Evidence for a narrow |S|=1 baryon state at a mass of 1528 MeV in quasi-real photoproduction

TL;DR

This paper reports evidence for a narrow S=+1 baryon state at 1528 MeV observed in quasi-real photoproduction, potentially corresponding to the exotic Theta+ pentaquark, with a significance of 4-6 sigma.

Contribution

First experimental evidence of a narrow S=+1 baryon at 1528 MeV in photoproduction, supporting the existence of the Theta+ pentaquark.

Findings

Peak observed at 1528 MeV in p K^0_S invariant mass spectrum.

Statistical significance of the peak is 4-6 sigma.

No signal found for Theta^{++} baryon, constraining its possible quantum numbers.

Abstract

Evidence for a narrow baryon state is found in quasi-real photoproduction on a deuterium target through the decay channel p K^0_S --> p pi^+ pi^-. A peak is observed in the p K^0_S invariant mass spectrum at 1528 +/- 2.6 (stat) +/-2.1 (syst) MeV. Depending on the background model,the naive statistical significance of the peak is 4--6 standard deviations and its width may be somewhat larger than the experimental resolution of sigma=4.3 -- 6.2 MeV. This state may be interpreted as the predicted S=+1 exotic Theta^{+}(uuddbar(s)) pentaquark baryon. No signal for an hypothetical Theta^{++} baryon was observed in the pK^+ invariant mass distribution. The absence of such a signal indicates that an isotensor Theta is excluded and an isovector Theta is unlikely.