Masses and magnetic moments of exotic fully heavy pentaquarks

Inspired by the observation of a resonant state $X(6600)$ of fully charm tetraquark by the CMS experiment of LHCb Collaboration in double $J\mathrm{/}\psi$ decay channel, we perform a systematical study of all configurations of fully heavy pentaquarks $P_{Q_1{Q}_2{Q}_3{Q}_4\overline{Q_5}}$ ($Q_i=c,b$%, $i=1,2,3,4,5$) in their ground states in unified framework of MIT bag model. The color-spin wavefunctions of pentaquarks, classified via Young tableau and presented in terms of the Young-Yamanouchi bases, are used to compute masses and magnetic moments of fully heavy pentaquarks via numerical variational method, predicting a set of masses ranging from $8.229$ GeV for the $P_{cccc\bar{c}}$ to $24.770$ GeV for the $P_{bbbb\bar{b}}$. Combining with computed masses of fully heavy mesons and baryons, we find that masses of fully heavy hadrons(mesons, baryons, tetraquarks and pentaquarks) with identical flavor rise almost linearly with the number of valence quarks in hadrons, being consistent with the heavy quark symmetry in the heavy-quark limit.