Search for the pentqauark partners: $\Sigma^0$, $\mathrm{N}^0$ and $\Theta^{++}$

In 1997 based on chiral soliton model a narrow exotic state, $\Theta^+$(1540), which has quark component $uudd\bar{s}$, was predicted in a SU(3)$_F$ antidecuplet of pentaquarks. If such state exists, other members of the antidecuplet could be expected to have sufficiently narrow widths to be observed as well. The Jefferson Laboratory experiment E04-012 focused on the search for $\Sigma^0$ and N$^0$ partner states in the missing mass spectra of the H(e,e'K$^+$)X and H(e,e'$\pi^+$)X channels. In addition, if the $\Theta^+$ has non-zero isospin then a hypothetical isospin partner, $\Theta^{++}$, might be expected in the H(e,e'K$^-$)X channel which was also studied. The experiment was performed in Hall~A at Jefferson Lab using a 5~GeV CW electron beam incident on a liquid hydrogen target. The two high resolution spectrometers were coupled to septum magnets to get 6 degrees' forward angles. Kaons were identified using a combination of a ring-imaging Cherenkov detector and two aerogel Cherenkov counters. The missing mass resolution was determined to be 3.5~MeV FWHM using neutron, $\Lambda$(1116) and $\Sigma$(1193) productions and provided a high sensitivity to narrow resonances. A precise measurement of the $\Lambda$(1520) resonance has also been conducted for a cross-section comparison. As a result, no significent narrow structures were observed in any of the three channels so a Feldman-Cousins approach was used to determine the 90\% confidence interval.