Abstract
Observations of exotic structures in the \$J/p\$ channel, that we refer to
as pentaquark-charmonium states, in \$Łambda\_b^0J/K^- p\$ decays are
presented. The data sample corresponds to an integrated luminosity of 3/fb
acquired with the LHCb detector from 7 and 8 TeV pp collisions. An amplitude
analysis is performed on the three-body final-state that reproduces the
two-body mass and angular distributions. To obtain a satisfactory fit of the
structures seen in the \$J/p\$ mass spectrum, it is necessary to include two
Breit-Wigner amplitudes that each describe a resonant state. The significance
of each of these resonances is more than 9 standard deviations. One has a mass
of \$4380829\$ MeV and a width of \$2051886\$ MeV, while the second
is narrower, with a mass of \$4449.81.72.5\$ MeV and a width of \$39\pm
519\$ MeV. The preferred \$J^P\$ assignments are of opposite parity, with one
state having spin 3/2 and the other 5/2.
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