Expectation values of twist fields and universal entanglement saturation of the free massive boson

摘要: The evaluation of vacuum expectation values (VEVs) in massive integrable quantum field theory (QFT) is a nontrivial renormalization-group "connection problem" -- relating large and short distance asymptotics general unsolved. This particularly relevant the context entanglement entropy, where VEVs branch-point twist fields give universal saturation predictions. We propose new method to compute associated continuous symmetries QFT. based on differential equation symmetry parameter, gives as infinite form-factor series which truncate at two-particle level free verify by studying U(1) models, are simply related fields. provide first exact formulae for such uncompactified boson model, checking against an independent calculation angular quantization. show that logarithmic terms, overlooked original work Callan Wilczek [Phys. Lett. B333 (1994)], appear both massless situations. implies that, agreement with numerical observations Bianchini Castro-Alvaredo [Nucl. Phys. B913 (2016)], standard power-law short-distance behavior corrected factor. discuss how this entropy single interval near-critical harmonic chains, including log corrections.