Finite-temperature form factors in the free Majorana theory

Abstract

We study the large distance expansion of correlation functions in the free massiveMajorana theory at finite temperature, alias the Ising field theory at zero magnetic field ona cylinder. We develop a method that mimics the spectral decomposition, or form factorexpansion, of zero-temperature correlation functions, introducing the concept of‘finite-temperature form factors’. Our techniques are different from those of previousattempts in this subject. We show that an appropriate analytical continuation offinite-temperature form factors gives form factors in the quantization scheme on the circle.We show that finite-temperature form factor expansions are able to reproduceexpansions in form factors on the circle. We calculate finite-temperature form factors ofnon-interacting fields (fields that are local with respect to the fundamental fermion field).We observe that they are given by a mixing of their zero-temperature form factors and ofthose of other fields of lower scaling dimension. We then calculate finite-temperature formfactors of order and disorder fields. For this purpose, we derive the Riemann–Hilbertproblem that completely specifies the set of finite-temperature form factors of general twistfields (order and disorder fields and their descendants). This Riemann–Hilbertproblem is different from the zero-temperature one, and so are its solutions. Ourresults agree with the known form factors on the circle of order and disorder fields.