Almost all known particles are unstable, from the neutron with an average lifetime of fifteen minutes to the pi meson which lasts less than a tenth of a microsecond. There is strong experimental evidence, however, that there are many more particles that have lifetimes of less than ten to the minus twenty-third seconds. These particles are called resonance particles because of one of the methods used to detect their presence. Their average lifetimes are so short that normal methods, such as bubble chambers, can not be used to detect them.
Particles are usually detected by using some means of making them leave trails in some medium. Particles in bubble chambers leave bubbles in a superheated liquid, particles in cloud chambers leave trails of fog in supercooled gas, and particles in spark chambers leave a trail of sparks behind them between electrically charged plates. However, a particle lasting less than an attosecond will move less than a third of a nanometer, even when traveling at nearly the speed of light. Most resonance particles have lives much shorter than a whole attosecond. That distance is much too small to cause any trail or spark to form in any particle detector.
A resonance particle can be detected in particle decays when the sum of the energies of some of the resulting particles tend towards certain values. When the total energy (meaning kinetic energy as well as rest mass energy) of a group of resulting particles tends to be a certain value, it is said to be because the result of the decay producing those particles actually produced an ultra-short-lived particle, which then broke down into those more long-lived particles. Their total energy adds up to the energy of the intermediate particle, which is constant.
A similar process takes place in chemistry, when complex chemical processes take place. When a chemical reaction involves several substances, it does not take place all at once. The various atoms would never come together at just the right times to produce the reaction. Rather, a series of simple reactions, each involving only one or two reactants, takes place in quick succession, with the final result being the same as if a single complex reaction had taken place. There are numerous intermediate chemicals which are created during the combustion of an octane molecule, for example, but because none of them survive to the end of the reaction, they can be safely ignored. Similarly, a negative kaon and proton don't decay immediately into two pions of opposite charge and a lambda baryon, but into a pion and an ultra-short-lived lambda of the opposite charge, which quickly breaks down into the final lambda and another pion.
Just as it is unimportant in most situations to know what series of reactions change gasoline into water and carbon dioxide, it is usually not necessary to know what short-lived particles may exist momentarily when particles decay. However, just as knowing the actual process by which a chemical reaction occurs can help to
explain why it occurs when it does or what environmental conditions are important, knowing what intermediate particles exist during the decay of a particle can help explain why some results are more likely than others, or why the results depend on various conditions. A negative kaon and proton may not form two pions and a lambda baryon, but rather may simply bounce off of each other, or form a neutron and a neutral kaon, or a sigma baryon and a pion. In all the decays producing new particles, a lambda is an intermediate particle. When the particles simply scatter, the energy wasn't right to produce the lambda. Depending on what energy the lambda has, it can decay into five different sets of particles. It is important to note that when the input energy isn't the correct amount to make a lambda, one is less likely to form, but it is certainly still possible. This is because the particle lasts for such a short amount of time that the uncertainty associated with it's energy is significant.
This likelyhood of certain decays happening at certain energy levels is most often illustrated with graphs of cross section vs. energy. Cross section is generally given in barns, which is a unit of area, although it is found by dividing the number of reactions per unit of time per nucleus by the incident intensity. The important thing about cross section in the context of resonance particles is that it can show the relative likelyhood of different reactions taking place at different energies. For example, the likelyhood of a proton and a kaon decaying into two pions and a lambda is zero for total energies less than 1.48 GeV. At energies higher than 1.48 GeV, the likelyhood of this decay taking place increases linearly, which is not unexpected. However, there is a spike in the cross section at 1.52 GeV, where the likelyhood of this decay taking place is more than four times greater than expected. This spike can be explained only by a resonance particle: that a lambda of energy 1520 MeV momentarily existed during the decay.
Unfortunately, a graph of cross section vs. total energy cannot be shown here. The shape of this graph is important. The spike in the graph due to the resonance particle in our example of the proton and kaon is not a sharp spike at 1520 MeV, but is a wide peak, with the resonance particle causing the cross section to deviate from the expected value a significant amount for energies within about 16 MeV of 1520 MeV. This spreading out of the spike is due to the uncertainty of the particle's energy, mentioned earlier. The longer a particle lasts, the more certain its energy is. Because the resonance particle can come into being when the energy is with 16 MeV of 1520 MeV, we have a good idea of what the uncertainty associated with the lambda's energy is. From this uncertainty, we can find the average lifetime of the particle, which in this case is about four times ten to the negative twenty-third seconds.
"Resonance particle" is the name given to those particles which do not exist long enough to be detected by ordinary means before decaying into other particles. They are detected by graphing cross section, or the likelihood of a decay occuring, vs. total energy of the particles in a particular decay. Spikes in the cross section are evidence that a resonance particle of rest energy equal to the center energy of the spike existed momentarily. The average lifetime of the particle can be determined from the width of the spike.
Neat Links:
Dr. Luis Alvarez's work work made accomplishments such as detecting resonance particles (first detected in 1960) possible.
Here is an example of current research on resonance particles.
Bibliography:
Concepts of Modern Physics, Fourth Edition. Arthur Beiser. McGraw-Hill Book Company. New York. 1987.
Elementary Modern Physics, Third Edition. Richard T. Weidner and Robert L. Sells. Allyn and Bacon, Inc. Boston. 1980.
Modern Physics. Hans C. Ohanian. Prentice-Hall, Inc. Englewood Cliffs, NJ. 1987.
Modern Physics. Paul A. Tipler. Worth Publishers, Inc. New York. 1978.