On the challenges of searching for GeV-scale long-lived particles at the LHC

TL;DR

This paper highlights the difficulty of detecting GeV-scale long-lived particles at the LHC with existing methods and proposes modifications to improve sensitivity for lighter LLPs.

Contribution

It identifies biases in current displaced vertex searches against light LLPs and suggests specific analysis modifications to enhance detection capabilities.

Findings

Current searches have low efficiency for light LLPs due to existing cuts.

Proposed modifications significantly improve sensitivity to low-mass and short-lived LLPs.

Projected limits show enhanced exclusion potential with new analysis strategies.

Abstract

Many models of dark matter predict long-lived particles (LLPs) that can give rise to striking signatures at the LHC. Existing searches for displaced vertices are however tailored towards heavy LLPs. In this work we show that this bias severely affects their sensitivity to LLPs with masses at the GeV scale. To illustrate this point we consider two dark sector models with light LLPs that decay hadronically: a strongly-interacting dark sector with long-lived exotic mesons, and a Higgsed dark sector with a long-lived dark Higgs boson. We study the sensitivity of an existing ATLAS search for displaced vertices and missing energy in these two models and find that current track and vertex cuts result in very low efficiency for light LLPs. To close this gap in the current search programme we suggest two possible modifications of the vertex reconstruction and the analysis cuts. We calculate projected exclusion limits for these modifications and show that they greatly enhance the sensitivity to LLPs with low mass or short decay lengths.