ttbar+Large missing energy from top-quark partners: a comprehensive study at NLO QCD

TL;DR

This study analyzes top-quark partner production at the LHC, emphasizing the importance of NLO QCD corrections and merged simulations for accurate modeling of ttbar plus missing energy signatures, crucial for new physics searches.

Contribution

It provides a comprehensive comparison of simulation methods, highlighting the inaccuracies of parton shower models and the necessity of NLO and merged samples for reliable predictions.

Findings

Parton shower simulations deviate significantly from NLO and merged results.

Merged samples agree well with NLO predictions for acceptances.

Scale variation underestimates the true uncertainty after cuts.

Abstract

We perform a detailed study of top-quark partner production in the ttbar plus large missing energy final-state at the LHC, presenting results for both scalar and fermionic top-quark partners in the semi-leptonic and dileptonic decay modes of the top quarks. We compare the results of several simulation tools: leading-order matrix elements, next-to-leading order matrix elements, leading-order plus parton shower simulations, and merged samples that contain the signal process with an additional hard jet radiated. We find that predictions from leading-order plus parton shower simulations can significantly deviate from NLO QCD or LO merged samples and do not correctly model the kinematics of the ttbar + ETmiss signature. They are therefore not a good framework for modeling this new physics signature. On the other hand, the acceptances obtained with a merged sample of the leading-order process together with the radiation of an additional hard jet are in agreement with the NLO predictions. We also demonstrate that the scale variation of the inclusive cross section, plus that of the acceptance, does not accurately reflect the uncertainty of the cross section after cuts, which is typically larger. We show the importance of including higher-order QCD corrections when using kinematic distributions to determine the spin of the top-quark partner.