Charged Lepton Flavour Violation (cLFV) process are highly suppressed in the Standard Model, even forbidden if neutrino mixing is not taken into account. Any observation of cLFV decay would be a clear hint of new Physics Beyond SM, since their expected branching fraction ($\mathcal{B}$) is well below any current experiment sensitivity.
The LHCb collaboration has performed many searches of cLFV involving heavy hadrons in $b\to s l^+l^-$, $b\to c l^-\nu_l$ and $\tau$ decays, setting upper limits to constrain BSM theories.