The first observation of the doubly charmed baryon Ξ_cc⁺⁺, which contains two heavy quarks, was reported by the LHCb collaboration in 2017 [1]. Recently, the lifetime of the doubly charmed baryon Ξ_cc⁺⁺ is measured [2], and a new decay mode of the Ξ_cc⁺⁺ baryon has been observed by the LHCb collaboration [3]. The LHCb-China group, including experimental particle physicists from University of Chinese Academy of Sciences (UCAS), Tsinghua University, Central China Normal University, and Wuhan University, has played a leading role in these studies.

In the Standard Model of particle physics, there are six different types of quarks: d, u, s, c, b, and t. While the first three quarks are classified as light, the last three are considered heavy, due to the very large mass difference between these two groups. Baryons are particles formed by three quarks, and all the baryons observed before contain at most one heavy quark. The existence of the Ξ_cc⁺⁺  baryon was already predicted by the quark model, and its discovery opens a new research field in the experimental particle physics.

The Ξ_cc⁺⁺ baryon is discovered with its decay to one baryon Λ_c⁺, one K^- meson, and two π⁺ mesons. The Ξ_cc⁺⁺ lifetime is measured to be 0.256 picoseconds, with small uncertainty. The lifetime measurement is important to confirm that the Ξ_cc⁺⁺ baryon decays via the weak interaction, one of the four fundamental interactions in nature. Figure 1 shows the reconstructed Ξ_cc⁺⁺ decay time distribution together with the fit results. A new decay mode has been observed, where the  Ξ_cc⁺⁺ baryon transforms into a Ξ_c⁺ baryon and a π⁺ meson. The Ξ_c⁺π⁺ mass distribution is shown in Figure 2, where the Ξ_cc⁺⁺ mass peak is clearly visible, providing an observation with a statistical significance of about six standard deviations. This confirms the previous discovery of the doubly charmed baryon Ξ_cc⁺⁺, and strengthens its credibility.

Figure 1 Reconstructed decay time distribution of the doubly charmed baryon Ξ_cc⁺⁺

Prof. Jibo He and Dr. Daniel Vieira, from University of Chinese Academy of Sciences, have made significant contributions to these studies. In May 2018, during the International Conference on Charm Physics (CHARM 2018) held in Novosibirsk (Russia), Dr. Daniel Vieira announced for the first time the lifetime measurement of the Ξ_cc⁺⁺ baryon. The announcement was part of his report summarizing the studies of the doubly charmed baryons at LHCb. In July 2018, Prof. Jibo He gave an overview talk on the experimental studies of the doubly charmed baryons at LHCb, in the International Conference on High Energy Physics (ICHEP 2018) held in Seoul (South Korea).

Figure 2: Invariant mass distribution of the Ξ_cc⁺⁺ baryon

The experimental studies of the doubly heavy baryons at UCAS is supported by the National Key R&D Program of China, the Key Research Program of Frontier Sciences of CAS, and the CAS President’s International Fellowship Initiative (PIFI).

[1] Public announcement of the first observation of the Ξ_cc⁺⁺ baryon at the LHCb website

[2] Public announcement of the measurement of the Ξ_cc⁺⁺ lifetime at the CERN website

[3] Public announcement of the observation of the new Ξ_cc⁺⁺ decay mode at the LHCb website