Evidence for a narrow baryonic state decaying to K0Sp and K0Sp̄ in deep inelastic scattering at HERA

Abstract A resonance search has been made in the K 0 S p and K 0 S p 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 GeV 2 . Recent results from fixed-target experiments give evidence for a narrow baryon resonance decaying to K + n and K 0 S p , interpreted as a pentaquark. The results presented here support the existence of such state, with a mass of 1521.5±1.5(stat.) +2.8 −1.7 (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 GeV 2 , 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 6×10 −5 .