting conformal line defect for free theories

Christopher Herzog
(KCL )
Abstract
We argue that interacting conformal line defects in free quantum field theories can exist, provided that correlation functions are not invariant under inversion symmetry.  Important for our demonstration is the existence of a special cross ratio for bulk-defect-defect three point functions that is invariant under the conformal group but picks up a sign under inversion. We examine the particular case of a free scalar field in detail, and we provide a toy model example where this bulk field interacts via a Yukawa term with fermions on the line.  We expect nontrivial line defects may also exist for free Maxwell theory in four dimensions and free bulk fermions.  Based on 2510.02871.


 

Tue, 10 Feb 2026
13:00
L2

Dynamics of the Fermion-Rotor System

Vazha Loladze
(Oxford )
Abstract

In this talk, I will examine the dynamics of the fermion–rotor system, originally introduced by Polchinski as a toy model for monopole–fermion scattering. Despite its simplicity, the system is surprisingly subtle, with ingoing and outgoing fermion fields carrying different quantum numbers. I will show that the rotor acts as a twist operator in the low-energy theory, changing the quantum numbers of excitations that have previously passed through the origin to ensure scattering consistent with all symmetries, thereby resolving the long-standing Unitarity puzzle. I will then discuss generalizations of this setup with multiple rotors and unequal charges, and demonstrate how the system can be viewed as a UV-completion of boundary states for chiral theories, establishing a connection to the proposed resolution of the puzzle using boundary conformal field theory.