Why do most particles disintegrate [the technical term is ``decay''] into other particles?
Particle physicists have discovered a slew of apparently elementary particles, and there may be more. But most of these types of particles aren’t just lying around on the floor waiting for us to sweep them up; we’ve had to build special machines like the Large Hadron Collider to produce, discover and study them. Why is that? Because most of these particles — with the exceptions of the ones out of which we ourselves are made, and a couple of others — fall apart (“decay”) into other particles in a tiny fraction of a second. I mean tiny: a millionth of a second is forever. Some of these particles survive only a trillionth of a trillionth of a second, or even less! (You may well wonder how we find such evanescent things! That’s another article; or you can read about how physicists are trying right now (August 2011) to find the Higgs particle.)
In this little article, using some pretty decent though imperfect analogies, I’m going to give you some insights into why decay is the ultimate fate of most elementary particles.
You may recall (or you may want to read this article [coming soon] or the first part of the Higgs Particle FAQ) that waves in a quantum world are actually made from particles; sound waves are made from phonons, light waves are made from photons, and so on. Or you can just accept this and read on.
Particle decay is to particles as “dissipation” is to waves, which is something with which you are very familiar (though you may not know it yet — read on!)
[ dissipate: to become scattered or dispersed; be dispelled; disintegrate: The sun shone and the mist dissipated.]
Nothing lasts forever, including the sound of a plucked string on a guitar or violin, or a struck note on a xylophone. In fact, before there is sound, there is vibration. The guitar string or xylophone key vibrates back and forth. Why do you hear a sound, even though the string is far from your ear? You hear it because the string, as it vibrates through the air, makes the air vibrate, creating waves that move through the air and reach your ears, making your ear-drums flap back and forth — a motion which your brain converts into your experience of a musical tone.
Why does the sound of the string gradually die off? When you plucked the string, you exerted yourself a little bit, and some of the energy you used was turned into energy of the vibrating string. Energy is conserved — it is neither created nor destroyed, though it can move from place to place and change from type to type. Little by little the energy which manifests itself in the vibrations of the string is lost, converted to other things. Some is lost to the vibrations of the air, and thus to the sound waves. Some is lost to friction and thereby to heat, which involves microscopic vibrations of molecules in the string and in the pegs that hold the string in place. This conversion of one type of vibration to many other types, and the transfer of energy from the large-scale motion of the vibrating string to other places, is called dissipation. Dissipation happens because the vibrating string is in contact with — has some sort of interaction with — other things, in particular the air and the pegs at its two ends, and also because of its own internal structure.
Particles decay by a similar sort of dissipation, but this is where quantum mechanics comes in and makes things different. While the vibrations of the string disappear gradually into broad waves of sound and the jiggling of hordes of atoms and molecules, a typical particle can decay suddenly into just two, or three, or maybe four lighter-weight particles. This is just the quantum version of dissipation; it is the same basic idea, with a quantum twist.
For example, a Higgs particle may decay suddenly into two particles of light (“photons”); a Z particle may decay suddenly to a muon and an anti-muon.
Terminology: Particles that decay rapidly are called “unstable”; particles that never decay are called “stable”. Particles that take a long time to decay are often called “metastable” or “long-lived” — but CAUTION: “long-lived” and “metastable” are relative terms whose precise meaning is context-dependent.
A small warning: I’ve had to white lie just a bit here. The phenomenon of dissipation that particles undergo is a quantum version of the dissipation that waves are subject to — that is true. But in order to appeal to your intuition, I have described a type of dissipation that you are familiar with, and though similar it is not quite the one that is responsible for most particle decays.
Almost all particles known to us decay, most very rapidly. The only known stable particles in nature (for reasons to be explained later) are
- the electron (and anti-electron)
- the lightest of the three types of neutrinos (and its anti-particle)
- the photon (which is its own anti-particle)
- the graviton (which has not yet been observed and won’t be detectable any time soon, though gravitational waves have been indirectly detected and probably will be observed soon)
Then there are some particles that might be stable but probably are just extremely long-lived — with lifetimes so long that only a small number of them have decayed since the Big Bang. These probably-metastable particles include
- the other neutrinos (and anti-neutrinos … I’m going to stop mentioning the anti-stuff, it goes without saying)
- The proton (which is not an elementary particle, see here)
- Many atomic nuclei (the cores of all the types of atoms we see around us)
The other rather long-lived particle is the neutron, which when on its own, outside an atomic nucleus, lives just 15 minutes or so. But neutrons inside many atomic nuclei can live far longer than the age of the universe; such nuclei provide them with a stable home.
What determines how quickly particles decay? Well, let’s ask what determines how fast the waves on the vibrating string dissipate. It has to do with what objects the string interacts with (the air, the pegs on its ends, itself) and how strongly the string interacts with those other things. Air is easy to push around, so a guitar string can ring for quite some time. But if you put the string in a bathtub, its vibrations would die away much faster, because the string, in making water ripples, would use up its vibrational energy much faster. And you yourself can make the dissipation occur much faster if you put your finger right on the edge of the string. This is because (as you can feel) the atoms and molecules in your finger start to absorb the energy. Since you are interacting more strongly with the string than anything else, you determine thereby how quickly the vibrations die out. The harder you press on the string, the more strongly you interact with it, and the more rapidly the sound stops.
What is true for wave dissipation is true for particle decay. Some types of particles interact with each other strongly, others less so. For instance, photons interact with ordinary solid matter strongly, which is why the earth is opaque to light; neutrinos interact with ordinary solid matter very weakly, which is why they usually travel straight through the earth. Quarks have very strong interactions with each other, which is why they are always stuck inside composite particles like protons. But quarks interact with electrons rather weakly, so electrons can easily fly free of quarks — and this is why electrons in atoms are found orbiting at a relatively great distance from the protons and neutrons that make up the tiny atomic nuclei.
Suppose a particle of one type (the “parent”) is able to decay to two or more particles of other types. The stronger is the interaction between these types of particles, the more likely the decay is to occur — and thus the more common is that type of decay, and the shorter is the “lifetime” of the parent particle. For instance, the Higgs particle interacts very weakly with light, which is why its decay to two photons is rare. But it interacts much more strongly with W particles, and so, if it is heavy enough to decay to W particles, it does so most of the time. See here or here for more details on the Higgs and its decays.
So now you know that the basic physics behind particle decay is a quantum version of what you see around you: the dissipation that is happening to vibrations of all types. You know now that the speed of dissipation has to do with how strongly a vibrating object interacts with other objects; and that in an analogous way, particles that have stronger interactions will typically decay faster than those that have weaker ones. But this is not the whole story. Quantum mechanics influences particle decay in ways that are not intuitive from daily life, and determines why some particles do not decay at all, or decay only very slowly. Fortunately these features can be mostly stated as rather simple rules.