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.
At left, a string is plucked; the energy expended in plucking the string is turned into the energy of the string's vibrations. Right: the string's vibrations serve to make waves in the air, and to warm up the pegs at the end of the string as well as the string itself, through friction. In this way the energy of the vibrations gradually dissipates into waves in the air and vibrations of microscopic molecules.
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.
Above: A Higgs particle, at rest. Below: The Higgs particle may spontaneously, rapidly and permanently disintegrate into two photons (particles of light.)
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.
To be honest, I see very little help from guitar string analogy for understanding ethropy. may be it’s me.
I assume you meant to write “entropy”? Thanks for your comment, because it reminds me to emphasize that entropy is *not* the point. Indeed, if I had wanted to explain entropy, I would have done so differently. But particle decay is not fundamentally about entropy — which is why I didn’t use the word “entropy” in the post.
While it is true that particle decay increases entropy (a tiny bit), this is a symptom, not a cause. The important point in the guitar string analogy is that vibrational energy is transferred from one object (the string) to a variety of others (air, molecules in the pegs), because of interactions between the first object and the others — not that entropy increases in the process (though it does). This is to be distinguished from some processes in nature for which the tendency of entropy to increase can be viewed as the main cause — as in some aspects of weather.
Another way to see that entropy isn’t crucial is to imagine the dominant loss of energy in the guitar string were to sound waves. In this case, entropy would increase very little — but the dissipation of energy would still be the cause for the sound dying off, and the analogy to particle decay would actually be better.
Experts will note that the real problem with the guitar string analogy is this: in particle decay, the transfer of energy involves non-linear wave equations, not linear ones. I am not currently aware of a process with which most people are familiar in which transfer of energy is through simple non-linear wave equations. Suggestions are welcome.
Well, let me then represent a general public. With string we can speak aboud two pieces of matter with different impulse (mv) properties. Their interaction is then an equalization of impulse with given effectiveness. As this process is less than 100% effective, we have transient process. Now particle, is considered as single object in empty space, first of all. So, from the string analogy, we can conclude, that instead if single particle there is already (before any decay) system of some objects and that these objects are not particles themselves yet and they has some energy properties that tends to be equalized. As we observe the decay, it can be noted that degree of freedom of the system is increased and concentration of energy decreased. And probability of the event replaced effectivness of macro world. That’s how it looks to non expert like me. Still not entirely clear why the same decays of same particles has different times depending on outer conditions, like neutron in the article.
As to nonlinear process, may be social activity, like forming of pairs/families out of groups of individuals.
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Prof. Matt Strassler
“In this example I’m going to map speed to the pitch of the note, length/postion to the duration of the note and number of turns/legs/puffs to the loudness of the note.” How to make sound out of anything.- http://lhcsound.wordpress.com/2010/08/10/how-to-make-sound-out-of-anything/
LHC Sound- http://lhcsound.hep.ucl.ac.uk/
You may find this of interest?
Best,
This blog is great! It’s amazing that I can actually understand most of this without having any previous physics knowledge. So probably my question is rather silly, but here it goes:
For what I can understand, the same particle can decay to different types of particles. As you said in a video a Z particle can decay to form a muon and an antimuon, an up quark and an up antiquark or other particles. How does the Z particle “decide” what to decay to? What factors influence the resulting particles of the decay process?
Quantum mechanics suggests that the “decision” is a random process… part of nature’s inherent randomness. If that sounds strange, it is — and it appears to be true. There are many other examples. If you remind me after the dust settles this week, I can give you more familiar ones.
I will remind you, it sounds promising! So (I guess it is impossible, but theoretically speaking), if we knew the exact conditions at which every different decay products happen, there would be no pattern at all? How do you know that all the variables are being controlled? I mean, I have read somewhere that gravity could be such a weak force because it may spread throughout other dimensions (This was my interpretation at least… I may be wrong). If this is to be true, wouldn’t it be possible that another uncontrolled dimension(s) is/are affecting the particle?
All of the probabilities in question can be calculated, if you have a set of appropriate equations to work with; then you can compare the probabilities you calculate with the probabilities you measure in data, and figure out which sets of equations do not describe the data, and which ones do a better job.
Dear Prof. Strassler,
I am trying to figure out how fast the Higgs decays and how far it travels in a detector at the LHC. Figure 2.5 in http://www.hep.lu.se/atlas/thesis/egede/thesis-node14.html gives decay widths of the Higgs as a function of its mass.
For a Higgs of 125 GeV, it is 0.002 GeV. Using the Heisenberg Uncertainty eq. lifetime*decay width = h-bar/2, I find lifetime = 1 zeptosecond. Is this correct? What kind of assumptions can I make about its speed so that I could calculate distance travelled?
Regards,
Marek
Marek — you can assume that a Higgs produced at the LHC typically travels at semi-relativistic speeds — speeds in the range of, say, 0.1 to 0.95 c. With so much energy flying around, it is hard to make a Higgs that moves very slowly (though it will happen occasionally.) And since the Higgs is a bit heavy it is rare that its speed is very close to c, though again this will certainly happen. A zeptosecond (0.000,000,000,000,000,000,001 seconds) is about right. At these speeds, time dilation only adds a factor of 2 or 3 at most to the Higgs’ lifetime. The corresponding distance that the Higgs can travel is larger than an atomic nucleus but smaller than an atom. Only a very fast (and also lucky) Higgs will travel further than an atom’s radius.
Hello Prof Strassler,
Is there anywhere outside of a particle smasher that you will find unstable particles that decay like this? Would these perhaps exist during a star collapse, supernova, or maybe right after the big bang?
Regards
Yes — definitely right after the big bang, and in supernovas, and also in other high-energy stellar environments; near black holes; and most relevant for us, when cosmic rays (high-energy protons, mostly) hit atoms in the top of the atmosphere and create showers of unstable particles that rain down on us from the sky. There is a muon from such a shower passing through your body (and doing a tiny little bit of biological damage that your body has to repair) every couple of seconds.
Not to mention all of the slow-decaying radioactive nuclei — including uranium, thorium, and radon — found naturally inside the earth.
That’s very interesting – thanks for the response!
Am I correct in understanding that all particle decays are reversable? So that if two energetic photons merged, it could produce a Higgs particle. I suppose entropy would become relevant because it is much more likely for a Higgs to decay than the reverse to occur.
Yes you are right. Two photons can make a Higgs particle just as two gluons can; the Higgs can decay to two gluons as well as to two photons. But indeed, a Higgs will decay on its own without your help, whereas if you want to make a Higgs particle you have to actively slam high-energy two photons or two gluons together. Since gluons are plentiful inside a proton while photons are not, we make most of our Higgs particles from gluons, but in principle we could make them from photons. Or from collisions of other particles, such as a muon and an antimuon; a muon-antimuon collider is under consideration for this purpose, but big technical problems are not yet solved.