Precision measurement of the Λ_c⁺ , Ξ_c⁺ and Ξ_c⁰ baryon lifetimes

The measurement reported in this paper was a followup measurement of the lifetimes of three other weakly-decaying charm baryons, the Λ_c⁺ , Ξ_c⁺ and Ξ_c⁰. Given the “surprise” seen with the Ω_c⁰ baryon, and the unique data sample that LHCb has to make these measurements, it seemed prudent to measure these charm baryon lifetimes.

The analysis is done in a very similar way to that which was done to measure the Ω_c⁰ baryon lifetime. Large and clean samples of Λ_c⁺, Ξ_c⁺ and Ξ_c⁰ are obtained from decays that have the signature of a semileptonic beauty-baryon decay. Namely, each charm baryon can be paired with a μ^– to form a displaced vertex with good fit quality. The most likely sources of these charm baryons are the semileptonic decays Λ_b⁰ → Λ_c⁺ μ^–νX , Ξ_b→Ξ_c⁺ μ^–νX, and Ξ_b → Ξ_c⁰ μ^–νX decays. The charm baryons are reconstructed in the following decay modes: Λ_c⁺ → p K^–Π⁺, Ξ_c⁺ → p K^–Π⁺ and Ξ_c⁰ → p K^–K^–Π⁺. The invariant mass spectra of the Λ_c⁺, Ξ_c⁺ and Ξ_c⁰ baryons are shown from left to right below.

 

 

 

 

 

The signal comprise about 300,000, 56,000 and 21,000 Λ_c⁺, Ξ_c⁺ and Ξ_c⁰ signal decays, respectively. For each of the signal decays, we also measure the decay time using the production and decay positions of the charm baryon. Then, as discussed in the Ω_c⁰ baryon lifetime summary, we use fully simulated signal decays to determine the lifetime that is most compatible with the observed decay time spectra.

The resulting decay time spectra and best fit lifetimes are shown in the figures below.

 

 

 

 

 

The best fit lifetimes are τ(Λ_c⁺) = 203.5 ± 1.0 ± 1.3 ± 1.4 fs,  τ(Ξ_c⁺) = 456.8 ± 3.5 ± 2.9 ± 3.1 fs, and   τ(Ξ_c⁰) = 154.5 ± 1.7 ± 1.6 ± 1.0 fs, where the uncertainties are statistical, experimental systematic, and the last is due to using D+ decays for normalization. All three measurements have significantly better precision than the previous world average. Interestingly, the Ξ_c⁰ lifetime value found here is 3.3 standard deviations away from the previous world average value of 112⁺¹³_-10 fs. The previous measurements were based on much smaller samples obtained in fixed target experiments, where 4, 42 and 110 signal decays were used in the measurement. This should be compared to our sample of about 22,000 signal decays!

A recent measurement of the Ξ_c⁰ lifetime using prompt Ξ_c⁰ decays found a result consistent with the LHCb value of 154.5 fs, thus confirming the larger value.

 

More details about the analysis can be found in the publication.

• Journal: Phys. Rev. D100, 032001 (2019)
• CERN Document System: LHCb-PAPER-2019-008.