What causes a particle to decay?
What causes a fundamental particle (e.g. one of the heavier quarks) to "decay" into other fundamental particles, and where do these new particles come from if they are not part of the original particle?
It seems to me that for a particle to decay, it must have some sort of internal or external force acting on it, but how is this possible if all forces are borne by other fundamental particles? And as to the new particles formed, I am perplexed by their ability to spring into existence after the original particle ceases to exist. I am a high school student with some knowledge of physics, but I have only recently begun reading about particle physics.
In a sense, particles will decay because they are lazy: they want to be in the lowest possible energy state they can reach. So, if the decay products have lower energy than the initial particle, the decay can happen spontaneously. That means that the particle can be sitting in the middle of nowhere with absolutely no forces acting on it, and it will still decay. Though it is not possible to predict the exact time at which the decay will happen, the particles have a characteristic lifetime which they typically live (this is very close to their half-life, if you've heard of that term before).
As an example, a neutron is slightly heavier than a proton, so it has slightly more energy than the latter. It turns out that left alone, a free neutron (one that isn't bound in a nucleus) will spontaneously decay into a proton, and electron and a neutrino (this is called "beta decay"). The characteristic time for the decay to occur is about 15 minutes.
Finally, what kinds of particles can decay in this way? It turns out that any particles that are composites of fundamental particles (like protons, neutrons, and atoms full of protons and neutrons) can decay in this manner. As for fundamental particles themselves, an electron for example can't spontaneously change into anything else in the same way that a neutron decays. The quarks which you mention are a more difficult case, because we don't think that quarks exist in isolation.
Where do the new particles come from? The best answer that I can give you is that they come from pure energy. Remember that Einstein proved that E=mc^2, that is that mass and energy are directly proportional to each other, with the speed of light squared being the proportionality constant. So, you can make matter out of energy and the other way around. So, one particle can "turn into" another type of particle if there is enough energy to do it (and certain particle physics book-keeping rules are met). In the case of the neutron and the proton this condition is satisfied, so the reaction can take place.
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