AmpPheno · Scattering Amplitudes as a Bridge Between Theory and Phenomenology, with Applications to Higher Spins

With the rapid updates of LHC analyses and the plans for future colliders, it is challenging to interpret experimental results with a fully general theoretical tool. The traditional EFT approach includes all symmetry-allowed operators in the Lagrangian, but entails an enormous parameter space. In contrast, scattering amplitudes are constrained by fundamental principles such as unitarity and causality, which often lead to strong bounds on the underlying theory. By working directly with physical observables, one avoids the redundancies of the traditional EFT approach. This proposal employs amplitude methods to derive concrete phenomenological implications.(1) Recursive amplitude construction: On-shell recursive amplitude computations depend on suitable momentum shifts. The applicant has proposed the first valid shift for massive particles. This project will employ this method for efficient calculations of multiparticle processes relevant to collider physics and other phenomenological applications.(2) Bottom-up EFT constraints: Using momentum-shift and positivity techniques, the project will extract model-independent features of candidate theories and derive comprehensive constraints on theories with different spins. This avoids Lagrangian assumptions and ensures general results.(3) Application to higher spins: Higher-spin states appear in nature in composite forms such as hadrons, nuclei, and spinning black holes, but are difficult to study due to theoretical pathologies. On-shell amplitudes offer a fresh perspective, allowing systematic modeling of such states within a consistent framework. This project aims to generalize the results on elementary higher spins previously obtained by the applicant to composite states and black holes. The objective is to improve composite-particle EFTs and black-hole potentials, which are crucial for nuclear physics and gravitational waves.