Prospective results for vector-boson fusion-mediated Higgs-boson searches in the four lepton final state at the High Luminosity Large Hadron Collider

The High Luminosity Large Hadron Collider is expected to be completed and operational in the second half of 2026, and will necessitate substantial upgrades to the ATLAS inner tracker detector. The impact of increased tracking coverage in the forward direction was investigated in terms of the separation of vector-boson fusion and gluon fusion-mediated Higgs-boson decays to four leptons in association with two jets. For an analysis dominated by statistical uncertainty, with vector-boson fusion production events treated as signal on top of gluon fusion background, the extension of tracking from pseudorapidity $|\eta| < 2.7$ to $|\eta| < 4.0$ improved the prospective vector-boson fusion discovery significance by 16%, while the relative uncertainty on the signal strength $\Delta\mu/\mu$ was reduced by 6%.