Probing Baryogenesis with Displaced Vertices at the LHC

Abstract

The generation of the asymmetric cosmic baryon abundance requires a departure from thermal equilib-rium in the early universe. In a large class of baryogenesis models, the baryon asymmetry results from the out-of-equilibrium decay of a new, massive particle. We highlight that in the interesting scenario where this particle has a mass at the weak scale, this out-of-equilibrium condition requires the proper decay length cτ & 1 mm. Such new fields are within reach of high-energy colliders such as the Large Hadron Collider (LHC), at which they can be pair produced leaving a distinctive, displaced-vertex signature. This scenario is realized in the recently proposed mechanism of baryogenesis where the baryon asymmetry is produced through the freeze-out and subsequent decay of a meta-stable weakly interacting massive particle (“WIMP baryogenesis”). In analogy to missing energy searches for stable WIMP dark matter, collider experiments such as the LHC are excellent probes of these new long-lived particles responsible for baryogenesis via the low-background displaced vertex channel. In our paper, we estimate the limits on simplified models inspired by WIMP baryogenesis from two of the most sensitive collider searches by CMS and ATLAS with 8 TeV LHC data. We also estimate the LHC reach at 13 TeV using current strategies, and demonstrate that up to two orders of magnitude of improvement in cross-section limits can be achieved by requiring two displaced vertices while simultaneously lowering kinematic thresholds. For meta-stable WIMPs produced through Standard Model weak gauge interactions, the high luminosity LHC has sensitivity to masses up to 2-2.5 TeV for lifetimes cτ ∼ 1 cm, while for singlet meta-stable WIMPs pair-produced through the off-shell-Higgs portal, the LHC is sensitive to production cross sections of O(10) ab for benchmark masses around 150 GeV. Our analysis and proposals also generally apply to displaced vertex signatures from other new physics such as hidden valley models, twin Higgs models and displaced supersymmetry.