The measurement reported in this paper was a followup measurement of the lifetimes of three other weakly-decaying charm baryons, the Λc+ , Ξc+ and Ξc0. Given the “surprise” seen with the Ωc0 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 Ωc0 baryon lifetime. Large and clean samples of Λc+, Ξc+ and Ξc0 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 Λb0 → Λc+ μ–νX , Ξb→Ξc+ μ–νX, and Ξb → Ξc0 μ–νX decays. The charm baryons are reconstructed in the following decay modes: Λc+ → p K–Π+, Ξc+ → p K–Π+ and Ξc0 → p K–K–Π+. The invariant mass spectra of the Λc+, Ξc+ and Ξc0 baryons are shown from left to right below.
The signal comprise about 300,000, 56,000 and 21,000 Λc+, Ξc+ and Ξc0 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 Ωc0 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(Λc+) = 203.5 ± 1.0 ± 1.3 ± 1.4 fs, τ(Ξc+) = 456.8 ± 3.5 ± 2.9 ± 3.1 fs, and τ(Ξc0) = 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 Ξc0 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 Ξc0 lifetime using prompt Ξc0 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.