Connecting particle physics and cosmology: Measuring the dark matter relic density in compressed supersymmetry models at the LHC

Abstract The identity of Dark Matter (DM) is one of the relevant topics in particle physics today. The R-parity conserving Minimal Supersymmetric Standard Model (MSSM), which naturally provides a DM candidate in the form of the lightest neutralino ( χ 1 0 ), is used as a benchmark scenario to show that a measurement of the DM relic density, Ω χ 1 0 h 2 , can be achieved from measurements at the CERN Large Hadron Collider. Focus is placed on compressed mass spectra regions, where the mass difference Δ m between the χ 1 0 and the supersymmetric partner of the tau lepton ( τ 1 ) is small and where the τ 1 - χ 1 0 coannihilation (CA) mechanism of the early Universe plays an important role. The technique for measuring Ω χ 1 0 h 2 relies on two proposed searches for compressed Supersymmetry (SUSY): (1) production via Vector Boson Fusion (VBF) processes; and (2) production with associated energetic jets from initial state radiation (ISR). These approaches allow for the determination of the relic abundance at the LHC for any model where CA is an important DM reduction mechanism in the early Universe. Thus, it is possible to confirm that the DM we observe today corresponds to χ 1 0 ’s created in the early Universe. We show that from measurements in the VBF and ISR SUSY searches at the LHC, the mass gap Δ m and the dark matter relic density can be measured with an uncertainty of 4.5% and 25%, respectively, assuming 13 TeV proton–proton data from the high-luminosity LHC. The precise measurement of a small Δ m value would also confirm the existence of τ 1 - χ 1 0 CA.