Amplifying muon-to-positron conversion in nuclei with ultralight dark matter

Abstract

We present an analysis of the lepton-number and lepton-flavour-violating process of muon-to-positron conversion $\hat{I}{¼}^{-}+N\to e^{+}+N^{\prime }$, in the presence of an ultralight scalar dark matter (ULSDM) field which couples to neutrinos. The ULSDM contributes to the effective off-diagonal Majorana mass $m_{\hat{I}¼e}$, therefore amplifying the rate of muon-to-positron conversion to experimentally observable levels. Using existing bounds from SINDRUM II, COMET, and Mu2e experiments, we derive novel constraints on the flavour-off-diagonal couplings of neutrinos to ULSDM. Our work reveals that upcoming experiments can provide stronger sensitivity to these new couplings than bounds arising from cosmological surveys and terrestrial experiments.

Invited edition for the special issue of Emerging Leaders 2025 in the Journal of Physics G: Nuclear and Particle Physics, 8 pages, 3 figures