Mass spectrum of spin-1/2 pentaquarks with a $c\bar{c}$ component and their anticipated discovery modes in $b$-baryon decays

The LHCb discovery of the two baryonic states $P_c^+(4380)$ and $P_c^+(4450)$, having $J^{P}=3/2^-$ and $J^{P}=5/2^+$, respectively, in the process $pp \to b \bar{b} \to Λ_b X$, followed by the decay $Λ_b \to J/ψ\; p\; K^-$, has motivated a number of theoretical models. Interpreting them as compact $\{\bar{c}\; [c u] \; [ud]; L_{\mathcal{P}}=0,1\}$ objects, the mass spectroscopy of the $J^P=3/2^-$ and $J^P=5/2^+$ pentaquarks was worked out by us for the pentaquarks in the $SU(3)_F$ multiplets, using an effective Hamiltonian based on constituent diquarks and quarks. Their possible discovery modes in $b$-baryon decays were also given using the heavy quark spin symmetry. In this paper, we calculate the mass spectrum of the hidden $c\bar{c}$ pentaquarks having $J^{P}=\frac{1}{2}^\pm$ for the $SU(3)_F$ multiplets and their anticipated discovery modes in $b$-baryon decays. Some of the $P_c^+(J^P=1/2^\pm$) pentaquarks, produced in the $Λ_{b}$ decays may have their masses just below the $J/ψ\, p$ threshold, in which case they should be searched for in the modes $ P_c^+(J^P=1/2^\pm) \to η_c\; p,\;\; μ^+ μ^- \;p,\;\; e^+ e^-\; p$.