Search for a Higgs boson decaying through Zd to 4 leptons

Abstract

The Standard Model (SM) is known to be incomplete (it cannot explain dark matter, dark energy, gravitational waves, matter-antimatter asymmetry, etc). The introduction of a Dark Sector via an additional U(1)d gauge symmetry added to the SM Lagrangian could be the long-awaited solution. In this model there is a dark vector boson Zd which can mix with the SM hypercharge gauge boson. This opens the Hypercharge Portal which can mediate the fluctuation of a Z to a Zd, or the decay of the Zd to SM leptons. If a dark Higgs singlet also exists, then this breaks the U(1)d symmetry, allowing for Higgs mixing (parametrised by ) between the SM and dark sector Higgs bosons, and thereby opening the Higgs portal as a discovery channel. Including dark fermionic fields in the Lagrangian allows for long-lived cold Dark Matter candidates. The various connections between the Dark and SM sectors allow descriptions of many key astro-physical phenomena. This contribution discusses an ATLAS search for the dark force boson Zd using its production via the Higgs Portal and its decay back to SM leptons: . The biggest deviation from the Standard Model expectation is from a single event at , with a local significance of 3.2 σ in the 2e2µ channel, and the corresponding global significance is approximately 1.9 σ.