Re-discovery of an exceptionally charming particle

[ m(Ξ_cc⁺⁺) = 3621.55±0.23±0.30 MeV/c² ]

A year ago the LHCb collaboration announced the observation of an exceptionally charmed particle, the Ξ_cc⁺⁺ baryon. This particle contains two charm quarks and one up quark, resulting in an overall doubly positive charge. A month ago the collaboration has presented the first measurement of the lifetime of this baryon. Today LHCb physicists submitted for publication a new observation of this particle using a different decay channel.

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.

21 November 2019 update: see Precision measurement of the Ξ_cc⁺⁺ mass paper.