First observation of an attractive interaction between a proton and a multi-strange baryon
Phys.Rev.Lett.
The collaboration

Abstract (data abstract)
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 p$-\Xi^{-}$ $\oplus$ $\overline{\mathrm{p}}-\overline{\Xi}^{+}$ pairs measured in p--Pb collisions at $\sqrt{s_{\mathrm{NN}}}=5.02$\,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 $[3.9, 4.7]$ is found. Since the measured p$-\Xi^{-}$ $\oplus$ $\overline{\mathrm{p}}-\overline{\Xi}^{+}$ 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 $[1.8, 2.8]$. The lattice potentials predict a shallow repulsive $\Xi^-$ interaction potential 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.