Pentaquarks and Tetraquarks at LHCb

Abstract

Exotic resonant structures found in $\Lambda^0_b$ and $\overline{B}^0$ decays into charmonium in the LHCb experiment are discussed. Examination of the $J/\psi p$ system in $\Lambda^0_b\to J/\psi K^- p$ 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 ($\Gamma$) 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 $\psi' \pi^-$ and was first observed by the Belle collaboration in $B^0\to \psi' K^+\pi^-$ decays. Using a sample of approximately 25,000 signal events, LHCb determines the $J^{P}$ to be $1^{+}$.Comment: 18 pages, 19 figures, presented at the European Physical Society Conference, Vienna Austria, July 22-19, and the DPF 2015 Meeting of the American Physical Society Division of Particles and Fields, Ann Arbor, Michigan, August 4-8, 201