The effective field theory of low scale see-saw at colliders

TL;DR

This paper investigates the collider phenomenology of right-handed neutrinos within the $ u$SMEFT framework, identifying weakly constrained scenarios and proposing new search strategies for rare decays involving photons.

Contribution

It introduces a comprehensive analysis of $ u$SMEFT operators with right-handed neutrinos, highlighting poorly constrained parameter regions and suggesting novel collider search methods.

Findings

Bounds on operator coefficients are generally at the TeV scale.

Some operator coefficients remain poorly constrained by current data.

Proposes new searches for rare top and tau decays involving photons.

Abstract

We study the Standard Model effective field theory ($\nu$SMEFT) extended with operators involving right-handed neutrinos, focussing on the regime where the right-handed neutrinos decay promptly on collider scales to a photon and a Standard Model neutrino. This scenario arises naturally for right-handed neutrinos with masses of the order $m_N \sim 0.1 \dots 10\, \text{GeV}$. We limit the relevant dimension-six operator coefficients using LEP and LHC searches with photons and missing energy in the final state as well as pion and tau decays. While bounds on new physics contributions are generally in the TeV scale for order one operator coefficients, some coefficients, however, remain very poorly constrained or even entirely evade bounds from current data. Consequently, we identify such weakly constrained scenarios and propose new searches for rare top and tau decays involving photons to probe potential new physics in the $\nu$SMEFT parameter space. Our analysis highlights the importance of performing dedicated searches for new rare tau and top decays.