Θ⁺Search at HERMES with Deuterium and Hydrogen Targets

Abstract

The previous search at HERMES for narrow baryon states excited in quasi-real photoproduction, decaying through the channel Θ + → pK 0 S → pπ + π − , has been extended. Improved decay-particle reconstruction, more advanced particle identification, and increased event samples are employed. The structure that was observed earlier at an invariant mass of 1528 MeV shifts to 1522 MeV in the new analysis of data with a deuterium target, with a drop of statistical significance to about 2σ. The number of events above background is 68 +98 −31 (stat) ± 13(sys). No such structure is observed in the hydrogen data set.