First observation of the doubly charmed baryon E+cc

We observe a signal for the doubly charmed baryon E + c c in the charged decay mode E + c c → A + c K - π + in data from SELEX, the charm hadroproduction experiment at Fermilab. We observe an excess of 15.9 events over anexpected background of 6.1 ′ 0.5 events, a statistical significance of 6.3σ. The observed mass of this state is 3519 ′ 1 MeV/c 2 . The Gaussian mass width of this state is 3 MeV/c 2 , consistent with resolution; its lifetime is less than 33 fs at 90% confidence.