Characterizing Dark Bosons at Chiral Belle

TL;DR

This paper investigates how polarized electron beams at Belle II can enhance the search for invisibly decaying dark bosons, enabling determination of their spin and coupling structures through mono-photon events.

Contribution

It introduces a method to use polarization dependence in mono-photon channels to identify dark boson properties at Belle II, considering various dark vector scenarios.

Findings

Polarization dependence can distinguish dark boson spin and couplings.

Constraints on dark boson couplings can be improved with polarization data.

Analysis of three dark vector scenarios demonstrates potential sensitivity.

Abstract

We explore the advantages of a polarized electron beam at Belle II, as proposed for ``Chiral Belle'', in the search for invisibly decaying (dark) bosons that weakly couple to the Standard Model. By measuring the polarization dependence of the production cross section of dark bosons in association with a photon, the dark boson's spin and Lorentz structure of its couplings can potentially be determined. We analyze the mono-photon channel, $e^+ e^- \rightarrow \gamma + \text{invisible}$, in detail, focusing on the production of an on-shell spin-1 boson. We explore this in the context of three separate scenarios for a new dark vector: a dark photon, a mass-mixed ``dark $Z$'', and a vector that couples to right-handed electrons, and estimate how well the couplings of such bosons to electrons can be constrained in the event of a positive signal.