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We extend a previous study of photoproduction of exotic baryon resonances tothe reaction $\gamma + d \to J/\psi + n + p$, which permits simultaneousinvestigation of the reactions $\gamma + p \to P_c^+ \to J/\psi~p$ \hbox{($n$spectator)} and $\gamma + n \to P_c^0 \to J/\psi~n$ \hbox{($p$ spectator).}Here $P_c^+$ is an exotic baryon with quark content $c \bar c uud$, and $P_c^0$is its hypothetical isospin partner with quark content $c \bar c ddu$. We find:\hbox{(1) The} cross section for $J/\psi~n$ photoproduction should be equal tothat for $J/\psi~p$ photoproduction if these processes are dominated by thephoton coupling to a $c\bar c$ pair. In that case the two processes are equalby isospin reflection. (2) If a $P_c^+$ candidate is a genuine $c \bar c u u d$resonance, its isospin partner $P_c^0 = c \bar c d d u$ should have the samemass (again by isospin reflection). (3) In the absence of Fermi motion, thecross section for photoproduction of $P_c$ off a deuteron should be nearly thesum of two equal cross sections: $\sigma(\gamma p \to P_c^+)$ (spectator $n$)and $\sigma(\gamma n \to P_c^0)$ (spectator $p$). (4) The effects of Fermimotion are significant. They include smearing, form-factor suppression andoffshellness. The upshot is that the resonance is significantly wider and thepeak cross section off a deuteron is expected to be considerably less thantwice that in $\gamma p$.

$J/\psi N$ photoproduction on deuterium as a test for exotic baryons