Testing the Spin of New Physics with Top Precision Measurements

Abstract

Precision measurements can reveal not only the presence of new physics, but also information about the dynamics that generated it. We study this question in top-pair production by comparing three top-philic scenarios: a colored scalar top partner, a colored vector-like fermion, and a spin-one $Z^\prime$ mediator. The first two contribute through loop corrections, while the $Z^\prime$ contributes at tree level. We show that the scalar and vector-like fermion scenarios match onto the same effective operator structure at low energies,making their distinction unfeasible using the EFT approach. However, near or above threshold their full loop amplitudes develop distinct momentum-dependent interference patterns. Using the top transverse momentum spectrum we find that resonant and loop-induced effects are relatively easy to distinguish, whereas separating the scalar and vector-like hypotheses is substantially more challenging. Nevertheless, their different analytic structures leave measurable information in the shape of the distribution. These results illustrate how precision differential measurements can probe the spin and dynamics of virtual new states even when they are not directly reconstructed, and motivate more powerful multidimensional analyses.