Two-particle correlation functions were measured for $\rm p\overline{p}$, $\rm p\overline{\Lambda}$, $\rm \overline{p}\Lambda$, and $\Lambda\overline{\Lambda}$ pairs in Pb-Pb collisions at $\sqrt{s_{\rm NN}}=2.76$ TeV and $\sqrt{s_{\rm NN}}=5.02$ TeV recorded by the ALICE detector. From a simultaneous fit to all obtained correlation functions, real and imaginary components of the scattering lengths, as well as the effective ranges, were extracted for combined $\rm p\overline{\Lambda}$ and $\rm \overline{p}\Lambda$ pairs and, for the first time, for $\Lambda\overline{\Lambda}$ pairs. Effective averaged scattering parameters for heavier baryon-antibaryon pairs, not measured directly, are also provided. The results reveal similarly strong interaction between measured baryon-antibaryon pairs, suggesting that they all annihilate in the same manner at the same pair relative momentum $k^{*}$. Moreover, the reported significant non-zero imaginary part and negative real part of the scattering length provide motivation for future baryon-antibaryon bound state searches.
PLB 802 (2020) 135223
HEP Data
e-Print: arXiv:1903.06149 | PDF | inSPIRE
Figure 1
Raw invariant mass distribution of $\rm p\uppi^{-}$ ($\rm \overline{p}\uppi^{+}$) pairs used to obtain the $\Lambda$ ($\overline{\Lambda}$) candidates for Pb--Pb collisions at $\sqrt{s_{\rm NN}}=5.02$ TeV in the 0--5\% centrality range. The dashed lines represent the selection width used in the analysis. Note that the mean value of the distribution is slightly shifted from the rest mass of $\Lambda$ ($\overline{\Lambda}$) by $\sim 1$ MeV/$c^2$ due to imperfectionsin the energy loss corrections that are applied in the track reconstruction. |  |
Figure 2
Illustration of the links between different baryon--antibaryon pairs through the residual correlation. Main contributions to the $\rm p\overline{p}$ correlation function are marked in blue, to the $\rm p\overline{\Lambda}$ in yellow and to the $\Lambda\overline{\Lambda}$ in red. Solid lines show connections between studied pairs, while dashed lines present other major residual contributions that are unique for a given system. |  |
Figure 3
Raw invariant mass distribution of $\rm p\uppi^{-}$ ($\rm \overline{p}\uppi^{+}$) pairs used to obtain the $\Lambda$ ($\overline{\Lambda}$) candidates for Pb--Pb collisions at $\sqrt{s_{\rm NN}}=5.02$ TeV in the 0--5\% centrality range. The dashed lines represent the selection width used in the analysis. Note that the mean value of the distribution is slightly shifted from the rest mass of $\Lambda$ ($\overline{\Lambda}$) by $\sim 1$ MeV/$c^2$ due to imperfectionsin the energy loss corrections that are applied in the track reconstruction. |  |
Figure 4
(Top) Comparison of extracted spin-averaged scattering parameters $\Re f_{0}$ and $\Im f_{0}$ for $\rm p\overline{\Lambda}\oplus \overline{p}\Lambda$, $\Lambda\overline{\Lambda}$ pairs and for effective $\rm B\overline{B}$ pairs, with previous analyses of $\rm p\overline{p}$ pairs (singlet) . (Bottom) Comparison of extracted spin-averaged scattering parameters $\Re f_{0}$ and $d_{0}$ for $\rm p\overline{\Lambda}\oplus \overline{p}\Lambda$, $\Lambda\overline{\Lambda}$ pairs as well as effective $\rm B\overline{B}$, with selected previous analyses of other pairs: $\rm pp$ (singlet) , $\rm p\overline{p}$ (singlet) , $\rm pn$ (singlet) , $\rm nn$ (singlet) , $\rm p\Lambda$ (singlet) , and $\Lambda\Lambda$ (spin-averaged) . (Note that the measurement of the $\Lambda\Lambda$ scattering parameters by the STAR experiment did not account for residual correlations. The recent analysis of $\Lambda\Lambda$ correlations by the ALICE Collaboration , properly taking into account those correlations, disfavours the STAR results.) |   |