Topological constraints on defect dynamics

Extended objects (defects) in quantum field theory exhibit rich, nontrivial dynamics describing a variety of physical phenomena. These systems often involve strong coupling at long distances, where the bulk and defects interact, making analytical studies challenging. By carefully analyzing the behavior of bulk symmetries in the presence of defects, we uncover robust topological constraints on defect RG flows. Specifically, we introduce the notions of defect anomalies and symmetry-reflecting defects, both of which are RG-invariant. Several known notions, such as higher-form symmetries, fractionalization, and projective lines, are revealed to be manifestations of defect anomalies, which also encompass novel phenomena and forbid trivial defect dynamics in the infrared. Meanwhile, symmetry-reflecting defects are shown to remain coupled at low energies, imposing powerful dynamical constraints. We verify our findings through concrete examples: exactly solvable defect RG flows in <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"><a:mrow><a:mo stretchy="false">(</a:mo><a:mn>1</a:mn><a:mo>+</a:mo><a:mn>1</a:mn><a:mo stretchy="false">)</a:mo><a:mi mathvariant="normal">d</a:mi></a:mrow></a:math> conformal field theories with symmetry-reflecting lines and a surface defect in <f:math xmlns:f="http://www.w3.org/1998/Math/MathML" display="inline"><f:mrow><f:mo stretchy="false">(</f:mo><f:mn>2</f:mn><f:mo>+</f:mo><f:mn>1</f:mn><f:mo stretchy="false">)</f:mo><f:mi mathvariant="normal">d</f:mi></f:mrow></f:math> scalar QED.