Violation of Eigenstate Thermalization Hypothesis in Quantum Field Theories with Higher-Form Symmetry.

We elucidate how the presence of higher-form symmetries affects the dynamics of thermalization in isolated quantum systems. Under reasonable assumptions, we analytically show that a p-form symmetry in a (d+1)-dimensional quantum field theory leads to the breakdown of the eigenstate thermalization hypothesis for many nontrivial (d-p)-dimensional observables. For discrete higher-form (i.e., p≥1) symmetry, this indicates the absence of thermalization for observables that are nonlocal but much smaller than the whole system size without any local conserved quantities. We numerically demonstrate this argument for the (2+1)-dimensional Z_{2} lattice gauge theory. While local observables such as the plaquette operator thermalize even for mixed symmetry sectors, the nonlocal observable exciting a magnetic dipole instead relaxes to the generalized Gibbs ensemble that takes account of the Z_{2} one-form symmetry.