Hadron Spectroscopy in Electrons-protons Collisions at Hera

Experimental evidence for an exotic baryon first came in 2003 from the observation of a S = +1 narrow resonance 1 at 1540± 10 MeV which can be associated with an exotic pentaquark state with content uudds̄. Confirmation came quickly from a series of experiments, with the observation of sharp peaks 2,3,4,5 in the nK and pK S invariant mass spectrum near 1540 MeV, in each case with a width limited by the experimental resolution. Strong evidence in support of a baryon decuplet comes from the reported observation of an exotic S = −2, Q = −2 baryon resonance in pp collisions . A narrow peak at a mass of about 1862 MeV in the Ξπ invariant mass spectrum is proposed as a candidate for the predicted exotic Ξ 3 2 baryon with a quark content dsdsū. At the same mass, a peak is