First observation of the $\rm p-\Xi^{-}$ interaction via two-particle correlations

The strong interaction among the different hadrons at relatively small momentum transfers is difficult to describe via direct solutions of Quantum Chromodynamics, due to the running of the coupling constant. Only very recently, attempts of first principle calculations based on lattice simulations have been made and these have yet to be tested. Typically, the description of the interaction among hadrons relies on phenomenological models and effective theories, where several free parameters have to be constrained from experimental observations. Since numerous measurements are available in the nucleonic sector, a sound understanding of N-N interactions is well established. In the strangeness sector however, the experimental knowledge is rather limited and the constraints to pin down precisely the interaction involving hyperons are missing. The lack of experimental data also hinders the validation of novel approaches to describe the baryon interaction based on lattice QCD, which currently provide applicable results for baryon pairs, including at least two or more strange quarks. Furthermore, the poor constraints available for descriptions of interactions involving hyperons, limit the understanding of their appearance and behavior in cold nuclear matter at extreme densities, which thereby hampers a solid modeling of the inner parts of neutron stars. Femtoscopy is based on a study of the two-particle correlation function, where final-state effects among specific particle pairs induce a signal that is characterized by the space-time extend of the particle emission source. Recent studies demonstrate that the strong interaction can be studied from this observable if the characteristics of the emission region are sufficiently constrained. This work extends this approach to investigate the strong interaction of $\rm p-\Xi^{-}$ pairs. In the first part of this analysis, the $\rm p-\rm p$, $\overline{\rm p}-\overline{\rm p}$, $\rm p-\Lambda$ and $\overline{\rm p}-\overline{\Lambda}$ correlation functions are measured in elementary p-Pb and pp collisions at the LHC with the ALICE detector. In a detailed study the source radius is measured as a function of the pair transverse mass $m_{\rm T}$ considering, for the first time in a quantitative way, the effect of strong resonance decays. After correcting for this effect, the radii extracted for pairs of different particle species agree. This indicates that protons, antiprotons, $\Lambda$s and $\overline{\Lambda}$s, as well as $\Xi^{-}$ and $\overline{\Xi}^{+}$ originate from the same source. In the second part, these results are applied to constrain the source size of $\rm p-\Xi^{-}$ pairs. The two-particle correlation function of $\rm p-\Xi^{-}$ and $\overline{\rm p}-\overline{\Xi}^{+}$ pairs is measured with ALICE in both collision systems. The comparison of the measurement with the prediction, assuming only an attractive Coulomb interaction, is not compatible with the data. The enhancement above the Coulomb prediction indicates the presence of an additional component due to the attractive strong interaction among a proton and a $\Xi^{-}$ baryon, which is observed in this way for the first time. Furthermore, the comparison of the data to predictions of this strong interaction from lattice QCD calculations by the HAL QCD collaboration exhibits a qualitative agreement. This serves as a first direct test of these first principle calculations, which supports the description of the N-$\Xi^{-}$ interaction in this approach. The lattice potentials are therefore useful to investigate the role of the $\Xi^{-}$ in the context of neutron stars. Consequently, its interaction within a neutron rich environment is expected to be more repulsive than typically assumed, which suggests a stiffening of the Equation of State of neutron star matter at large densities.