Polarized ZZ pairs in gluon fusion and vector boson fusion at the LHC

Pair production of helicity-polarized weak bosons $(V_{\lambda }=W_{\lambda }^{\pm },Z_{\lambda })$ from gluon fusion $(gg\to V_{\lambda }{V}_{{\lambda }^{\mathrm{\prime }}}^{\mathrm{\prime }})$ and weak boson fusion $(V_1{V}_2\to V_{\lambda }{V}_{{\lambda }^{\mathrm{\prime }}}^{\mathrm{\prime }})$ are powerful probes of the Standard Model, new physics, and properties of quantum systems. Measuring cross sections of polarized processes is a chief objective of the Large Hadron Collider’s (LHC) Run 3 and high luminosity programs, but progress is limited by the simulation tools that are presently available. We propose a method for computing polarized cross sections that works by directly modifying Feynman rules instead of (squared) amplitudes. The method is applicable to loop-induced processes, and can capture the interference between arbitrary polarization configurations, interference with non-resonant diagrams, as well as off-shell/finite-width effects. By construction, previous results that work at the (squared) amplitude level are recoverable. As a demonstration, we report the prospect of observing and studying polarized $Z_{\lambda }{Z}_{{\lambda }^{\mathrm{\prime }}}$ pairs when produced via gluon fusion and electroweak processes in final-states with four charged leptons at the LHC, using the new method to simulate the gluon fusion process. Our Feynman rules are publicly available as a set of $\mathit{\text{Universal}}\mathit{\text{FeynRules}}\mathit{\text{Object}}$ libraries called $\mathtt{\text{SM\_Loop\_VPolar}}$.