Multi Singlet Scalar Dark Matter Extensions of the Standard Model

Abstract

Despite extensive experimental evidence supporting the existence of dark matter (DM), its fundamental nature remains unknown. A common assumption is that DM consists of one or more undiscovered elementary particles. One of the simplest ways of extending the Standard Model (SM) of particle physics with a dark matter particle candidate, without breaking the SU(3)c × SU(2)L × U(1)Y gauge symmetry of the SM or spoiling renormalizability, is by adding a real scalar singlet (RSS) field and imposing an additional unbroken Z2 discrete symmetry that ensures the stability of the corresponding new spin 0 particle. However, this minimal Higgs-portal extension is highly constrained by the observed relic density and direct detection experiments. Being experimentally excluded for DM masses below 3535 GeV, this model cannot be probed through collider searches at the Large Hadron Collider (LHC). In this work, after re-constraining the real scalar singlet SM extension (SM+RSS) with the most recent experiments, we study multi-real-scalar-singlet SM extensions (SM+nRSS) featuring n = 2, 3 real scalar SU(3)c × SU(2)L × U(1)Y singlets and unbroken× n r=1 Z (r) 2 ≡ Z(1) 2 × · · · × Z(n) 2 discrete symmetries. These models provide n = 2, 3 (respectively) stable scalar DM particle candidates that in addition to coupling to the Higgs boson, also couple to each other. In both (n = 2 and n = 3) cases, we obtain a lower non-excluded mass range for at least one of the DM candidates, well below 1 TeV. The corresponding allowed free parameter space regions are, however, close from being excluded by the most recent (2024) LUX-ZEPLIN results. Therefore, not only can these (n = 2, 3) multi-real-scalar-singlet SM extensions be probed in upcoming direct detection experiments, but also through collider searches targeted at the lighter DM particle candidates at the LHC or future colliders.