Interpretation of LHCb Hidden-Charm Pentaquarks within the Compact Diquark Model

TL;DR

This paper interprets the LHCb observed hidden-charm pentaquarks as compact diquark-diquark-antiquark states, predicting a rich spectrum of such pentaquarks with specific quantum numbers and decay properties, guiding future experimental searches.

Contribution

It introduces a detailed diquark model for hidden-charm pentaquarks, calculating their mass spectrum and quantum numbers, and predicts additional states below decay thresholds.

Findings

Predicted a spectrum of hidden-charm pentaquarks with specific quantum numbers.

Identified some states as potentially below decay thresholds, implying narrower widths.

Provided guidance for future experimental searches at LHC and Tera-Z factories.

Abstract

The LHCb collaboration have recently updated their analysis of the resonant $J/\psi\, p$ mass spectrum in the decay $\Lambda_b^0 \to J/\psi\, p\, K^-$, making use of their combined Run~1 and Run~2 data. In the updated analysis, three narrow states, $P_c (4312)^+$, $P_c (4440)^+$, and $P_c (4457)^+$, are observed. The spin-parity assignments of these states are not yet known. We interpret these narrow resonances as compact hidden-charm diquark-diquark-antiquark pentaquarks. Using an effective Hamiltonian, based on constituent quarks and diquarks, we calculate the pentaquark mass spectrum for the complete $SU (3)_F$ lowest $S$- and $P$-wave multiplets, taking into account dominant spin-spin, spin-orbit, orbital and tensor interactions. The resulting spectrum is very rich and we work out the quark flavor compositions, masses, and $J^P$ quantum numbers of the pentaquarks. However, heavy quark symmetry restricts the observable states in $\Lambda_b$-baryon, as well as in the decays of the other weakly-decaying $b$-baryons, $\Xi_b$ and $\Omega_b$. In addition, some of the pentaquark states are estimated to lie below the $J/\psi\, p$ threshold in $\Lambda_b$-decays (and corresponding thresholds in $\Xi_b$- and $\Omega_b$-decays). They decay via $c \bar c$ annihilation into light hadrons or a dilepton pair, and are expected to be narrower than the $P_c$-states observed. We anticipate their discovery, as well as of the other pentaquark states present in the spectrum at the LHC, and in the long-term future at a Tera-$Z$ factory.