Hit multiplicity approach to b-tagging in FCC-hh

The FCC-hh project aims to study proton-proton collisions at $\sqrt{s}$=100 TeV. The ability to tag heavy flavour jets of high energy produced in those collisions will be crucial for many beyond the Standard Model searches such as Z' production. Tagging highly energetic jets at FCC-hh is challenging not only due to the presence of up to 1000 pile-up interactions per bunch crossing, but also due to the dense environments inside the boosted jets and the very displaced secondary vertices arising from the B- and C-hadron decays. In particular, the difficulties of reconstructing tracks from B-hadron decay products in such environments have a dramatic impact on the b-tagging performance. A new approach for tagging high energy b-jets without reconstructing tracks and secondary vertices has been suggested. The so-called "hit multiplicity jump" tagger is based on the number of hits in the different detector layers. It makes use of the fact that a long-lived particle decaying between two layers will cause the hit multiplicity to increase between those layers. This document describes the adaptation of this approach to the FCC-hh detector and its test on central jets with transverse momentum of up to 5 TeV for the first time. The approach of combining the information on the number of hits in each detector layer in a multivariate discriminator achieves a performance that is not only comparable but potentially superior to the discrimination achieved by the traditional approach based on track and secondary vertex reconstruction.