$K\to\pi\nu\bar\nu$ spectra and NA62 interpretation

TL;DR

This paper analyzes the $K^+ o ext{pi}^+ uar{ u}$ decay spectrum to set model-independent constraints on new physics, considering both Majorana and Dirac neutrinos and sterile neutrinos.

Contribution

It introduces a framework to incorporate sterile neutrino effects into $K$ decay spectra analysis within the weak effective theory at dimension-six.

Findings

Current constraints on $K^+ o ext{pi}^+ uar{ u}$ decays are established.

Projected future measurements will tighten bounds on new physics.

The approach accounts for neutrino mass effects, including sterile neutrinos.

Abstract

Using the measured and projected invisible mass spectrum of the $K^+\to\pi^+\nu\bar\nu$ mode, we determine the current and future constraints within the model-independent framework of the weak effective theory at dimension-six. We work in two different operator bases depending whether neutrinos are Majorana or Dirac fermions. This makes it possible to transparently incorporate mass effects of additional sterile neutrinos for all operators.