Diffusion Monte Carlo calculations of fully-heavy multiquark bound states

We use a diffusion Monte Carlo method to solve the many-body Schr\"odinger equation describing fully heavy tetraquark systems. This approach allows us to reduce the uncertainty of the numerical calculation at the percent level, accounts for multiparticle correlations in the physical observables, and avoids the usual quark clustering assumed in other theoretical techniques applied to the same problem. The interaction between particles was modeled by the most general and accepted potential---i. e. , a pairwise interaction including Coulomb, linear-confining and hyperfine spin-spin terms. This means that, in principle, our analysis should provide some rigorous statements about the mass location of the all-heavy tetraquark ground states, which is particularly timely due to the very recent observation made by the LHCb Collaboration of some enhancements in the invariant mass spectra of J/ pairs. Our main results are as follows: (i) The cccc, ccbb (bbcc), and bbbb lowest-lying states are located well above their corresponding meson-meson thresholds. (ii) The J^PC=0^++ cccc ground state with preferred quark-antiquark pair configurations is compatible with the enhancement (s) observed by the LHCb Collaboration. (iii) Our results for the cccb and bbcb sectors seem to indicate that the 0^+ and 1^+ ground states are almost degenerate, with the 2^+ located around 100 MeV above them. (iv) Smaller mass splittings for the cbcb system are predicted, with absolute mass values in reasonable agreement with other theoretical works. (v) The 1^++ cbcb tetraquark ground state lies at its lowest S-wave meson-meson threshold, and it is compatible with a molecular configuration.