Eur. Phys. J. C (2020) 80: 743
https://doi.org/10.1140/epjc/s10052-020-8339-2

Regular Article - Theoretical Physics

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

¹ Department of Physics, Institute for Particle Physics Phenomenology, Durham University, South Road, Durham, DH1 3LE, UK
² CAFPE and Departamento de Física Teórica y del Cosmos, Universidad de Granada, 18071, Granada, Spain

^* e-mail: mikael.chala@durham.ac.uk

Received: 8 July 2020
Accepted: 9 August 2020
Published online: 14 August 2020

Abstract

We study the Standard Model effective field theory (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 . 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 SMEFT parameter space. Our analysis highlights the importance of performing dedicated searches for new rare tau and top decays.