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Description
The existence of the nucleonic pentaquark resonances $P_{c\bar{c}}(4312)^+$, $P_{c\bar{c}}(4380)^+$, $P_{c\bar{c}}(4440)^+$, $P_{c\bar{c}}(4457)^+$, $P_{c\bar{c}s}(4338)^0$ and $P_{c\bar{c}s}(4459)^0$, established by the LHCb collaboration, has been one of the major discoveries in hadron physics in the latest years. Most of these states (5 out of 6) can be understood as meson-baryon bound states.
By unitarizing the scattering amplitude in the $t$-channel vector-meson exchange interaction model one can investigate the dynamically generated resonances. In the recent works [1,2] we revisited the procedure of the unitarized coupled-channel hidden gauge formalism, which has been a very successful approach in explaining many exotic hadrons in the charm and hidden charm sectors. Employing realistic regularization parameters, we predicted two double strangeness pentaquarks of molecular nature.
Now we go one step further and propose an improved unitarization method. The unitarization procedure requires the regularization of the meson-baryon loop function, commonly done using either a cut-off ($G^{\text{CO}}$) or dimensional regularization ($G^{\text{DR}}$). Although both schemes should yield similar results, some unphysical structures in the T-matrix were found that could not be associated to any resonance or bound state. Now we introduce a novel hybrid loop function ($G^{\text{HY}}$), which combines both dimensional and cut-off regularizations. This approach avoids the generation of unphysical poles and, thus, enables a much cleaner analysis of the scattering amplitude. We shall show that with the use of hybrid loop function the basic properties of the dynamically generated states remain unaltered. Such an improved procedure allowed us to predict two new pentaquark states in the $S = -1$, $I = 1$ sector.
These recently predicted molecular-type pentaquarks (two in $S = -2$, $I = 1/2$ and two in $S = -1$, $I = 1$) are dynamically generated in a very specific way via a strong non-diagonal attraction between the two heaviest meson-baryon channels [1,2]. This effect was overlooked before, because other research groups were discouraged by the repulsive character of the diagonal kernel coefficients, and because the complex structure of the scattering amplitudes obtained with both $G^{\text{DR}}$ and $G^{\text{CO}}$, with unphysical structures, has obscured these physical states.
We hope that our work would stimulate experiments looking for these new pentaquark states, the discovery of which would enrich the family of observed exotic baryons.
References
[1]. J.A. Marsé-Valera, V.K. Magas and A. Ramos, Phys. Rev. Lett. 130 (2023) no.9, 9.
[2]. J.A. Marsé-Valera, V.K. Magas and A. Ramos, Phys. Rev. D 111 (2025) no.5, 5.