Flavor Generations and the W, Z Bosons
The data show that generation plays an important role in the intensity of weak decay of particles. Table 1 contains relevant details of charged meson weak decay [1].
Table 1: Mean life (sec.) of weakly decaying charged mesons
The column ‘Generation’ shows the highest generation of the meson’s quark constituents. The penultimate column shows the main hadronic products of the meson decay. This column indicates that the main products of the decay belong to mesons that contain the heaviest quark possible.
Energy considerations show that the 
does not decay into hadrons. The s quark belongs to the second generation. Therefore, the 
decay is an inter-generation process, and its lifetime is similar to that of the . The c quark belongs to the second generation. Therefore, the decay of the 
meson is an intra-generation
process, and its life-time is 4 orders of magnitude shorter than that of the meson.
The b quark belongs to the third generation. Therefore, the decay of the 
meson is an inter-generation process, and its lifetime is similar to that of the meson. It can be concluded that the meson lifetime decreases with the increase of its mass but an intra-generation decay is much faster than an inter-generation decay.
Therefore, elements of a consistent weak interaction theory must include quantities that represent generation. The presently accepted weak interaction theory – the electroweak theory – uses the 
particles as the fields that mediate weak interaction (see [2], pp. 307, 308).
Let us examine the Z particle which is produced in an 
collision (see [3], p. 430). The leptonic decay mode of the Z [1] is generation-invariant and each of the
combinations 
takes the same value. The quark decay mode 
is larger than the 
by a factor of 1.25.
In summary, the strength of the weak interaction decay of mesons heavily depends on generation, while the decay of the Z boson indicates that its generation dependance is marginal. The leptonic decay of the 
shows similar features [1]. Thus, the three generations of the leptonic 
decay products 
have practically the same intensity.
Research topic: The electroweak theory states that the , Z bosons mediate weak interaction. Explain the discrepancy between the marginal generation dependency of the decay of these particles, and the dramatic (4 orders of magnitude) generation dependence of the data of table 1.
References
[1] M. Tanabashi et al. (Particle Data Group), Phys. Rev. D 98, 030001 (2018). http://pdg.lbl.gov/2018/listings/contents_listings.html
[2] S. Weinberg, The Quantum Theory of Fields, Vol. II (Cambridge University Press, Cambridge, 1995).
[3] M. Thomson, Modern Particle Physics (Cambridge University Press, Cambridge UK, 2013).