A Search for Charged Excitation of Dark Matter with the KamLAND-Zen Detector

Abstract

There are many theories where a dark matter particle is part of a multiplet with an electrically charged state. If WIMP dark matter ($\ddot{I}{‡}^0$) is accompanied by a charged excited state ($\ddot{I}{‡}^{-}$) separated by a small mass difference, it can form a stable bound state with a nucleus. In supersymmetric models, the $\ddot{I}{‡}^0$ and the $\ddot{I}{‡}^{-}$ could be the neutralino and a charged slepton, such as the neutralino-stau degenerate model. The formation binding process is expected to result in an energy deposition of {\it O}(1--10 MeV), making it suitable for detection in large liquid scintillator detectors. We describe new constraints on the bound state formation with a xenon nucleus using the KamLAND-Zen 400 Phase-II dataset. In order to enlarge the searchable parameter space, all xenon isotopes in the detector were used. For a benchmark parameter set of $m_{\ddot{I}{‡}^0}=100$ GeV and $\hat{I}”m=10$ MeV, this study sets the most stringent upper limits on the recombination cross section $⟨\ddot{I}ƒv⟩$ and the decay-width of $\ddot{I}{‡}^{-}$ of $2.0\times 10^{-31}$ $\mathrm{c}{\mathrm{m}}^3/s$ and $1.1\times 10^{-18}$ GeV, respectively (90\% confidence level).