For beta decay of mesons, the heavy quark approximation and surely other techniques can be used to show that the decay width scales as the quintic power of mass. This can be checked from pdg files very easily with this gnuplot script:
set logscale xy
set key noautotitle
plot "<cat Downloads/mass_width_2020.mcd|grep -v ^* | cut -c 32-" using "%20lf%*8lf%*8lf%16lf"
replot 2.4952*(x/91.1876)**3
replot 2.99591E-19*(x/0.105658)**5
The plot includes all the particles having mass and decay width listed in the particle data group datafile: mesons and baryons, some of their excited states, and also the Z0 and the W and the two unstable leptons, muon and tau. And the top quark. Higgs boson does not appear because its decay width is not yet in the table, proton does not appear because it is stable AFAWK, and well, the neutron does not appear because I have zoomed it out)
Units in the table are GeV (mass in the horizontal, decay width in the vertical). And you can appreciate the aforementioned quintic scaling as the blue line here.
But surprise, there is also an alignment for neutral, purely electromagnetic, decays. It is a cubic scaling. Is it an artifact, or can it be shown analytically?
To put more detail: the particles on the green line are $\pi^0, \eta, \Sigma^0, J/\Psi, \Psi(2S)$ and, rather peculiarly, the gauge bosons.
| Particle | Mass (Gev) | Total Width (GeV) | "Reduced Width" $MeV^{-2}$ |
|---|---|---|---|
| $\pi^0$ | 0.135 | 7.73E-9 | 3.1 |
| $\eta$ | 0.548 | 1.31E-6 | 8.0 |
| $\Sigma$$\Sigma^0$ | 1.19 | 8.90E-6 | 5.3 |
| $D^*$ | 2.01 | 8.34E-5 | 10.3 |
| $J/\Psi$ | 3.10 | 9.29E-5 | 3.1 |
| $\Psi(2S)$ | 3.69 | 2.94E-4 | 5.9 |
| $B(s2)^*$ | 5.84 | 1.49E-3 | 7.5 |
| $Z^0$ | 91.2 | 2.50 | 3.3 |
The particles above the green line are all the mesons and baryons that have strong decay. For some of these, when the partial EM decay width is known, usually to $\gamma\gamma$, it also aligns on the green line. I have checked this for $\omega, \phi$ and $\eta'$.
It would seem that the hadronic scale, extracted via the "reduced width" $\Gamma/M^3$, has some general bound, the only exception being the three first states in the bottom quark sector: $\Upsilon(1S,2S,3S)$ happen to have some extra of stability allowing them to lie under the cubic line. But note $\Upsilon(4S)$ is already above it.
