Evidence for a narrow baryonic state decaying to KS0p and KS0(p)over-bar in deep inelastic scattering at HERA

A resonance search has been made in the K(S)(0)p and K-S(0)(p) over bar 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, Q(2), above 1 GeV2. Recent results from fixed-target experiments give evidence for a narrow baryon resonance decaying to K(+)n and K(S)(0)p, 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 Q(2) and, for Q(2) > 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). (C) 2004 Elsevier B.V. All rights reserved.