Determine the mass-dependent background spin shape in situ

Determine the Standard Model background spin-shape nuisance directly from data through sideband channels and a simultaneous mass-dependent fit of the low sideband, signal region, and high sideband, rather than relying on the simplified profiled template.

Background

The analysis models the dominant systematic uncertainty with a yield-preserving deformation of the reconstructed Standard Model background helicity-correlation template. This nuisance is constrained by an assumed Gaussian prior motivated by sideband counting statistics, but the constraint is not obtained from an explicit fit to sideband data.

Because the reconstructed Standard Model helicity correlation varies strongly with the top-pair invariant mass, a reliable experimental implementation must determine how the background spin shape interpolates into the signal region. The paper identifies a simultaneous fit of the low sideband, signal region, and high sideband as the required unresolved extension.

References

The profiled scan is more realistic than the unprofiled bias study, but it does not yet determine the nuisance from data like sideband channels. Because \langle\rangle_{\rm bkg} runs strongly with t, the decisive experimental step is a simultaneous mass-dependent fit of low sideband, signal region, and high sideband.

— Distinguishing a pseudoscalar from a vector $t\bar t$ resonance with top-quark spin correlations at the HL-LHC  (2609.11641 - Bellagroudi et al., 10 Sep 2026) in Section V, subsection “Background spin shape control”