Speaker
Description
Near the tt_bar production threshold, residual QCD attraction between the top and antitop binds them into short-lived, toponium-like quasi-bound states, an effect recently proposed as consistent with excess near-threshold tt_bar events reported at the LHC. Because the leading production channel is gluon fusion, any such enhancement has a direct implication for the determination of the gluon parton distribution function. We build on the Green’s-function reweighting method for incorporating these non-relativistic QCD threshold effects into off-shell tt_bar matrix elements. We independently implement and validate a Lippmann-Schwinger solver, and in doing so pin down the exact Coulomb potential. We extend the treatment beyond the color-singlet channel considered previously to include the color-octet contribution and compute the resulting K-factors for both. These reweighted predictions are now being passed into the CT25NNLO global PDF fit to quantify the resulting shift in the extracted gluon distribution, the first step toward assessing whether toponium threshold effects are significant enough to warrant inclusion in precision PDF analyses.