Pentaquarks and Tetraquarks at LHCb

Exotic resonant structures found in $Λ^0_b$ and $\overline{B}^0$ decays into charmonium in the LHCb experiment are discussed. Examination of the $J/ψp$ system in $Λ^0_b\to J/ψK^- p$ decays shows two states each of which must be composed of $uudc\overline{c}$ quarks, and thus are called charmonium pentaquarks. Their masses are $4380\pm 8\pm 29$~MeV and $4449.8\pm 1.7\pm 2.5$~MeV, and their corresponding widths ($Γ$) are $205\pm 18\pm 86$ MeV, and $39\pm 5\pm 19$ MeV. The preferred $J^P$ assignments are of opposite parity, with one state having spin 3/2 and the other 5/2. Models of internal binding of the pentaquark states are discussed. Finally, another mesonic state is discussed, the $Z(4430)^-$ that decays into $ψ' π^-$ and was first observed by the Belle collaboration in $B^0\to ψ' K^+π^-$ decays. Using a sample of approximately 25,000 signal events, LHCb determines the $J^{P}$ to be $1^{+}$.

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