Triangle Seminars
Wednesday, 11 Feb 2026
Chiral anomalies and CPT
📍 London
Shu-Heng Shao
(MIT, Cambridge)
Abstract:
We discuss how the vector and axial U(1) symmetries of a massless Dirac fermion in 1+1d are realized in Hamiltonian lattice systems. Interestingly, these two lattice charges do not commute and form a non-abelian algebra, first discussed by Onsager. We prove that these symmetries force the low-energy theory to be massless, reminiscent of consequences from perturbative anomalies of continuous global symmetries in quantum field theory. This lattice anomaly is of order 2, but when a lattice CPT symmetry is further imposed, the anomaly becomes of infinite order (i.e., torsion-free), matching the continuum result.
We discuss how the vector and axial U(1) symmetries of a massless Dirac fermion in 1+1d are realized in Hamiltonian lattice systems. Interestingly, these two lattice charges do not commute and form a non-abelian algebra, first discussed by Onsager. We prove that these symmetries force the low-energy theory to be massless, reminiscent of consequences from perturbative anomalies of continuous global symmetries in quantum field theory. This lattice anomaly is of order 2, but when a lattice CPT symmetry is further imposed, the anomaly becomes of infinite order (i.e., torsion-free), matching the continuum result.
Posted by: Jesse van Muiden
Non-Lorentzian Holography and Near-BPS Physics in String Theory
📍 East of England
Joseh Smith
(University of Birmingham)
Abstract:
In recent years there has been a renewed interest in understanding string theory close to BPS bounds using the tools of non-Lorentzian physics. In these regimes the underlying geometry is deformed, becoming a generalisation of Newton-Cartan geometry based around a brane-like foliation of spacetime. By considering such limits on both sides of a Lorentzian holographic duality we can construct dualities between non-Lorentzian QFT and string theory on non-Lorentzian spacetimes, which in the IR reduces to solutions of exotic non-Lorentzian supergravity theories. In this talk I will show how these theories, along with their BPS brane solutions, can be constructed using the explicit example of the M5-brane limit of eleven-dimensional supergravity. We will apply these lessons to D-brane limits from the perspective of both the gravitational solution and the worldvolume QFT to construct proposals for non-Lorentzian holography. We will see that both limits exhibit the same symmetry structure as the gravitational solutions, with the QFT dynamics reducing to motion on the moduli space of solitons representing quarter-BPS brane bound states. Time-permitting, I will finish with a discussion of upcoming work on including quantum effects on the QFT side using the example of Galilean Yang-Mills, which is proposed to be dual to non-relativistic string theory on a near-horizon D2-brane background.
In recent years there has been a renewed interest in understanding string theory close to BPS bounds using the tools of non-Lorentzian physics. In these regimes the underlying geometry is deformed, becoming a generalisation of Newton-Cartan geometry based around a brane-like foliation of spacetime. By considering such limits on both sides of a Lorentzian holographic duality we can construct dualities between non-Lorentzian QFT and string theory on non-Lorentzian spacetimes, which in the IR reduces to solutions of exotic non-Lorentzian supergravity theories. In this talk I will show how these theories, along with their BPS brane solutions, can be constructed using the explicit example of the M5-brane limit of eleven-dimensional supergravity. We will apply these lessons to D-brane limits from the perspective of both the gravitational solution and the worldvolume QFT to construct proposals for non-Lorentzian holography. We will see that both limits exhibit the same symmetry structure as the gravitational solutions, with the QFT dynamics reducing to motion on the moduli space of solitons representing quarter-BPS brane bound states. Time-permitting, I will finish with a discussion of upcoming work on including quantum effects on the QFT side using the example of Galilean Yang-Mills, which is proposed to be dual to non-relativistic string theory on a near-horizon D2-brane background.
Posted by: Julian Kupka
A universal sum over topologies in 3d gravity
📍 London
Lorenz Eberhardt
(University of Amsterdam)
Abstract:
I will describe recent progress toward a well-defined sum over topologies in AdS3 quantum gravity and its interpretation from the boundary perspective. A minimal set of consistency requirements on the boundary ensemble (crossing symmetry and typicality) is formulated and shown to admit a direct bulk realization in terms of elementary surgery moves on three-manifolds. These moves generate a large and unavoidable class of bulk geometries that must be included in any reasonable definition of the gravitational path integral. The resulting manifolds are always on-shell (hyperbolic), though the construction does not exhaust all hyperbolic topologies. I will illustrate the structure of the resulting sum with explicit examples, including handlebody-knots, and discuss implications for the ensemble interpretations of AdS3 gravity.
Based on joint work with A. Belin, S. Collier, D. Liska, and B. Post (arXiv:2601.07906).
I will describe recent progress toward a well-defined sum over topologies in AdS3 quantum gravity and its interpretation from the boundary perspective. A minimal set of consistency requirements on the boundary ensemble (crossing symmetry and typicality) is formulated and shown to admit a direct bulk realization in terms of elementary surgery moves on three-manifolds. These moves generate a large and unavoidable class of bulk geometries that must be included in any reasonable definition of the gravitational path integral. The resulting manifolds are always on-shell (hyperbolic), though the construction does not exhaust all hyperbolic topologies. I will illustrate the structure of the resulting sum with explicit examples, including handlebody-knots, and discuss implications for the ensemble interpretations of AdS3 gravity.
Based on joint work with A. Belin, S. Collier, D. Liska, and B. Post (arXiv:2601.07906).
Posted by: Jesse van Muiden