The Ξ_cc⁺⁺ was first observed via its decay into a Λ_c⁺ baryon and three lighter mesons K^–, π⁺ and π⁺, with the Λ_c⁺ baryon decaying in turn into a proton p, a K^– meson and a π⁺ meson. The Ξ_cc⁺⁺ has now been “re-discovered” using a different decay, Ξ_cc⁺⁺→ Ξ_c⁺π⁺ (see graph), in which the Ξ_c⁺ baryon decays into a proton, a K^– meson and a π⁺ meson.

The left image shows the Ξ_c⁺π⁺ invariant mass distribution. The Ξ_cc⁺⁺ peak is clearly visible, with a statistical significance of 5.9σ against the background-only hypothesis. The mass peak represents an independent observation of the Ξ_cc⁺⁺ baryon, since the selected events are entirely different from those used in the previous study. The measured Ξ_cc⁺⁺ mass is 3620.6±1.5±0.4±0.3 MeV/c², fully consistent with the value of the previous measurement.

This measurement provides important information towards an improved understanding of the decays of doubly charmed baryons. Read more in the LHCb paper, and also in the CERN Update and the CERN Courier article.