New Laboratory Constraints on Neutrinophilic Mediators

TL;DR

This paper refines constraints on neutrinophilic mediators by accounting for quantum loop effects, resolving infrared divergence issues, and extending experimental limits on these particles beyond previous bounds, impacting future neutrino interaction studies.

Contribution

It demonstrates the exact cancellation of infrared divergences in decay processes involving light mediators by including 1-loop corrections, leading to more accurate laboratory constraints.

Findings

Infrared divergences cancel when including 1-loop corrections.

Updated constraints extend beyond parent particle masses.

Implications for future neutrino interaction detection.

Abstract

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. Including these cancellation effects, we then update the existing laboratory constraints on neutrinophilic scalar mediators, thereby extending the limits far beyond the decaying parent particle mass and excluding a wider range of parameter space. These new ``physical'' limits derived here have significant implications for the future detection prospects of nonstandard neutrino (self-)interactions.