New precision measurement of the Λb lifetime

19 July 2013

The value of the Λb 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 |Vcb| and |Vub| 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 Λb 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 Λb→ Λclν gave values of the ratio of the lifetimes, τ(Λb)/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(Λb), from the CDF experiment at the Tevatron using Λb→Λc π (Aaltonen et al. 2002) and from ATLAS and CMS at the LHC using Λb→ 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 Λb→ 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 Λb 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 Λb decays in 1.0 fb–1 of LHCb data. Use of ring-imaging Cherenkov detectors in the experiment removes most of the backgrounds from Bs→ J/ψ K+ K and B0→ J/ψ K+π decays.

The collaboration finds τ(Λb)/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 |Vcb| and |Vub|. 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(Λb)=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.

Further reading

For more about CKMfitter, see; for UTfit see
G Aad et al. ATLAS collaboration 2013 Phys. Rev. D87 032002.
T Aaltonen CDF collaboration 2002 Phys. Rev. Lett. 104 102002.
J Beringer et al. Particle Data Group 2012 Phys. Rev. D86 010001.
S Chatrchyan et al. CMS collaboration 2013 arXiv:1304.7495.
LHCb collaboration 2013 LHCB-PAPER-2013–032, in preparation.

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