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Q & A: Unstable elementary particles

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Most recent answer: 10/22/2007
Q:
Dear Sir I know there are over 200 subatomic particles that have been detected. How many of these subatomic particles that have been detected actually exist out in nature in some form or fashion naturally or in a natural decaying process or some other circumstance in nature? How many of the particles only have been detected in particle accelerators or are known to exist in the particle accelerators only and do not ever occur or come into existence outside of the particle accelerators or outside of a human manipulating machine? Scienties presently do not know why 2nd and 3rd generation particles even exist since they are not part of any stable matter in existence? My second question is what are the 2nd and 3rd generation particles that exist in nature in unstable form or unstable matter for any period of time, or do all 2nd and 3rd generation particles only exist within the manipulations of the particle accelerator machines? Any answers or insight you may have on these questions would be greatly appreciated. If possible can you send the answers to me email address also. Thanks Mike
- Mike (age 39)
Grayson County College, Sherman, Texas
A:
Yup, quite a few particles have been discovered in accelerator-based experiments over the years. Actually, some of them, including our first glimpse of the second generation, the muon, were first spotted in cosmic-ray experiments.

Most of the 200+ particles are not "elementary" but are composed of constituent parts. We now know that protons are made up of three quarks -- two "up" and one "down", while neutrons are made of two downs and an up. You can put three up-type quarks together and get a Delta++. Pions are made of a quark and an antiquark bound together with the strong force. Stir in three generations of quarks and you can make lots of interesting combinations.

Exotic particles of all three generations are made in collisions of cosmic rays and the molecules in the atmosphere. We also have found out that neutrinos, once thought to be massless, slowly change from one generation to another and back again, and so neutrinos of all three generations populate the universe. Manmade accelerators are not the only places where these things exist, but they provide the best places to study these particles because we can produce enough of them to measure their properties accurately and produce them in controlled environments where we can understand signals and backgrounds (where signals are evidences for the particle we want to study and backgrounds are the other particles whose interactions sometimes produces data that are indistinguishable from signals). With an accelerator you can do all kinds of good experiments and use control samples, while with cosmic rays you only get one kind of mixture of events at an unknown rate and energy spectrum and sample composition.

As for not knowing "why" the second and third generations exist, I happily agree, and even would say that we don't even know "why" any particle ought to exist. One can make an argument that a system with just one or two generations of particles has to obey "CP symmetry" (swapping particles for antiparticles and reversing the signs of the x,y, and z axes), making matter and antimatter behave in necessarily symmetric ways. Our universe has matter in it but no antimatter, and we deduce that CP symmetry doesn't govern the interactions of the particles in the universe. A three-generation set is the minimum which allows CP symmetry to be broken in the quarks, but we suspect that even this isn't enough to explain the preponderance of matter over antimatter in the universe.

The universe is filled with even stranger stuff still, things we haven't yet made in the laboratory (or haven't been able to separate from the backgrounds). Mysterious dark matter particles float around, interacting with ordinary matter only gravitationally, affecting galaxy distributions, and even how fast stars rotate around galactic cores.

In the very early univese, all generations of particles were common, as enough energy was available to freely produce all of them. At short times after the big bang, compared to the lifetimes of these particles, you can even think of these particles as "stable" (as compared to the age of the universe when it was very young). They contributed to the evolution of the early universe just as much as our more mundane particles we observe today.

Tom

(published on 10/22/2007)

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