When is the weak force triggered? I've been trying to understand the weak force (very much at a layman's level), and I've seen a lot of descriptions that say more or less the same thing. But I've had difficulty tracking down an answer to one of my primary questions: when does the weak force actually come into play? What situations trigger it? Thanks!
 A: Anything that interacts weakly, so, leptons and quarks, interacts weakly all the time. Weak transitions occur if they result in products that are energetically possible--if the circumstances of the reaction allow it.
However, Weak interactions are small/rare/invisible, if they compete in a context where the other two hugely stronger interactions are involved: electromagnetism & the strong interactions. So weak decays are very-very rare.
They still have a chance. They break symmetries such as parity, charge conjugation, and CP, so they can reveal themselves acting in settings where the EM and strong interactions  are excluded because they preserve such symmetries. In addition, neutrinos only interact weakly.
So the answer to your question is weak interactions are "triggered" all the time, in the roiling world of quantum possibilities, but they are so weak that they happen very rarely in everyday contexts... (except the sun!). Think of a flakey trigger that only engages once in a blue moon, randomly.
A: The weak force isn't triggered any more than the electromagnetic force is triggered. The EM force interacts with anything that has an electrical charge, and likewise the weak force interacts with anything that has a weak charge. I think this is every fundamental particle apart from photons, gluons and the Z boson.
However the weak force is very short range so particles have to get very close for the weak force to have any significant effect. In this context close means less than the size of a proton i.e. about a femtometre. By contrast electromagnetic interactions of fundamental particles are strong at about the size of molecules i.e.  a few nanometres. The weak force in't actually that weak - it just appears much weaker than the EM force because it's so short ranged.
A: Some trivia to sweat over before getting to the point -
The weak interaction acts upon left-handed fermions and right-handed antifermions by exchanging force-carrier particles known as the $W^{\pm}$ and $Z$ bosons. These bosons are heavy, with masses about 100 times the mass of a proton, and it is their heaviness that defines the extremely short-range nature of the weak interaction and that makes the weak interaction appear weak at the low energies associated with radioactivity. The weak interaction's efficiency is limited to a range of $10^{-17}$ m, or roughly $1$% of an average atomic nucleus' diameter. In fact, the electromagnetic force is $100,000$ times more powerful than the weak interaction during radioactive decay. However, it is now understood that the electromagnetic force and the weak interaction are fundamentally equivalent in strength, and it is thought that these two seemingly disparate forces are actually different manifestations of the same electroweak force.
Getting to the point -
Most subatomic particles are unstable and decay by the weak interaction, even if they cannot decay by the electromagnetic force or the strong force. Thus it is not entirely appropriate to say that the weak interaction is triggered somehow. The Higgs mechanism provides an explanation for the presence of three massive $W^{\pm}$ and $Z$ bosons and the massless photon - the force carrier of electromagnetism.
The Higgs Mechanism -
Essentially, the Higgs mechanism adds a field (called the Higgs field) that permeates all space. Below some extremely high temperature, this field causes a symmetry to be broken during interactions. This symmetry breaking is spontaneous as we assume the Higgs field to be present everywhere from the beginning rather than be entered abruptly in between. This triggers the Higgs mechanism, wherein the electroweak symmetry is broken, i.e, roughly speaking, electromagnetism and weak interactions are separated (in context of the Standard Model). This causes the $W^{\pm}$ and $Z$ bosons it interacts with to have mass, and as I already mention, it is their heavy masses that make the weak interactions manifest as such.
In summary, the heaviness of the force-carrying particles, which is due to the Higgs mechanism defines the scale and range of the weak interaction, wherever it occurs. One could interpret this mechanism itself as your so-called trigger. However, it is more appropriate to see this as a fundamental interaction that simply can occur, in particular on certain types of particles at relevant scales and ranges.
Does this answer your question?
