Are particle decay times universal constants?

Question

Basically what the title says. For neutrinos, for example.

Answer 1

Physicists think that the half lives of various kinds of unstable particles, measured in the rest frame of the particle, are the same everywhere in the universe. In that sense you could call them universal constants. But physicists don’t tend to do that because these half lives are calculable in terms of more fundamental constants like elementary particle masses, coupling constants, Planck’s constant, etc.

So these half lives are not independent constants but rather derived ones.

Note: There is no evidence that neutrinos decay. But other particles such as muons do.

Answer 2

Particle decay rates are not universal constants. However, under most conditions they are the same, so it is not an unreasonable assumption.

How likely a particle (or atomic nucleus) is to decay depends to some extent on its environment. The most obvious case is that neutrons in atomic nuclei or neutron stars are stabilized by their environment: there are no easily accessible lower energy states they can decay to.

A bit less "cheating" is the Purcell effect, where the spontaneous emission rate of excited atoms is enhanced by putting them in a cavity. Presumably this would also affect decay rates of resonance particles.

A non-intuitive result of relativistic quantum field theory is that accelerations can change the decay rates of elementary particles. This is an effect unrelated to time dilation (where the decay rate of a fast particle as seen from a frame "at rest" is decreased). Instead it is linked to the Unruh effect where acceleration makes the experienced fields in a vacuum change, allowing particles to jump to excited states by interacting with the "hot" vacuum. In the case of particle decay this can not only change decay rates but possibly allow decays that are forbidden in inertial frames. This includes proton decay, where the vacuum provides enough energy to make heavier decay products than the initial proton. The effect for protons is very slight (multiplying the decay rate by a factor of $1+(a/M)^4$; LHC accelerations have $a/M\approx 10^{-11}$). Due to the equivalence principle there is also a similar gravitational effect, although it is minuscule even for neutron stars and most black holes.

Technically, decay rates can be calculated using Fermi's Golden Rule as $\Gamma = 2\pi |T_{12}|^2\rho_2$ where $\rho_2$ is the density of decayed states and $T_{12}$ is the transition matrix element one derives from the perturbed Hamiltonian. While calculating the the transition matrix may be somewhat complex, the relevant parts here are the perturbation of the Hamiltonian and the density of possible post-decay states. Given an environment these will be affected and the rate will be changed relative to a particle in vacuum.

In the vacuum case the numbers only depend on particle masses, the relevant fields and other conserved quantities. So in that case we might say they are universal since they are derived from things we usually treat as universal constants.