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

Abstract

The first collider search for dark matter arising from a strongly coupled hidden sector is presented and uses a data sample corresponding to 138 fb−1, collected with the CMS detector at the CERN LHC, at $$ \sqrt{s} $$ s = 13 TeV. The hidden sector is hypothesized to couple to the standard model (SM) via a heavy leptophobic Z′ mediator produced as a resonance in proton-proton collisions. The mediator decay results in two “semivisible” jets, containing both visible matter and invisible dark matter. The final state therefore includes moderate missing energy aligned with one of the jets, a signature ignored by most dark matter searches. No structure in the dijet transverse mass spectra compatible with the signal is observed. Assuming the Z′ boson has a universal coupling of 0.25 to the SM quarks, an inclusive search, relevant to any model that exhibits this kinematic behavior, excludes mediator masses of 1.5–4.0 TeV at 95% confidence level, depending on the other signal model parameters. To enhance the sensitivity of the search for this particular class of hidden sector models, a boosted decision tree (BDT) is trained using jet substructure variables to distinguish between semivisible jets and SM jets from background processes. When the BDT is employed to identify each jet in the dijet system as semivisible, the mediator mass exclusion increases to 5.1 TeV, for wider ranges of the other signal model parameters. These limits exclude a wide range of strongly coupled hidden sector models for the first time.

Leptons lurking in semi-visible jets at the LHC

“…In our study, anti- jets [ 50 , 51 ] with (AK8 jets) are reconstructed requiring a minimum for clustering of 200 GeV. We use AK8 jets since semi-visible jets are expected to be wider than typical SM jets because they arise from a multi-step process beginning with dark quarks produced via and ending up with SM hadrons after hadronization in the hidden sector and in the SM [ 52 ]. All the samples have been normalized to the LHC Run 2 integrated luminosity .…”

“…Following the most recent recommendations from the LHC DM Working Group [ 53 ], the coupling has been assumed to be flavour-universal and set to 0.25. Taking into account the number of flavours and colours in the dark sector as in [ 52 ], the coupling has been fixed to 0.4. For this set of couplings, the constraints from the di-jet searches are still weak [ 30 , 52 , 54 ].…”

“…Taking into account the number of flavours and colours in the dark sector as in [ 52 ], the coupling has been fixed to 0.4. For this set of couplings, the constraints from the di-jet searches are still weak [ 30 , 52 , 54 ]. The number of stable and unstable dark hadrons produced in the dark hadronization process can vary according to the details of the dark sector.…”

“…In this study, we have considered the same background processes as in the CMS semi-visible jets search [ 52 ]. All the background samples have been generated at LO.…”

“…Furthermore, in the fully hadronic SVJ search [ 52 ], the background from boosted top decays in semi-leptonic final states is reduced by applying a veto on mini-isolated leptons [ 62 , 63 ].…”