EXOTIC subatomic particles, such as pions, kaons and hyperons, are produced constantly in the Earth’s atmosphere. Cosmic rays – high-energy particles (mainly protons) from outer space – bombard atoms in the upper atmosphere, causing spectacular nuclear disintegrations. The debris from these collisions includes particles that are too short-lived to form a substantial part of familiar matter, but which are nevertheless as real and detectable as the stable protons, neutrons and electrons.
Physicists first discovered these ‘exotic’ particles in studies of cosmic rays during the 1930s and 40s. The muon appeared first, in experiments in 1936 which revealed a charged particle some 210 times as heavy as the electron, or 11 per cent the mass of the proton. We now know that the muon behaves almost exactly like a heavy electron. The only difference is that being heavier, the muon can decay into an electron, which it does in a time of 2.2 microseconds through the agency of the weak force.
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From 1936 to 1947, the muon’s true identity was mistaken, however. At first, physicists believed that it was the particle we now know as the pion. The pion is only about 30 per cent heavier than the muon, but it behaves very differently. Whereas the muon is uninfluenced by the strong force that works inside the nucleus, the pion plays a role in binding protons with neutrons. This means that high-energy muons can penetrate far into matter before they interact or decay; indeed, some cosmic-ray muons travel hundreds of metres below ground. Pions, on the other hand, feel the effect of the nuclei they encounter in matter and are much more readily absorbed. Moreover, while muons can be positively or negatively charged, pions can also have no charge at all.
The pion was discovered in 1947, in photographic emulsion which Cecil Powell’s team from the University of Bristol had exposed to cosmic rays on the Pic du Midi in the Pyrenees. The emulsion revealed the decay of one particle to another of slightly smaller mass; the first was the pion, its decay product a muon.
Around the same time, Clifford Butler and George Rochester at the University of Manchester observed the decay of a heavier particle, now called the kaon, which weighs in at about half the proton’s mass. Like the pion, the kaon can be positively charged, negatively charged or neutral. But being heavier than the pion, the kaon can decay to pions, although the most common mode of decay for charged kaons is to a muon.
The studies of cosmic rays also revealed short-lived particles heavier than protons – the hyperons. The lightest of these, the lambda, is a neutral particle 19 per cent heavier than the neutron. The lambda most often decays to a proton and a negative pion, but it can also decay to a neutron and a neutral pion. Still heavier hyperons include the sigma, which can be positive, negative or neutral. These are about 27 per cent heavier than the proton.
We now know that the pions, kaons and hyperons differ from the muon in that they are composite particles built from quarks, as are the proton and neutron.
The hyperons, like the proton and neutron, contain three quarks; the pions and kaons, on the other hand, are each built from a quark bound with an antiquark. Particles built from quarks all feel the strong interaction – the force between quarks. This is the way that they interact with atomic nuclei. However, like the muon they are also subject to the weak force, which governs their decays to other particles, after lifetimes of typically a few billionths of a second or less.