How Often Do Bottom Quark To Up Quark Transitions Occur?

The CKM matrix measures the probability of one kind of quark transitioning into another kind of quark in a W boson mediated interactions.

The global average value for the V(ub) element according to the Particle Data Group without doing a global fit on the entire matrix is:

This is a relative precision of about 5%. According to the latest measurement from the Belle II experiment "the magnitude of the Cabibbo-Kobayashi-Maskawa matrix element,

V(ub) = (3.55±0.12±0.13±0.17)×10^−3, is extracted. Here, the first uncertainty is statistical, the second is systematic and the third is theoretical." 

The magnitude of the probability of a bottom to up quark transition (which mutes the complex number components of the CKM matrix element that reflects CP violation in the transition probability) is the square of the magnitude of this CKM matrix element (i.e. this happens in about 13 per million transitions).

Combining the uncertainties in quadrature, the measurement is 3.55 ± 0.245 x 10^-3. This is consistent with the PDG value at the one sigma level.

Replicating the existing value from other experiments was no small feat.

We present an analysis of the charmless semileptonic decay B0→π−ℓ+νℓ, where ℓ=e,μ, from 198.0 million pairs of BB¯ mesons recorded by the Belle II detector at the SuperKEKB electron-positron collider. The decay is reconstructed without identifying the partner B meson. The partial branching fractions are measured independently for B0→π−e+νe and B0→π−μ+νμ as functions of q2 (momentum transfer squared), using 3896 B0→π−e+νe and 5466 B0→π−μ+νμ decays.