Measurement of the ratio of branching fractions of the decays $\Lambda_b^0 \!\to \psi(2S) \Lambda$ and $\Lambda_b^0 \!\to J\!/\!\psi \Lambda$

Using pp collisions corresponding to 3 fb$^{−1}$ integrated luminosity, recorded by the LHCb experiment at centre-of-mass energies of 7 and 8 TeV, the ratio of branching fractions $ \mathrm{\mathcal{B}}\left({\Lambda}_b^0\to \psi (2S)\Lambda \right)/\mathrm{\mathcal{B}}\left({\Lambda}_b^0\to J/\psi \Lambda \right)=0.513\pm 0.023\left(\mathrm{stat}\right)\pm 0.016\left(\mathrm{syst}\right)\pm 0.011\left(\mathrm{\mathcal{B}}\right) $ is determined. The first uncertainty is statistical, the second is systematic and the third is due to the external branching fractions used.