First Observation of an Attractive Interaction between a Proton and a Cascade Baryon

This Letter presents the first experimental observation of the attractive strong interaction between a proton and a multistrange baryon (hyperon) ${\mathrm{\Xi }}^{-}$. The result is extracted from two-particle correlations of combined $p\text{−}{\mathrm{\Xi }}^{-}\oplus \overline{p}\text{−}{\overline{\mathrm{\Xi }}}^{+}$ pairs measured in $p\text{−}\mathrm{Pb}$ collisions at $\sqrt{s_{\mathrm{NN}}}=5.02\mathrm{TeV}$ 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 $p\text{−}{\mathrm{\Xi }}^{-}\oplus \overline{p}\text{−}{\overline{\mathrm{\Xi }}}^{+}$ 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 ${\mathrm{\Xi }}^{-}$ interaction within pure neutron matter 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.

Figure 1

The (a) $p\text{−}p$ and (b) $p\text{−}{\mathrm{\Xi }}^{-}$ correlation functions shown as a function of $k^{*}$. Statistical (bars) and systematic uncertainties (boxes) are shown separately. The filled bands denote the results from the fit with Eq. (1). Their widths correspond to one standard deviation of the systematic error of the fit. The HAL-QCD curve uses potentials obtained from Ref. [42]. The dashed line in the right panel shows the contribution from misidentified $p\text{−}{\tilde{\mathrm{\Xi }}}^{-}$ pairs from the sidebands scaled by its $\lambda$ parameter. See text for details.

Figure 2

Predictions for the $\mathrm{\Xi }$-nucleon potential from the HAL-QCD Collaboration [42] for the different spin ($S$) and isospin ($I$) states. The error bands refer to different Euclidean times considered in the calculation. The inset shows the correlation function computed with the central value of the potential for each of the different states and a source radius of 1.4 fm.