Charmed baryons strike back

Last year, the LHCb collaboration announced the first observation of the Ξ_cc⁺⁺ baryon, a doubly charmed particle (CERN Courier July/August 2017 p8). It was identified via the decay Ξ_cc⁺⁺ → Λ_c⁺ K^–π⁺π⁺, with the Λ_c⁺ baryon subsequently decaying to pK^–π⁺. Since then, LHCb has carried out a campaign of further studies to pinpoint the properties of this special particle, namely looking for additional Ξ_cc⁺⁺ decays and, more importantly, measuring its lifetime.

LHCb has now reported a first measurement of the Ξ_cc⁺⁺ lifetime, exploiting the same decay mode and using a data sample and an event selection similar to those used in the first observation. The experimental technique used is to measure the decay-time distribution relative to that of another decay with a similar topology, Λ_b⁰ → Λ_c⁺π^–π⁺π^–. As the lifetime of the Λ_b⁰ is already known with high precision from previous measurements, once the ratio of efficiencies for reconstructing the Ξ_cc⁺⁺ and Λ_b⁰ decays is determined, it is possible to derive the lifetime of the Ξ_cc⁺⁺ baryon from its decay-time distribution (see figure).

The lifetime value that is obtained is 256⁺²⁴ _–22 (stat) ± 14 (syst) fs. Relatively large lifetimes like this are a distinctive feature of weak interactions. In addition, LHCb has also observed a new Ξ_cc⁺⁺ decay: Ξ_cc⁺⁺ →  Ξ_c⁺π⁺, with a statistical significance of about six standard deviations, thus confirming the first observation of Ξ_cc⁺⁺ in an independent analysis. The baryon’s mass is measured to be 3620.6 ± 1.5 (stat) ± 0.5 (syst) MeV/c², which is consistent with the previous result.

Turning to a separate analysis, a puzzling result has emerged at LHCb while measuring the lifetime of another charmed baryon: the Ω_c⁰. The sample of LHCb data used for this measurement comprises about 1000 Ω_b⁻ → Ω_c⁰μ⁻ν_μX signal decays, where the Ω_c⁰ baryon is detected via the decay Ω_c⁰ → pK⁻K⁻π⁺ and X represents possible additional undetected particles in the decay. The Ω_c⁰ lifetime is determined from the observed decay-time distribution to be 268 ± 24 (stat) ± 10 (syst) fs (see figure).

Quite surprisingly, this value is nearly four times larger than, and inconsistent with, the current world average of 69 ± 12 fs. This average is based on three experimental results from fixed-target experiments, namely E687, FOCUS and WA89, where each experiment observed only a few dozen events with relatively large background. The new measurement from LHCb redefines the lifetime hierarchy of charmed baryons, placing the Ω_c⁰ baryon as having the second largest lifetime after the Ξ_c⁺ baryon, i.e. τ(Ξ_c⁺) > τ(Ω_c⁰) > τ(Λ_c⁺) > τ(Ξ_c⁰). This result may lead to reconsideration of the relative importance of the roles of spectator quarks and of non-perturbative effects in the decay dynamics of hadrons containing heavy quarks.