HL-LHC sensitivity to an ultraheavy S u u diquark in the u χ channel

We study the HL-LHC sensitivity to an ultraheavy diquark S u u produced in up-quark fusion and decaying through a vectorlike quark (VLQ) χ as S u u → u χ , with χ → W b , Z t , h 0 t . For fully hadronic decays of the W , Z and top quark, these modes give rise to multijet final states with different jet multiplicities. Within the same model framework used previously for the S u u → χ χ six-jet channel, we consider S u u masses in the multi-TeV range and VLQ masses of the order of a few TeV, and simulate proton–proton collisions at s = 13.6 TeV with integrated luminosities up to the HL-LHC target. The analysis employs a machine learning-based discriminant adapted from the six-jet study to the S u u → u χ topology, and is used to derive the corresponding discovery reaches and exclusion limits. We find that the u χ channel provides a complementary signature for ultraheavy diquark searches, especially in regions where the branching ratio B ( S u u → u χ ) is sizable, thereby broadening the sensitivity program for S u u production at the HL-LHC.