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

TL;DR

This paper investigates the photoproduction of exotic baryons involving J/psi on deuterium, testing isospin symmetry and the existence of isospin partner states, with implications for understanding exotic pentaquark states.

Contribution

It extends previous studies to deuterium, analyzing isospin relations and the effects of Fermi motion on the photoproduction cross sections of exotic baryons.

Findings

Cross sections for J/psi n and J/psi p photoproduction should be equal under certain conditions.

The isospin partner of a P_c^+ state should have the same mass if it exists.

Fermi motion significantly broadens the resonance and reduces the peak cross section.

Abstract

We extend a previous study of photoproduction of exotic baryon resonances to the reaction $\gamma + d \to J/\psi + n + p$, which permits simultaneous investigation 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 to that for $J/\psi~p$ photoproduction if these processes are dominated by the photon coupling to a $c\bar c$ pair. In that case the two processes are equal by 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 same mass (again by isospin reflection). (3) In the absence of Fermi motion, the cross section for photoproduction of $P_c$ off a deuteron should be nearly the sum 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 Fermi motion are significant. They include smearing, form-factor suppression and offshellness. The upshot is that the resonance is significantly wider and the peak cross section off a deuteron is expected to be considerably less than twice that in $\gamma p$.