LSZ in Action: Extracting Form Factors from Correlators Nonperturbatively in 2d ${\phi }^4$ Theory

In this paper, we compute multiparticle form factors of local operators in 2d ${\phi }^4$ theory using a recently proposed method [1] for efficiently implementing the LSZ prescription with Hamiltonian Truncation methods, and we adopt Lightcone Conformal Truncation (LCT) in particular for our calculations. We perform various checks of our results at weak and strong coupling, and elucidate the parametric behavior of truncation errors. This opens up the possibility to compute S-matrix in various strongly coupled models using the LSZ method in LCT.