Resolution of a b-baryon Lifetime Puzzle!
Measurement of the Λb Lifetime in Λb–>J/Ψ Λ0 at the CDF experiment.
The weak decay of quarks depends on fundamental parameters of the standard model, including elements of the Cabibbo-Kobayashi-Maskawa (CKM) matrix which describe mixing between quark families. Extraction of these parameters from weak decays is complicated since observed quarks are not free but are confined within color-singlet hadrons, as described by quantum chromodynamics (QCD). An essential tool used in this extraction is the heavy quark expansion (HQE) technique. In HQE, the total decay width of a heavy hadron is expressed as an expansion in inverse powers of the heavy quark mass mq. Lifetime ratios of b-flavored hadrons are predicted to be unity through O(1/mb), and O(1/m2b) corrections are small (< 2%). Detailed analysis of O(1/m3b) corrections to the lifetime ratio lead to an expected value of τ(Λb)/τ(B0b) ≃ 0.94. By 2006, this theoretical prediction has been in poor agreement with measurements for more than a decade, with the world average being τ(Λb)/τ(B0b) = 0.804 ± 0.049.
On the CDF experiment, I lead an analysis of τ(Λb) in exclusive decay Λb–>J/Ψ Λ0. A diagram of this decay channel is shown below:
The muons from the J/Ψ decay are used to trigger the detector on Λb events and measure the decay vertex. The full reconstruction of the decay allows for measurement of the Λb momentum and therefore the cτ distribution, shown below for B0b (left) and Λb (right):
τ(Λb)/τ(B0b) = 1.041 ± 0.057 (stat. + syst.)
At the time of publication of this result PRL 98 (2007) 122001, this was the single most precise measurement of τ(Λb) and higher than the previous world average by 3.2σ. Our measurement resolved the “Λb Lifetime Puzzle” in favor of earlier theory calculations of τ(Λb).