States, symmetries and correlators of T T and J T symmetric orbifolds

We derive various properties of symmetric product orbifolds of TT over bar and JT over bar -deformed CFTs from a field-theoretical perspective. First, we generalise the known formula for the torus partition function of a symmetric orbifold theory in terms of the one of the seed to non-conformal two-dimensional QFTs; specialising this to seed TT over bar and JT over bar -deformed CFTs reproduces previous results in the literature. Second, we show that the single -trace TT over bar and JT over bar deformations preserve the Virasoro and Kac-Moody symmetries of the undeformed symmetric product orbifold CFT, including their fractional counterparts, as well as the KdV charges. Finally, we discuss correlation functions in these theories. By extending a previously-proposed basis of operators for JT over bar -deformed CFTs to the single -trace case, we explicitly compute the correlation functions of both untwisted and twisted-sector operators and compare them to an appropriate set of holographic correlators. Our derivations are based mainly on Hilbert space techniques and completely avoid the use of conformal invariance, which is not present in these models.