INTRODUCTION.
When scientists first began looking at cosmic rays showers with an instrument such as the cloud chamber and discovered the positron, discoveries came thick and fast. With an instrument, such as the cloud chamber, capable of measuring both ionization and the amount of bending in a magnetic field, entirely new and unexpected results began to show up. These results were made during the 1930s and 1940s, and they led to the creation to an entirely new area of research, called particle physics.
Cosmic rays were discovered during the search for the cause of ionization chamber leakage. Some ionization chamber leakage was expected, since no insulator is perfect. But when it was found that the leakage rate was greater than could be accounted for by imperfect insulation, it was assumed that the difference was due to ionization from traces of natural radioactivity in the earth. To test this assumption, ionization chambers were carried above the earth in ballons. During the first part of the ascent, the ionization did indeed decrease slightly. But in 1909, Gockel found that at greater heights the ionization began to increase, and at 2.5 miles the ionization was greater than at the surface of the earth. In 1910, Hess and Kohlhauser carried a chamber even higher and, after extensive study, proposed that there was a very penetrating radiation coming outside the earth and they were called cosmic rays. It was estimated that these rays were at least ten times as penetrating as the most energetic gamma rays known.
In the years since the discovery of cosmic rays, experimental observations has shown a great deal about the properties of cosmic rays. Two kinds of cosmic rays have been distinguished: primary and secondary. Primary cosmic radiation is incident on the earth's atmosphere at enormous energies; they consist of the nuclei of atoms, stripped of all electrons. About 77 percent of cosmic-ray primaries are protons, 20 percent are alpha particles, and the rest are very small amounts of the nuclei of elements of higher atomic number, including elements thoughtout the periodic table. When a cosmic-ray particle enters the earth atmosphere, it may strike the nucleus of an atom of the air. The result of such a collision is the creation of a number of other particles. These newly created particles, called secondary cosmic rays, then strike other nuclei and create more particles, leading to showers of particles. The vast majority of cosmic rays observed at sea level are secondaries, not primaries.
In 1932, the American physicist Carl David Anderson (1905-1991) at California Institute of Technology investigated the characteristics of the electrons produced by secondary cosmic rays, using a large cloud chamber in a horizontal magnetic field. The field caused the electrons coming in from above to travel in curved paths. Knowing the strength of the magnetic field and measuring the curvature of the paths along which the electrons moved, Anderson, expecting to determine the velocity, and thus the energy, of the electrons. observed a number of tracks that curved in wrong direction. These particles, who curved in the wrong direction, might be negative electron moving upward rather downward. In order to determine the direction of motion, a lead plate was inserted in the cloud chamber. The particle was expected to lose energy traversing the lead plate, thus its path would be more tightly curved after going through the lead. This would give a clear indication of the direction of travel. He found that the particles who tracks curved in the wrong direction came from above. This meant that the particle was not negatively but positively charged. It had the same mass as an electron, but electrical properties were reversed. This particle was a positive electron, which Anderson called the positron. Its path had probably been observed before but its significance was not realized until Anderson did. He is therefore rightly given credit for the discovery of positron and was awarded the Nobel Prize for physics in 1936.
In the early 1930s, cloud chamber experiments of the type that led to the discovery of the positron were being performed in many laboratories around the world. By 1934 it had become apparent that something was wrong in the way these experiments were being interpreted. Particles were seen whose ability to create ions in the cloud chamber did not correspond with the behavior of electrons, positron, or protons. These new particles, which were called "penetrating rays" (because they could penetrate through the atmosphere to the apparatus at sea level), had either positive or negative electrical charges. When the new quantum mechanics was used to calculate the progression of electrons through a cosmic ray shower, it showed that very few would be expected to reach sea level with the energies like those that were being seen. On the other hand, the energies that the particles lost through ionization in the cloud chambers were too low for them to be protons. This meant either that the penetrating rays were not made up of any known particles or that the quantum mechanics gave incorrect answers when applied to particles of very high energies. The resolution of this problem became one of central issues in cosmic ray physics of the late 1930s.
The fact that problem such as this kind existed illustrates an important point about how scientific research is carried out, a point that is often ignored when looking back at important achievements. Often the significant results of the research is picked out and then put together into a chain of inductive reasoning that appears, in retrospect, to be very precise and logical. In reality, however, the scientist is often confronted with evidence that does not fit into previous knowledge. In this case, for example, the problem could arise because the theory might be wrong (after all, it was only in 1936 that the quantum mechanical theory of electron showers was worked out). On the other hand, the technical details of the analysis of the cloud chamber pictures made it very diffcult to determine the mass of the particles going through at high speed. The path of such particles would only slightly bend by the magnetic field, and normal experimental errors could easily mask significant details. Only when all these ambiguities had been removed could the conclusion be drawn that the particles seen had a mass different from both the electron and the proton.
By 1938, enough experimental evidence had been accumulated to convince the physicists that the problem of explaining the penetrating rays did not lie in an error in quantum mechanics. In an effort to get a final resolution to the problem, Seth Neddermeyer and Carl Anderson, working with the California Institute of Technology cloud chamber, tried a new mode of operation. They inserted a Geiger counter into the chamber and arrange it to expand and photograph the drops only when a particular reading was seen on the counter, a reading that indicated that a very slowly moving particle was in the chamber. In this way they hoped to obtain a series photographs of the tracks of the penetrating rays, which could be easy to analyze. And it worked. In a letter to Physical Review on June 6, 1938, they reported a single event in which a penetrating ray of positive charge was slowed down enough by passing through the Geiger counter so that it came to a complete stop in the cloud chamber. This was the best possible experimental situation to determine the mass of a particle, and when Neddermeyer and Anderson analyzed the photograph, they reported that a new particle had been discovered, with a mass about 240 times that of the electron. (The modern value is 210 times). They named their new discovery the "mesotron" from the Greek root "meso" which means "middle". This name was later shortened to "meson", a term more convenient and more correct linguistically. The particle is customarily represented by the Greek letter μ, and therefore now called the "mu-meson" or "muon".
It turned out that there were actually two mu-mesons, one with a positive charge and one with a negative charge, and their masses were identical. Were these, then, the particles that Yukawa had predicted in 1934 to be responsible for the strong force? But there had been many predictions of an intermediate mass particles. By 1938, Yukawa's prediction was only one among many and his suggestion, coming as it did from a research center far away, was not even the most prominent one in the minds of the American scientists. The Japanese physicist Hideki Yukawa (1907-1981) proposed in 1934 the first significant theory of the strong force to solve the problem: What holds the nucleus together? In the nucleus, the positively charged protons should repel one another violently, as they were packed together in such close proximity in the nucleus. Evidently, there must be some other force, more powerful than the electrical force of repulsion. This force binds the protons and neutrons together; the physicists called it the strong force. But if there is such a potent force, why hasn't it been noticed in everyday life? Virtually every other force, gravity, electricity and magnetism is experienced directly. The answer must be that, powerful though it be, this strong force is of very short range. This strong force is confined within the nucleus, beyond it this strong force drops off rapidly to zero. Gravitational and electromagnetic forces have an infinite range, but the range of the strong force is about the size of the nucleus itself. Yukawa assumed that the proton and neutron are attracted to one another by some form of a field, just as the electron is attracted to the nucleus by an electric field and the moon is attracted to the earth by a gravitational field. But this nuclear field should be quantized, and about it Yukawa asked the question: What must be properties of its quantum - the particle (similar to the photon) whose exchange would account for the known features of the strong force? For example, the short range of the strong force indicated that its mediator would be rather heavy; Yukawa calculated that its mass should be nearly 300 times that of the electron, about a sixth of mass of the proton. Because its mass fell between that of the electron and the proton, Yukawa's particle became known as the meson (meaning "middle-weight"), just as the electron was called a lepton (meaning "light-weight"), and the proton and neutron were called baryons (meaning "heavy-weight"). Now, Yukawa knew that no such particle had ever been observed in the laboratory, and he therefore assumed that his theory was wrong. But at the time a number of laboratories were studying cosmic rays, and by 1937 two groups (Anderson and Neddermeyer in California, and Street and Stevenson in the East) had identified particles that matched Yukawa's description.
For a while everything seemed to be in order. But as more detailed study of cosmic rays were undertaken, a disturbing discrepancies began to appear. They had the wrong lifetimes and they were significantly lighter than what Yukawa had predicted; and worst still, different mass measurements were not consistent with one another. In 1946 (after World War II and the development of the "atomic" or more accurately the "nuclear" bomb), decisive experiments were carried out in Rome, Italy, demonstrating that the cosmic rays particles interacted very weakly with atomic nuclei. Now if this was really Yukawa's particle, the transmitter of the strong force, then why had not the interaction been more dramatic? This puzzle was finally solved in 1947, when Cecil F. Powell (1903-1969) and Giuseppe (Beppo) Occhialini (1907-1993) at the University of Bristol in England discovered that there were two middle-weight particles in the cosmic rays, which they called pion (π) and muon (μ). (Robert E. Marshak reached the same conclusion on theoretical grounds). The mass of the pion is about 135 Mev and the mass of the muon is 100 Mev. Actually Yukawa's meson is the pion; it is produced copiously in the upper atomsphere, but ordinarily disentegrates long before reaching the ground. Powell's group exposed their photographic emulsions on mountain tops. One of the decay products of these pions is the lighter and longer lived muon; and it is primarily the muons that are observed at sea level. Thus in the search for Yukawa's meson, the muon was found to be an impostor, having nothing to do with the strong nuclear interactions. In fact, the muon behaves in every way like a heavier version of an electron and properly belongs the lepton family (though some people today still call them "mu-meson").
In the 1950s, machines were becoming available that were capable of accelerating electrons and protons to energies comparable to those of cosmic rays. These particle accelerators were capable of creating pions for use in physics research, so that detailed study of their properties could be done. It turned out that there were three kinds of pions: pions with a positive electrical charge, pions with a negative electrical charge, and pions that are electrically neutral. These are denoted respectively by the symbols: π+, π-, and π0. The mass of the charged meson is 273 times that of the electron, and each of the charged mesons decays by the reaction
π → μ + ν,
with a lifetime of 10-8 seconds. The π0 has a mass 265 times that of the electron and decays by the reaction
π0 → 2 photons
in about 10-16 seconds. It is now believed that most of the strong force is generated by the exchange of mesons in the nucleus.
Why was it not discovered sooner? Given the importance of the meson in the theory of the nuclear force, why did it take a decade to solve the pi-mu problem? Part of the answer is historical; the decade between the discovery of the meson and the identification of the pion with the Yukawa meson spans World War II, a period when the attention of most of physicists were forcused elsewhere. But perhaps the more important reason is that the properties of the pi-meson itself. When such a meson is created by cosmic ray collisions high in the atmosphere, one of two things can happen: it can decay before it hits the ground or it can interact with a nucleus in the atmosphere. Typically. a pi-meson of moderate energy will travel only a few meters or tens of meters before it decays, and even if it has energy high enough to bring it to sea level before decay, it would be able to travel only a few hundred meters before it interacted with a nucleus. In either case, the original meson will not be able to reach ground level and therefore would not be seen in a ground-based cloud chamber. And since most pi-mesons in a cosmic ray shower are created at high altitudes, before the energy of the particles are degraded below the level needed for particle production, pi-mesons are never seen at sea level. This fact alone explains why the Bristol group found evidence for the pion when they exposed emulsions on mountain tops rather than at sea level. This also explains why, in the acknowledgements of one of their original pi-meson papers, the authors thank the leaders of the mountaineering expedition for carrying some plates to the top of Mount Kilimanjaro in Tanzania - a height of 19,000 feet. But even a cloud chamber on top of a mountain would not stand a very good chance of detecting and identifying a pi-meson. Since the material in the cloud chamber is not very dense, a meson passing through the chamber would have a very low probability of encountering a nucleus and interacting. It would then be very difficult to tell the difference between a fast muon and fast pion by looking at droplets. It is critical that a particle be brought to rest in a cloud chamber in order that it may be identified. And the first identification of the mu-meson depended upon a lucky event in a ground level experiment. Given the difficulties attendant on operating cloud chambers on top of a mountain and the resulting paucity of high-altitude cloud chamber data, it is not too surprising that there was no equivalent lucky event for the discovery of the pi-meson. With emulsions, however, the situation is different. They are quite dense, so that pions entering the emulsion are likely to be stopped. And in addition it is possible to make much more accurate determination of a particle mass by counting silver grains through a microscope than by analyzing droplets.
C. F. Powell and others developed nuclear emulsion technique which used special photographic emulsions that contained about 80 percent silver bromide by dry weight and are about 1 mm thick. These can be stacked to make a sensitive region of any size. Such blocks of emuslion are continuously sensitive and are portable enough to be carried by ballons and other high-altitude aircraft. Ordinary photographic emulsions are unsuitable because they contain too small a fraction of sensitive silver halides and they are too thin. Even though it requires elaborate techniques to develop, piece together, and read these nuclear emulsions, the resultant "tracks" closely resemble those formed in cloud and bubble chambers. The important characteristics of the tracks, such as their density, range, direction, and deviation in direction, often permits the positive identification of the particle forming the track.
By 1948 the riddle of the mesons had been solved. Not one, but two groups of particles with a mass between that of the electron and the proton had been found. The pions are the particles predicted by Yukawa, and all three of them are routinely exhanged within the nucleus to generate the strong interaction. It was inevitable that the existence of such particles were to be discovered. And the number of elementary particles had increased by one (it is customary in particle physics to refer to all the pion family as single particle). And with this increase in the complexity of the universe, there is gained an understanding of the strong interaction.
The case of the mu-meson is not so clear. It added another elementary particle to our fast growing collection, but it is not obvious what role it plays. It is not essential to our understanding of the nucleus, and in many ways in seems that God, having created the electron, went ahead and repeated the process for a particle 200 times heavier. One of the major unsolved mysteries of particle physics remains the question why the muon should exist at all. Physicist Isidor Isaac (I. I.) Rabi (1898-19??) of Columbia University expressed the sentiments of the physics community about the mu-meson very well with the query: "Who ordered this?"
In the 1930s, physicists who were studying the radioactive decay of the nuclei of the atoms found something that was puzzling. By precise measurements they found that there was more energy before disintegration of the nuclei than after disintegration; this appeared to be a violation of the fundamental law of the conservation law of energy. In 1930 the theoretical physicist Wolfgang Pauli proposed a solution of the problem by suggesting that a elusive particle was carrying off the missing energy. He proposed that there must exist a neutral particle which has no mass, no charge, no magnetic properties, and so little interaction of with matter so that it could traverse a solid wall 1014 miles thick before interacting with an atom. It was thus about as close to nothing as is possible while being something. It was considered to be something because it had energy, momentum, and angular momentum. Pauli called this particle a "neutron", because it was electrically neutral; this name lead to confusion when another neutral particle, with mass and magnetic properties was discoveried by Chadwick in 1932 and was also named the neutron. Enrico Fermi, when asked by a group of students whether Chadwick's particle was Pauli's neutron, answered that it was not, and that Pauli's neutral particle was a neutrino (which means "a little neutral one" in Fermi's native Italian). At the time that Pauli made his suggestion, it seemed like a cheat to explain away the energy problem by postulating the existence of a new particle that was nearly impossible to detect. But eventually these particles were directly detected and today physicists make radiation beams of them. The experimental evidence that lead Pauli to make this proposal came from the study of beta decay, the emission of an electron in radioactive decay. The simplest example of beta decay is the decay of a free neutron. A neutron, free from the neuleus of an atom and not bound in it, decays with a half-life of about 15 minutes into a proton and an electron.
neutron → proton + electron
When the decay of the neutron occurs in a nucleus of an atom, the effect is to increase the nuclear charge by one unit without changing its mass number, for example,
90Th234 → e- (beta ray) + 91Pa234.
When very careful measurement of the mass of the original nucleus, the mass of the product nucleus, and the mass and energy of emitted electron, an anomaly was found. A few of the emitted electrons have just the right amount of energy so that the energy of the starting nucleus (mass included) and the final energy of all the products (mass included) are the same. But many more of the emitted electrons have less than this amount, and no energy is detected anywhere else. Therefore, it seems that either energy is not conserved or some unseen particle is carrying off the missing energy. This particle must be electrically neutral to advoid detection. A photon has many of these required properties, but it cannot penetrate matter. If a photon was carrying away the missing energy, it would be absorbed in the apparatus of the experiment and the energy would be detected as a rise in temperature. Pauli therefore proposed that a new particle carried away the energy. The precise properties that he gave to this particle were necessary to account for the experimental observations - electrical neutrality, masslessness, and the ability to penetrate matter. A similar anomalty is found for momentum. The momentum before beta decay and afterwards were found not to be the same. Again Pauli proposed that the new particle carried away the missing momentum, if the concervation of momentum is not to fail. And if the angular momentum or spin of the electron, proton, and neutron are taken into account, then the new particle must carry away angular momentum, if the conservation of angular momentum is not to fail. Thus Pauli proposed that this new particle, later called the neutrino, had the properties that it carries away energy, momentum, and angular momentum; it needed no other properties.
By 1950, there was complelling theoretical evidence for the existence of neutrinos, but there was no direct experimental verification. A skeptic might have argued that the neutrino was nothing but a book-keeping device, a purely hypothetical particle whose sole function was to rescue the conservation laws of physics. It left no tracks, it did not decay; in fact, no one had ever seen a neutrino do anything.
In 1956 the experimental discovery of the neutrino occurred. Actually, it turn out that the neutrino-particle was an antineutrino. That is,
neutron → proton + electron + antineutrino.
In 1956 Clyde Cowan and Fred Reines, working near a nuclear reactor at the Savannah River, South Carolina, found evidence that the following reaction occurred:
antineutrino + proton → neutron + positron,
About once every 20 minutes they observed that a neutron and a positron appeared simultaneously in their apparatus. These are exactly the products predicted if an antineutrino reacts with a proton. This is the reverse of the reaction that occurs in the beta decay. Reines and Cowan found that the such products occurred only when the reactor was running, confirming that the reactor was the source of whatever was happening. A nuclear reactor provides the best man-made source of neutrinos, from the beta decay of the products of nuclear fission in the reactor. Thus they confirmed that neutrinos do really exist, and that they were not the figment of Paul's imagination.
The decay of a pion or muon requires a neutrino to be given off, in order that energy and momentum be conserved. A pion decays into a muon and a neutrino, and a muon decays into an electron and two neutrinos.
π+ → μ+ + neutrino
π- → μ- + antineutrino
and
μ+ → electron + neutrino + antineutrino
μ- → positron + neutrino + antineutrino
Back in 1953, Konopinski and Mahmoud had introduced a beautifully simple rule for determining which reaction will occur and which will not. To accomplish this, they assigned a lepton number L = +1 to the electron, to the muon, and to the neutrino, and L = -1 to the positron, the positive muon, and the antineutrino (all other particles were given a lepton number of zero). Then they proposed the law of the conservation of lepton number (analogous to the law of the conservation of charge); in view of this law of conservation of lepton number, the charged pion decay and the charged muon decay were written as given above. What property then distinguishes the neutrino from the antineutrino? The cleanest answer is lepton number; if its lepton number is +1, then it is a neutrino, and if its lepton number is -1, it is antineutrino. These numbers are experimentally determinable, just as electric charge is, by watching how the particles interacts with others.
But experimentally the decay of the muon into an electron plus a proton is never observed. This process would have been consistent with the conservation of charge and the conservation of lepton number. Now there is a reliable rule of thumb in particle physics (generally attributed to Richard Feynman) which says that whatever is not expressly forbidden is manadory. The absence of occurence of the decay of the muon into an electron plus a proton suggests a law of the conservation of "mu-ness"; but then how do we explain the observed decay of the muon? The answer occurred to number of physicists in the late 1950s and early 1960s: suppose that there are two different kinds of neutrinos: one associated with the the electron (the electron-neutrino) and one with the muon (the muon-neutrino). If we assign a muon number Lμ = +1 to muon- and muon-neutrino, and at the same time an electron number Le = -1 to muon+ and antimuon-neutrino, and refine the conservation of lepton numbers into two separate laws: the conservation of electron number and the conservation of muon number, then we can account for all the allowed and forbidden processes.
The neutrinos associated with these processes was first observed in 1962 at the Brookhaven National Laboratory. The surprising result was that the neutrino associated with the muon decay is not the same as the electron's neutrino. So now there are two kinds of neutrinos, each with its own antineutrino. They will be designated νe and νμ and the reaction written as
muon+ → positron + electron-neutrino + antimuon-neutrino
muon- → electron + antielectron-neutrino + muon-neutrino
By 1962, the lepton family had grown to eight particles: the electron, the muon, their respective neutrinos, and their corresponding antiparticles. The leptons are characterized by the fact that they do not participate in strong interactions. For the next 14 years, as far as the leptons were concerned, things were pretty quite.