Search for Rare Higgs Boson Production and Beyond the Standard Model Physics with Bottom Quarks and Tau Leptons at the CMS Experiment

The fundamental constituents of matter, called fermions, as described by the Standard Model of particle physics, are grouped in three generation with increasing mass. The heaviest fermions are particularly interesting, as they have the strongest coupling to the Higgs boson and are often most sensitive to theories going beyond the Standard Model. The heaviest lepton is the tau lepton and the heaviest quarks are the top and the bottom quarks. In this thesis, two searches are described with tau leptons and bottom quarks measured in the CMS detector from proton-proton collisions taken in the years 2016 to 2018. The CMS experiment is one of the main experiments at the Large Hadron Collider and measures the properties of particles produced in proton-proton or heavy ion collisions. One search is for a rare and so far not observed process predicted by the Standard Model, the Higgs boson production in association with bottom quarks. The Higgs boson is identified through its decay to a tau lepton pair or a W boson pair. This is the first search for the single Higgs boson production in association with bottom quarks, resulting in an upper limit on the signal cross section. The observed (expected) upper limit at the 95% confidence level on the signal strength is 3.7 (6.1) times the value expected in the Standard Model. The second search is for vector-like leptons, predicted by the 4321 model, a beyond the Standard Model theory that introduces additional gauge symmetries. The model also predicts leptoquarks, which couple leptons to quarks with the strongest coupling to third generation fermions. The flavor-dependent coupling strengths can address flavor hierarchies and potential lepton flavor violations. The vector-like leptons couple through leptoquarks to Standard Model fermions and are expected to have mass of smaller than 2 TeV. A search for vector-like leptons in a fully hadronic final state, performed by the CMS Collaboration, found an excess of data events with a deviation from the background-only hypothesis. The excess is checked by the described search in a final state with one electron or muon, one hadronic tau lepton decay and at least three bottom quarks. In the future, the High Luminosity Large Hadron Collider will increase the proton collision rate to deliver large amounts of data that enables the CMS and other experiments to search for processes even more rare. The change in conditions requires an upgrade of the CMS detector, including the innermost part, the silicon pixel detector. The best pixel sensors for the upgrade are evaluated in a research and development campaign, which includes testing pixel sensors in testbeams. My contribution to the testbeam campaign is described in this thesis.