Speaker
Description
Accurate phase-space integration for multi-particle processes at multi-TeV lepton colliders can be challenging because of complicated resonance structures, strongly forward-peaked distributions, and large gauge cancellations among individual Feynman diagrams. We present a multi-channel phase-space method in which each integration channel is enhanced according to a single Feynman diagram evaluated in the Feynman-diagram (FD) gauge. In this gauge, individual amplitudes retain a close correspondence with the associated physical subprocesses, and their squared amplitudes provide efficient importance-sampling functions over the relevant regions of phase space.
We demonstrate the method for lepton-collider processes involving top-quark pair production in association with a Higgs boson, including vector-boson-fusion and related channels, in the SMEFT with a complex top-Yukawa coupling. Some of these processes contain lepton-mass singularities arising from t-channel photon exchange and require an accurate treatment of extremely forward charged leptons. We therefore introduce a dedicated phase-space parametrization for the forward region and modify the HELAS (HELicity Amplitude Subroutines) library so that helicity amplitudes involving momentum transfers as small as the squared electron mass can be evaluated reliably even at multi-TeV collision energies.
The resulting framework provides stable and efficient phase-space integration for processes that are difficult to simulate with conventional methods and constitutes an important step toward reliable event generation for future high-energy lepton-collider studies.