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

Abstract We study the Standard Model effective field theory ( $$\nu $$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mi>ν</mml:mi></mml:math> 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 $$m_N \sim 0.1 \dots 10\, \,\text {GeV}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msub><mml:mi>m</mml:mi><mml:mi>N</mml:mi></mml:msub><mml:mo>∼</mml:mo><mml:mn>0.1</mml:mn><mml:mo>⋯</mml:mo><mml:mn>10</mml:mn><mml:mspace/><mml:mspace/><mml:mtext>GeV</mml:mtext></mml:mrow></mml:math> . 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 $$\nu $$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mi>ν</mml:mi></mml:math> SMEFT parameter space. Our analysis highlights the importance of performing dedicated searches for new rare tau and top decays.