New precision measurement of the Λ_b lifetime

The value of the Λ_b lifetime has long been controversial but the situation has recently been clarified by a new measurement from the LHCb experiment.

There are many ways in which the decays of b quarks are used to search for physics beyond the Standard Model. One strategy, used by the CKMfitter and UTfit teams, is to compare the consistency of various sets of measurements and for this a knowledge of the elements |V_cb| and |V_ub| of the Cabibbo-Kobayashi-Maskawa (CKM) matrix is essential. One way of determining these from data is to use a theoretical framework called the heavy quark expansion (HQE).

An early prediction from this model was that the Λ_b lifetime was almost equal to that of the B⁰ meson but shorter by 1–2%. However, measurements from CERN’s Large Electron–Positron collider using the semileptonic decay Λ_b→ Λ_clν gave values of the ratio of the lifetimes, τ(Λ_b)/τ(B⁰), of around 0.8. This caused concern over the applicability of HQE and various attempts were made to explain the observations. More recent measurements of τ(Λ_b), from the CDF experiment at the Tevatron using Λ_b→Λ_c π^– (Aaltonen et al. 2002) and from ATLAS and CMS at the LHC using Λ_b→ J/ψΛ⁰ (Aad et al. 2013, Chatrchyan et al. 2013) have indicated larger values but with relatively large uncertainties.

The LHCb collaboration has discovered a new decay mode Λ_b→ J/ψ pK^– that is ideally suited for measuring the lifetime, by determining it relative to that for the decays B⁰→ J/ψ K^*0, K^*0→K⁺π^–. In both the Λ_b and B⁰ decays, four charged tracks are produced at the position of the b-hadron’s decay. This minimizes systematic uncertainties in the ratio and provides excellent decay-time resolutions of around 40 fs in each mode. The figure shows the signal yield of more than 15,000 Λ_b decays in 1.0 fb^–1 of LHCb data. Use of ring-imaging Cherenkov detectors in the experiment removes most of the backgrounds from B_s→ J/ψ K⁺ K^– and B⁰→ J/ψ K⁺π^– decays.

The collaboration finds τ(Λ_b)/τ(B⁰) = 0.976±0.012±0.006, where the first uncertainty is statistical and the second is systematic (LHCb collaboration 2013). The result demonstrates consistency with the original HQE prediction and should help to resolve issues involving measurements of the CKM parameters |V_cb| and |V_ub|. Using the precisely measured value of τ(B⁰)=1519±7 fs from the Particle Data Group (Beringer et al. 2012) yields a value for τ(Λ_b)=1482±18±12 fs. This result is about twice as precise as the best previous measurement.

Much of the early work on the HQE was done by Nikolai Uraltsev, whose passing earlier this year is much lamented by the community.