Primary Vertex Selection in VBF Higgs to Invisibles at the HL-LHC with the ATLAS Experiment

ATLAS has developed a new approach for vertex selection in VBF invisible events at HL-LHC conditions, exploiting its new forward tracking capabilities, integrating calorimeter and tracking information to mitigate the impact of pileup vertex merging, and introducing a new way to apply pile-up jet suppression methods for the selection of VBF jets. The new algorithm is insensitive to pileup density and improves the average vertex selection efficiency from 86\% to 95\% under tight VBF event selection cuts.