Study of the $Λ$-$Λ$ interaction with femtoscopy correlations in pp and p-Pb collisions at the LHC


This work presents new constraints on the existence and the binding energy of a possible $\Lambda$-$\Lambda$ bound state, the H-dibaryon, derived from $\Lambda$-$\Lambda$ femtoscopic measurements by the ALICE collaboration. The results are obtained from a new measurement using the femtoscopy technique in pp collisions at $\sqrt{s}=13$ TeV and p-Pb collisions at $\sqrt{s_{\mathrm{NN}}}=5.02$ TeV, combined with previously published results from p-Pb collisions at $\sqrt{s}=7$ TeV. The $\Lambda$-$\Lambda$ scattering parameter space, spanned by the inverse scattering length $f_0^{-1}$ and the effective range $d_0$, is constrained by comparing the measured $\Lambda$-$\Lambda$ correlation function with calculations obtained within the Lednicky model. The data are compatible with hypernuclei results and lattice computations, both predicting a shallow attractive interaction, and permit to test different theoretical approaches describing the $\Lambda$-$\Lambda$ interaction. The region in the $(f_0^{-1},d_0)$ plane which would accommodate a $\Lambda$-$\Lambda$ bound state is substantially restricted compared to previous studies. The binding energy of the possible $\Lambda$-$\Lambda$ bound state is estimated within an effective-range expansion approach and is found to be $B_{\Lambda\Lambda}=3.2^{+1.6}_{-2.4}\mathrm{(stat)}^{+1.8}_{-1.0}\mathrm{(syst)}$ MeV.


Phys. Lett. B 797 (2019) 134822
HEP Data
e-Print: arXiv:1905.07209 | PDF | inSPIRE

Figure 1

Results for the fit of the pp data at $\sqrt{s}=13$ TeV. The p-p correlation function (left panel) is fitted with CATS (blue line) and the $\Lambda$-$\Lambda$ correlation function (right panel) is fitted with the Lednicky model (yellow line). The dashed line represents the linear baseline from Eq. 5, while the dark dashed-dotted line on top of the $\Lambda$-$\Lambda$ data shows the expected correlation based on quantum statistics alone, in case of a strong interaction potential compatible with zero

Figure 2

$\Lambda$-$\Lambda$ correlations measured in pp collisions at $\sqrt{s}=13$ TeV (left panel) and pPb collisions at $\sqrt{s_\mathrm{NN}}=5.02$ TeV (right panel) together with the functions computed by the different models [20]. The tested potentials are converted to correlation functions using CATS and the baseline is refitted for each model. The effects of momentum resolution and residuals are included in the theory curves.

Figure 3

Exclusion plot for the $\Lambda$-$\Lambda$ scattering parameters obtained using the $\Lambda$-$\Lambda$ correlations from pp collisions at $\sqrt{s}=7$ and 13 TeV as well as pPb collisions at $\sqrt{s_{\mathrm{NN}}}=5.02$ TeV. The different colors represent the confidence level of excluding a set of parameters, given in $n\sigma$. The black hashed region is where the Lednicky model produces an unphysical correlation. The two models denoted by colored stars are compatible with hypernuclei data, while the red cross corresponds to the preliminary result of the lattice computation performed by the HAL QCD collaboration. For details regarding the region at slightly negative $f^{-1}_0$ and $d_0< 4$, compatible with a bound state, refer to Fig. 4.

Figure 4

The region of the 1$\sigma$ confidence level from Fig. 3, displayed in the ($B_{\Lambda\Lambda},d_0$) plane. The inner (dark) region corresponds to the statistical uncertainty of the method, while the outer (light) region includes the systematic variations. The red star corresponds to the parameters with the lowest $\chi^2$.