Search for a Narrow Baryonic Resonance Decaying to K⁰p or K⁰p in Deep Inelastic Scattering at HERA

Recently several experiments reported evidence of an exotic baryon, made up from 5 quarks, the strange pentaquark Θ⁺. This baryon was observed in the decay channels K⁺n and K⁰_sp. H1 used the latter decay channel to search for this particle, exploring the good track reconstruction capabilities of the detector. The search was made in the DIS regime with the data sample collected in the years 1996-2000.

This exotic baryon has been reported with masses in the range of 1520 to 1540 MeV and with a narrow width, consistent with the experimental resolution in most of the observations. H1 carried out the search in the mass region from 1480 to 1700 MeV with the assumption that the width of the resonance is small and limited by the experimental resolution. While the K⁰_s was identified via the subsequent decay to charged pions, the proton (antiproton) was identified via the measurements of the ionisation energy loss (dE/dx) in the tracker. In total 133000 K⁰_s candidates were reconstructed with only 3% background under the K⁰_s peak. The efficiency of the proton selection varies between 65% and 100% and was tested using protons from Lambda decays.

No signal from a narrow resonance was observed in the mass range from 1480 to 1700 MeV. With the assumption that the resonance is produced with kinematics similar to those of known strange baryons, mass-dependent upper limits at 95% confidence level on the production cross section are derived. The figure shows the combined mass spectra of K⁰_sp and K⁰_sp in the studied mass range. The upper limit on the cross section is shown as a curve in the lower plot. The invariant mass spectra are also studied separately for K⁰_sp and K⁰_sp. No narrow resonance is observed here either.