$\Lambda NN$ and $\Sigma NN$ systems at threshold

We calculate the hypertriton binding energy and the $\Lambda d$ and $\Sigma d$ scattering lengths using baryon-baryon interactions obtained from a chiral constituent quark model. We study consistently the $\Lambda NN$ and $\Sigma NN$ systems analyzing the effect of the $\Sigma \leftrightarrow \Lambda$ conversion. Our interactions correctly predict the hypertriton binding energy. The $(I,J)=(0,3/2)$ $\Lambda NN$ channel is also attractive and it might have a bound state. From the condition of nonexistence of a (0,3/2) $\Lambda NN$ bound state, an upper limit for the spin-triplet $\Lambda N$ scattering length is obtained. We also present results for the elastic and inelastic $\Sigma N$ and $\Lambda N$ cross sections. The consistent description of the $\Sigma N$ scattering cross sections imposes a lower limit for the corresponding spin-triplet scattering lengths. In the $\Sigma NN$ system the only attractive channels are $(I,J)=(1,1/2)$ and $(0,1/2)$, the $(1,1/2)$ state being the most attractive one.