Search for exotic long-lived particles using disappearing tracks with the CMS experiment in proton-proton collisions at $\sqrt{s}$=13 TeV

A search for heavy charged long-lived particles is presented in the context of theories beyond the Standard Model (BSM) with a small mass splitting between the two lightest newly proposed particles. Such BSM models can be targeted using the signature of a track that disappears in the inner tracking detector. The lightest particle in the final state of the long-lived particle decay presents a candidate for dark matter, whose existence is strongly hinted at by astrophysical evidence. A small mass splitting between the two lightest new particles can for example occur between the chargino and a neutralino in the context of the minimal supersymmetric Standard Model. Given a sufficiently small mass splitting in the range of m${}_{\pi }$ ≲ Δm ≲ 200 MeV, the chargino is expected to decay in the CMS tracker volume into soft leptons, which are not reconstructed, or hadrons and a lightest supersymmetric particle, leaving a short track that then seems to disappear. The signature is a reconstructed track with missing hits in the outer layers of the tracker, with little or no energy deposited in the calorimeter in the neighborhood of the track's trajectory. Events are selected with one or more disappearing tracks in final states with at least one jet as well as with and without leptons. The leptonic channel serves to target top and bottom squark production, for which leptons can arise due to leptonic top quark decays. For both event topologies, the analysis is binned in terms of number of jets, missing transverse momentum, and the number of b-quark-tagged jets in order to provide sensitivity to the chargino production mechanism. A further binning in terms of the energy loss dE/dx provides sensitivity to the chargino mass, as the chargino is assumed to be highly ionizing. Data-driven methods are used to determine the Standard Model backgrounds, which can arise either as the result from mal-reconstructed leptons, or from tracks reconstructed from random combination of hits. Results are presented using 137 fb${}^{-1}$ of proton-proton collision data at $\sqrt{s}$ = 13 TeV collected with the CMS experiment during Run 2, with mass exclusion limits reaching up to squark masses and dark matter masses of m${}_{\overline{b}}$ ≈ 1.575 TeV and m${}_{{\chi }_1^0}$ ≈ 1.15 TeV, as well as of m${}_{\overline{t}}$ ≈ 1.6 TeV and m${}_{{\chi }_1^0}$ ≈ 1.2 TeV. Mass exclusion limits for the gluino mass and dark matter mass reach up to m${}_{\overline{g}}$ ≈ 2.25 TeV and m${}_{{\chi }_1^0}$ ≈ 1.7 TeV.