A mystery of the beauty baryon lifetime resolved

(Image: LHCb)

τb)/ τ(B0) = 0.976±0.012±0.006 ]
τ((Λb) = 1.482±0.018±0.012 ps ]


The LHCb Collaboration has just published an important precise measurement of the Λb beauty baryon lifetime. The Λb is a particle composed of the up (u), down (d) and beauty (b) quarks, so it can be understood as being like a neutron (composed of udd quarks) in which one of the d quarks has been replaced by the beauty (b) quark. Therefore the Λb baryon is about 6 times heavier than the neutron. The lifetime of the Λb baryon was first measured by experiments which took data at the electron-positron collider LEP in the 1990s. The results of the measurements were puzzling. It was a real nightmare for theoretical physicists. In fact, the calculations they were using, the Heavy Quark Expansion HQE, predicted that the Λb lifetime should be very similar to that of the B0 meson, but the LEP experiments found the Λb lifetime to be about 20% shorter than the B0lifetime.

The LHCb Collaboration has recently discovered a new decay mode 
Λb → J/ψpK-, J/ψ→μμ. The image shows the signal yield of more than 15,000 Λb decays in 1.0 fb-1 of LHCb data. This decay allows the precise measurement of the Λb decay point from the intersection of four charged tracks. The B0 decay point was also measured precisely using four charged tracks of the B0 decay into J/ψK*0, K*0→K+π-. In this way the LHCb collaboration made the most precise measurement of the Λb to B0 lifetime ratio to be 0.976±0.012±0.006, close to 1 and in agreement with the original HQE prediction. The mystery of the Λblifetime is now resolved. Using previous determinations of the B0meson lifetime, the Λb lifetime was found to be 1.482±0.018±0.012 ps.

Read more in the LHCb publication.