First measurements of the absolute branching fraction of ${\mathrm{\Lambda }}_c(2625{)}^{+}\to {\mathrm{\Lambda }}_c^{+}{\pi }^{+}{\pi }^{-}$ and upper limit on ${\mathrm{\Lambda }}_c(2595{)}^{+}\to {\mathrm{\Lambda }}_c^{+}{\pi }^{+}{\pi }^{-}$

The absolute branching fraction of the decay ${\mathrm{\Lambda }}_c(2625{)}^{+}\to {\mathrm{\Lambda }}_c^{+}{\pi }^{+}{\pi }^{-}$ is measured for the first time to be $(50.7\pm {5.0}_{\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{t}\mathrm{.}}\pm {4.9}_{\mathrm{s}\mathrm{y}\mathrm{s}\mathrm{t}\mathrm{.}})\mathrm{\%}$ with 368.48 pb${}^{-1}$ of $e^{+}{e}^{-}$ collision data collected by the BESIII detector at the center-of-mass energies of $\sqrt{s}=4.918$ and $4.950$ GeV. This result is lower than the naive prediction of 67%, obtained from isospin symmetry, by more than $2\sigma$, thereby indicating that the novel mechanism referred to as the $\mathit{\text{threshold}}\mathit{\text{effect}}$, proposed for the strong decays of ${\mathrm{\Lambda }}_c(2595{)}^{+}$, also applies to ${\mathrm{\Lambda }}_c(2625{)}^{+}$. This measurement is necessary to obtain the coupling constants for the transitions between $s$-wave and $p$-wave charmed baryons in heavy hadron chiral perturbation theory. In addition, we search for the decay ${\mathrm{\Lambda }}_c(2595{)}^{+}\to {\mathrm{\Lambda }}_c^{+}{\pi }^{+}{\pi }^{-}$. No significant signal is observed, and the upper limit on its branching fraction is determined to be 80.8% at the 90% confidence level.