Search Talks

Various operations on two-dimensional quantum field theories such as the GSO projection and various dualities involve a sum over spin structures. This talk will discuss a subtle mod-16 anomaly in the spin structure, extending the usual mod-8 anomaly of fermion parity, which can obstruct this sum. As such one must account for this anomaly to define these operations properly or face surprising consequences. We will see how this phenomenon is captured cleanly by homotopy theory, specifically the non-vanishing cohomology of a sequence of cobordism groups. From there, we will discuss applications in diverse dimensions and with additional symmetries, and the corresponding SymTFT.

Semisimple categorical symmetries, and their physical implications have been studied in great detail in the continuum and on the lattice. In particular, the nice properties of semisimple categories allows one to construct hamiltonians which enjoy this symmetry, and study its symmetric gapped phases. Using the anyonic chain framework in (1+1)d, I will present a model that has a finite but non-semisimple categorical symmetry. Sacrificing Hermiticity, I will present several symmetric, frustration-free, gapped Hamiltonians with real spectra and analyze their ground state subspaces. I then discuss two intriguing phenomena. First, I will identify a smooth path of gapped symmetric Hamiltonians whose ground states transform inequivalently under the symmetry. Second, I will discuss a model where a product state and the so-called W state spontaneously break the symmetry, and propose an explanation for the indistinguishability of these two states in the infinite-volume limit in terms of the symmetry category.

The amplituhedron was introduced by Arkani-Hamed and Trnka as a geometric object which "encodes" scattering amplitudes in N=4 SYM theory. One of their motivations was the Britto-Cachazo-Feng-Witten (BCFW) recursion for amplitudes, which in particular produced many different formulas for the same amplitude. Their expectation was that the amplitude is the "volume" of the amplituhedron, and the different BCFW formulas correspond to different ways of decomposing the amplituhedron into small pieces. This expectation is in fact correct. I'll discuss some of the mathematics going into the proof, some unexpected phenomena arising along the way, and some lingering questions. This is joint work with (various subsets of) Chaim Even-Zohar, Tsviqa Lakrec, Matteo Parisi, Ran Tessler, and Lauren Williams.