Search for resonant production of strongly-coupled dark matter in proton-proton collisions at 13 TeV

The first collider search for dark matter arising from a strongly-coupled hidden sector is presented, using a data sample corresponding to $137~\mathrm{fb}^{-1}$ collected with the CMS detector at the CERN LHC at a center-of-mass energy of $13~\mathrm{TeV}$. The hidden sector is hypothesized to couple to the standard model (SM) via a heavy leptophobic $\mathrm{Z}^{\prime}$ mediator. The resonant production and decay of such a mediator in proton-proton collisions would result in two ``semi-visible'' jets, which contain both visible matter and invisible dark matter. This would lead to moderate missing energy aligned with one of the jets, a signature ignored by most dark matter searches. The observed dijet transverse mass spectrum is smoothly falling, as expected from the SM; no structure compatible with the signal is observed. Assuming the same couplings as the SM $\mathrm{Z}$ boson, mediator masses up to $3.9~\mathrm{TeV}$ are excluded at $95\%$ confidence level, depending on the other signal model parameters. To enhance the sensitivity of the search for this particular class of models, a boosted decision tree (BDT) is trained using jet substructure variables to distinguish between semi-visible jets and standard model jets from background processes. When the BDT is employed to select events with jets identified as semi-visible, the mediator mass exclusion increases to $5.1~\mathrm{TeV}$, for wider ranges of the other signal model parameters.