Search for Dark Matter Production in Association with a Higgs Boson with the ATLAS Detector at the LHC

Many cosmological and astrophysical observations indicate the existence of Dark Matter (DM) in the Universe with an abundance of about five times the one of visible matter. Several theories beyond the Standard Model (SM) of particle physics predict the existence of DM particles, and in particular, their associated production with a Higgs boson at the Large Hadron Collider (LHC). A search for DM particles has been performed in the mono-Higgs channel in proton-proton collisions at a center-of-mass energy of 13 TeV. The Higgs boson decaying into a $b\bar{b}$ quark pair is produced in association with large missing transverse energy related to the DM particles. The search for DM in the mono-Higgs channel is complementary to searches for DM in association with other SM particles at the LHC. For high missing transverse energy, the Higgs boson can be highly boosted requiring novel jet substructure analysis techniques for the reconstruction of the $h\to b\bar{b}$ decays. Data of the ATLAS experiment at the LHC corresponding to integrated luminosities of 36.1fb$^{-1}$ and 79.8fb$^{-1}$ recorded from 2015 to 2017 have been analysed. In the absence of a signal, the results are interpreted in terms of exclusion limits for simplified DM models with heavy mediators of the interaction between DM and SM particles, including models with a $Z'_B$ boson from a $\mathrm{U}(1)_\mathrm{B}$ gauge symmetry generated by baryon number $\mathrm{B}$ or with additional vector or pseudoscalar mediators in two Higgs-doublet models. Vector mediator masses up to 2.6 TeV are excluded at 95$\%$ CL and pseudoscalar mediator masses up to 260 GeV based on the 36.1/fb dataset. 90$\%$CL upper limits on the spin-independent DM-nucleon scattering cross section in the $Z'_B$ model have been derived and compared with direct search results. For DM masses below 4 GeV, the constraints on the $Z'_B$ mediated DM-nucleon cross section are more stringent than direct detection limits. Furthermore, upper limits on the visible production cross section with reduced model dependence are placed for associated production of DM particles with the Higgs boson. Using the increased dataset of 79.8/fb and improved reconstruction techniques for the boosted Higgs decays into $b\bar{b}$ based on variable-radius jets, vector mediator masses up to 2.8 TeV have been excluded at 95$\%$ CL.