A ug 2 00 5 Confirmation of the Doubly Charmed Baryon Ξ + cc ( 3520 ) via its Decay to pD + K − The SELEX

We observe a signal for the doubly charmed baryon Ξcc in the decay mode Ξ + cc → pDK to complement the previous reported decay Ξcc → Λc Kπ in data from SELEX, the charm hadro-production experiment at Fermilab. In this new decay mode we observe an excess of 5.62 events over a combinatoric background estimated by event mixing to be 1.38 ± 0.13 events. The mixed background has Gaussian statistics, giving a signal significance of 4.8σ. The Poisson probability that a background fluctuation can produce the apparent signal is less than 6.4× 10. The observed mass of this state is 3518 ± 3 MeV/c, consistent with the published result. Averaging the two results gives a mass of 3518.7± 1.7 MeV/c. The observation of this new weak decay mode confirms the previous SELEX suggestion that this state is a double charm baryon. The relative branching ratio for these two modes is 0.36 ± 0.21.