Dark Higgs-strahlung at Belle II: A distinctive dark sector signature with displaced vertices and missing energy

TL;DR

This paper proposes a novel search strategy for dark Higgs-strahlung at Belle II, focusing on distinctive displaced vertices and missing energy signatures to improve detection of dark sector particles.

Contribution

It introduces the concept of dark Higgs-strahlung signatures with displaced vertices at electron-positron colliders, expanding the search methods for dark sector particles beyond existing approaches.

Findings

Dark Higgs-strahlung can produce unique displaced vertex signatures.

Photon energy and missing mass peaks help suppress background.

Belle II can significantly enhance dark sector model coverage.

Abstract

Dark photons with kinetic mixing are compelling mediators for the interactions between dark matter and Standard Model particles. While most experimental searches focus on fully visible or fully invisible decays of dark photons, we explore processes that involve dark Higgs-strahlung, i.e. the emission of a dark Higgs boson connected to the mass generation of the dark photon. If the dark Higgs boson is the lightest dark sector particle, it is expected to be long-lived and decay into Standard Model particles via Higgs mixing. At electron-positron colliders, dark Higgs-strahlung may occur either in isolation (leading to a single displaced vertex and missing energy) or accompanied by a photon from initial-state radiation. Both signatures offer distinctive kinematic features, such as peaks in photon energy or missing invariant mass, which enable efficient background suppression and enhances sensitivity beyond existing searches. Our study suggests that Belle II could significantly improve coverage of dark sector models by targeting this previously unexplored final state and that combining dark Higgs-strahlung events with and without additional photon offers great potential for reconstructing the properties of the dark sector.