Global bounds on heavy neutrino mixing

General bounds on the mixing between heavy and active neutrinos from a complete set of 28 observables are derived. Two scenarios are analysed and compared: (i) a general scenario in which an arbitrary number of heavy neutrinos is integrated out without any further assumption; (ii) only three heavy neutrinos are included and the light neutrino masses and mixings as measured in neutrino oscillations experiments are generated. A $1 − 2 \sigma$ preference for non-zero heavy mixing is founded and implies some interesting correlations in the more constrained scenario (ii). At the $2 \sigma$ level, the bounds on all the squared mixings are extremely similar in both scenarios. If only Lepton Flavour Conserving processes are included, they range from $\mathscr{O} ( 10^{− 2})$ to $\mathscr{O}(10^{−4})$. Adding Lepton Flavour Violating observables as $\mu \rightarrow e \gamma$ to the analysis improves the bound in the $\mu − e$ sector by one order of magnitude.