Joint Theory Seminar: Maxime Trepanier

Title: Probing nonlagrangian theories via surface operators

Abstract: Surface operators are string-like defects that appear in many guises in a wide
array of QFTs. The most intriguing context in which they arise might be the
so-called 6d N=(2,0) superconformal field theories, which are mysterious
strongly-coupled theories predicted by string theory with no lagrangian
description. In this talk I will explain how surface operators in these
theories are analogous to Wilson lines in gauge theories and define a notion of
string potential. I will then discuss two powerful nonperturbative approaches
to study them: the chiral algebra, which provides an alternative to
supersymmetric localization, and the conformal bootstrap. Using these I will
extract exact OPE coefficients for protected operators, as well as defect CFT data
for nonprotected operators at large N. These methods provide concrete results
for the 6d theories and are also applicable more generally, in particular to 4d
N=2 theories.