Evidence for a narrow baryonic state decaying to K0sp and K0s anti p in deep inelastic scattering at HERA

A resonance search has been made in the Ks0p and Ks0p invariant-mass spectrum measured with the ZEUS detector at HERA using an integrated luminosity of 121 pb-1. The search was performed in the central rapidity region of inclusive deep inelastic scattering at an ep centre-of-mass energy of 300-318 GeV for exchanged photon virtuality, Q2, above 1 GeV2. Recent results from fixed-target experiments give evidence for a narrow baryon resonance decaying to K+n and Ks0p, interpreted as a pentaquark. The results presented here support the existence of such state, with a mass of 1521.5 ± 1.5(stat.)-1.7+2.8(syst.) MeV and a Gaussian width consistent with the experimental resolution of 2 MeV. The signal is visible at high Q2 and, for Q2 > 20 GeV2, contains 221 ± 48 events. The probability of a similar signal anywhere in the range 1500-1560 MeV arising from fluctuations of the background is below 6 x 10-5. (orig.)