  This work presents the first experimental observation of the attractive
strong interaction between a proton and a multi-strange baryon (hyperon)
$\Xi^-$. The result is extracted from two-particle correlations of combined
$\rm{p}-\Xi^{-}$$\oplus$$\rm{\bar{p}}-\bar{\Xi}^{+}$ pairs measured in p-Pb
collisions at $\sqrt{s_{\rm{NN}}}=5.02$ TeV at the LHC with ALICE. The measured
correlation function is compared with the prediction obtained assuming only an
attractive Coulomb interaction and a standard deviation in the range
$[3.6,5.3]$ is found. Since the measured
$\rm{p}-\Xi^{-}$$\oplus$$\rm{\bar{p}}-\bar{\Xi}^{+}$ correlation is
significantly enhanced with respect to the Coulomb prediction, the presence of
an additional, strong, attractive interaction is evident. The data are
compatible with recent lattice calculations by the HAL-QCD Collaboration, with
a standard deviation in the range $ [1.8,3.7]$. The lattice potential predicts
a shallow repulsive $\Xi^-$ interaction within pure neutron matter at
saturation densities and this implies stiffer equations of state for
neutron-rich matter including hyperons. Implications of the strong interaction
for the modeling of neutron stars are discussed.
