Dark matter searches in invisible Higgs boson decays with the CMS experiment: Analysing Higgs boson decays to invisible particles in ttH and VH final states and combining all final states using Run 1 and Run 2 CMS data

Dark matter (DM) is a well-known but as-of-yet unidentified phenomenon that con- stitutes more than a quarter of the universe. It is dark in that it neither absorbs nor emits light; it is electromagnetically inert. Its effects, however, are observable: spiral galaxies behave in such a way that their rotation is more like what is expected in the presence of substantially more mass than that which is visible. In the Bullet Cluster, where two galaxies are colliding, gravitational lensing techniques show how the centre of mass of these galaxies is consistent with that of two large bodies of DM-dominated mass. A high energy environment such as that of the Large Hadron Collider (LHC) at CERN would be ideal to produce DM particles with a mass O(TeV), where proton- proton collisions take place and purpose-built detectors such as the Compact Muon Solenoid (CMS) detector can record the collisions and reconstruct the particles pro- duced. One of the primary aims in the current frontier of high energy physics is to detect DM directly, although so far this has not been achieved. In the LHC, efforts to discover DM rely on production from standard model (SM) particles, including via models where the Higgs boson acts as a mediator. With the CMS detector, 165 fb −1 data has been recorded across the periods 2011–2013 (Run 1) and 2015–2018 (Run 2), with which in light of the absence of DM discovery limits are set on the branching fraction B(H → inv), the probability of decays of Higgs bosons to invisible particles not detected in the CMS detector. In the standard model, given the only invisible decays of the Higgs boson are to neutrinos, B(H → inv) is approximately 0.12%. Using 2016–2018 data, B(H → inv) is set at 54% observed (39% expected) at the 95% confidence level (CL) in the combined ttH and VH channel. Using both Run 1 and Run 2 data from the CMS detector, this is set at 15% observed (8% expected) at the 95% CL, from a combination of channels in which a Higgs boson is produced in association with another or other particles.