Constraints from $\Lambda$ hypernuclei on the $\Lambda NN$ content of the $\Lambda$-nucleus potential
A depth of $D_{\Lambda}\approx -28$ MeV for the $\Lambda$-nucleus potential was confirmed in 1988 by studying $\Lambda$ binding energies deduced from $(\pi^+,K^+)$ spectra measured across the periodic table. Modern two-body hyperon-nucleon interaction models require additional interaction terms, most likely $\Lambda NN$ three-body terms, to reproduce $D_{\Lambda}$. In this work we apply a suitably constructed $\Lambda$-nucleus density dependent optical potential to binding energy calculations of observed $1s_{\Lambda}$ and $1p_{\Lambda}$ states in the mass range $12\leq A\leq 208$. The resulting $\Lambda NN$ contribution to $D_{\Lambda}$, about 14 MeV repulsion at symmetric nuclear matter density $\rho_0=0.17$ fm$^{-3}$, makes $D_{\Lambda}$ increasingly repulsive at $\rho\gtrsim 3\rho_0$, leading possibly to little or no $\Lambda$ hyperon content of neutron-star matter. This suggests in some models a stiff equation of state that may support two solar-mass neutron stars.