Precision measurements of top-quark pair production at the LHC can reveal not only the existence of physics beyond the Standard Model, but also the spin and dynamics of the particles that generate it. A recent study compares three top-philic scenarios: a colored scalar acting as a top partner, a colored vector-like fermion, and a spin-one Z′ mediator. While the Z′ contributes at tree level, the first two act through loop corrections, and distinguishing them is key to identifying what kind of new physics might be at play.

The problem is that, at low energies, the scalar and the vector-like fermion reduce to the same operator structure within the effective field theory (EFT) framework. In that regime, no precision measurement can separate them: both leave exactly the same imprint. The authors show that discrimination becomes possible only near or above the top-antitop production threshold, where the full loop amplitudes develop momentum-dependent interference patterns that do differ between the two scenarios.

Using the top-quark transverse momentum spectrum, the study concludes that distinguishing resonant effects (such as those of the Z′) from loop-induced effects is relatively straightforward. Separating the scalar hypothesis from the vector-like fermion one, however, is substantially more challenging, although their different analytic structures leave measurable information in the shape of the differential distribution. These results illustrate how precision differential measurements can probe the spin and dynamics of virtual new states even when they cannot be directly reconstructed, and motivate more powerful multidimensional analyses in the future.