Multilepton dark matter signals

J A Aguilar-Saavedra,J Quilis,R Ruiz De Austri,J A Casas

doi:10.1007/jhep04(2020)069

Abstract

The signatures of dark matter at the LHC commonly involve, in simplified scenarios, the production of a single particle plus large missing energy, from the undetected dark matter. However, in Z′-portal scenarios anomaly cancellation requires the presence of extra dark leptons in the dark sector. We investigate the signatures of the minimal scenarios of this kind, which involve cascade decays of the extra Z′ boson into the dark leptons, identifying a four-lepton signal as the most promising one. We estimate the sensitivity to this signal at the LHC, the high-luminosity LHC upgrade, a possible high-energy upgrade, as well as a future circular collider. For Z′ couplings compatible with current dijet constraints the multilepton signals can reach the 5σ level already at Run 2 of the LHC. At future colliders, couplings two orders of magnitude smaller than the electroweak coupling can be probed with 5σ sensitivity.

Highlights

There must be at least one extra scalar, S, whose vacuum expectation value (VEV) breaks the U(1)Y group
We investigate the signatures of the minimal scenarios of this kind, which involve cascade decays of the extra Z boson into the dark leptons, identifying a four-lepton signal as the most promising one
The most obvious of such interactions are those mediated by a Higgs or a Z boson (Higgs and Z portals)

Summary

The model

The simplest extension of the SM that accommodates a leptophobic Z with axial coupling to DM has the following characteristics. Let us mention that there are two additional terms, consistent with the gauge symmetry, that could be added to the previous Lagrangian, namely −λLχLχLS − λRχRχRS∗ plus their Hermitian conjugate These terms induce a splitting of the two lightest degrees of freedom of the DM particle, spoiling its axial coupling to the Z. The expressions for the mass eigenvalues are lengthy and not very illuminating, but they get greatly simplified in the limit where left and right angles are equal, which occurs for y2 = y4 ⇒ θLN = θRN ≡ θN , y1 = y3 ⇒ θLE = θRE ≡ θE This assumption has very mild implications on the collider phenomenology, the most important effect being the modification of angular distributions in decay chains.

Interactions in the mass basis

Constraints from dark matter

Features of the four-lepton signal

Benchmarks

Discovery potential

Summary and discussion