New Laboratory Constraints on Neutrinophilic Mediators

Understand

Neutrinophilic mediators are well-motivated messenger particles that can probe some of the least known sectors of fundamental physics involving nonstandard interactions of neutrinos with themselves and potentially with dark matter.

• In particular, light mediators coupling to the active neutrinos will induce new decay modes of the Standard Model mesons (e.g., $\pi^\pm, K^\pm \to \ell^\pm + \nu + \phi$), charged leptons (e.g., $\tau^\pm \to \pi^\pm + \nu + \phi$), and gauge bosons (e.g., $Z \to \nu + \bar\nu + \phi$).
• A common lore is that these decays suffer from infrared divergences in the limit of the vanishing mediator mass, i.e., $m_\phi \to 0$.
• Here, we show that including the 1-loop contributions of these mediators to the standard 2-body decays (e.g., $\pi^\pm,\,K^\pm \to \ell^\pm + \nu$, etc.), the infrared divergence from the 3-body decay cancels out exactly by virtue of the Kinoshita-Lee-Nauenberg theorem.