32.6

Elementary Particles

| Setting the Stage

By 1932 the electron, the proton, and the neutron had been discovered and were thought to be nature’s three elementary particles, in the sense that they were the basic building blocks from which all matter is constructed. Experimental evidence obtained since then, however, shows that several hundred additional particles exist, and scientists no longer believe that the proton and the neutron are elementary particles.
Most of these new particles have masses greater than the electron’s mass, and many are more massive than protons or neutrons. Virtually all the new particles are unstable and decay in times between about 10−6 and 10−23 s.
Often, new particles are produced by accelerating protons or electrons to high energies and letting them collide with a target nucleus. For example, Figure 32-11 shows a collision between an energetic proton and a stationary proton. If the incoming proton has sufficient energy, the collision produces an entirely new particle, the neutral pion . The particle lives for only about 0.8 × 10−16 s before it decays into two -ray photons. Since the pion did not exist before the collision, it was created from part of the incident proton’s energy. Because a new particle such as the neutral pion is often created from energy, it is customary to report the mass of the particle in terms of its equivalent rest energy (see Equation 28.5). Often, energy units of MeV are used. For instance, detailed analyses of experiments reveal that the mass of the particle is equivalent to a rest energy of 135.0 MeV. For comparison, the more massive proton has a rest energy of 938.3 MeV. Analyses of experiments also provide the electric charge and other properties of particles created in high-energy collisions. In the limited space available here, it is not possible to describe all the new particles that have been found. However, we will highlight some of the more significant discoveries.

Figure 32-11 When an energetic proton collides with a stationary proton, a neutral pion () is produced. Part of the energy of the incident proton goes into creating the pion.

| Neutrinos

In 1930, Wolfgang Pauli suggested that a particle called the neutrino (now known as the “electron neutrino”) should accompany the decay of a radioactive nucleus. As Section 31.5 discusses, the neutrino has no electric charge, has a very small mass (a tiny fraction of the mass of an electron), and travels at speeds approaching (but less than) the speed of light. Neutrinos were finally discovered in 1956. Today, neutrinos are created in abundance in nuclear reactors and particle accelerators and are thought to be plentiful in the universe.

| Positrons and Antiparticles

The year 1932 saw the discovery of the positron (a contraction for “positive electron”). The positron has the same mass as the electron but carries an opposite charge of +e. A collision between a positron and an electron is likely to annihilate both particles, converting them into electromagnetic energy in the form of rays. For this reason, positrons never coexist with ordinary matter for any appreciable length of time. The mutual annihilation of a positron and an electron lies at the heart of an important medical diagnostic technique, as Conceptual Example 7 discusses.

| Conceptual Example 7 | | | Positron Emission Tomography

Positron emission tomography, or PET scanning, as it is known, utilizes positrons in the following way: Certain radioactive isotopes decay by positron emission—for example, oxygen . Such isotopes are injected into the body, where they collect at specific sites. The positron () that is emitted during the decay of the isotope encounters an electron () in the body tissue almost at once. The resulting mutual annihilation produces two -ray photons (), which are detected by devices mounted on a ring around the patient, as Figure 32-12a shows. The two photons strike oppositely positioned detectors and, in doing so, reveal the line on which the annihilation occurred. Such information leads to a computer-generated image that can be useful in diagnosing abnormalities at the site where the radioactive isotope collected (see Figure 32-13). How does the principle of conservation of linear momentum explain the fact that the photons strike detectors located opposite one another?

Figure 32-12 (a) In positron emission tomography, or PET scanning, a radioactive isotope is injected into the body. The isotope decays by emitting a positron, which annihilates an electron in the body tissue, producing two -ray photons. These photons strike detectors mounted on opposite sides of a ring that surrounds the patient. (b) A nurse is talking to a patient about to undergo a PET scan of the brain. (CC Studio/SPL/Photo Researchers)

Figure 32-13 Positron emission tomography (PET) provides an important medical diagnostic technique. This PET scan of the chest, axial section, shows cancer in a lung (see arrow). (© ISM/Phototake)

The physics of PET scanning.

Reasoning and Solution The momentum-conservation principle states that the total linear momentum of an isolated system remains constant (see Section 7.2). Therefore, the total momentum of the -ray photons will be equal to the total momentum of the positron and electron before annihilation, provided that the system is isolated. An isolated system is one on which no net external force acts. The positron and the electron do exert electrostatic forces on one another, however, since they carry electric charges. But these are internal, not external, forces and cannot change the total linear momentum of the two-particle system. The total photon momentum, then, must equal the total momentum of the positron and the electron, which is nearly zero, to the extent that the two particles have much less momentum than the photons do. With a total linear momentum of zero, the momentum vector of one photon must point opposite to the momentum vector of the other photon. Thus, the two photons depart from the annihilation site traveling in opposite directions and strike oppositely located detectors.
Related Homework: Problem 38

The positron is an example of an antiparticle, and after its discovery, scientists came to realize that for every type of particle there is a corresponding type of antiparticle. The antiparticle is a form of matter that has the same mass as the particle but carries an opposite electric charge (e.g., the electron–positron pair) or has a magnetic moment that is oriented in an opposite direction relative to the spin (e.g., the neutrino–antineutrino pair). A few electrically neutral particles, like the photon and the neutral pion , are their own antiparticles.

| Muons and Pions

In 1937, the American physicists S. H. Neddermeyer and C. D. Anderson discovered a new charged particle whose mass was about 207 times greater than the mass of the electron. The particle is designated by the Greek letter (mu) and is known as a muon. There are two muons that have the same mass but opposite charge: the particle and its antiparticle . The muon has the same charge as the electron, whereas the muon has the same charge as the positron. Both muons are unstable, and have a lifetime of 2.2 × 10−6 s. The muon decays into an electron (), a muon neutrino , and an electron antineutrino (), according to the following reaction:

The muon decays into a positron (), a muon antineutrino (), and an electron neutrino (ve):

Muons interact with protons and neutrons via the weak nuclear force (see Section 31.5).
The Japanese physicist Hideki Yukawa (1907–1981) predicted in 1935 that pions exist, but they were not discovered until 1947. Pions come in three varieties: one that is positively charged, the negatively charged antiparticle with the same mass, and the neutral pion, mentioned earlier, which is its own antiparticle. The symbols for these pions are, respectively, , , and . The charged pions are unstable and have a lifetime of 2.6 × 10−8 s. The decay of a charged pion almost always produces a muon:

As mentioned earlier, the neutral pion is also unstable and decays into two -ray photons, the lifetime being 0.8 × 10−16 s. The pions are of great interest because, unlike the muons, the pions interact with protons and neutrons via the strong nuclear force.

| Classification of Particles

It is useful to group the known particles into three families—the photons, the leptons, and the hadrons—as Table 32-3 summarizes. This grouping is made according to the nature of the force by which a particle interacts with other particles. The photon family, for instance, has only one member, the photon. The photon interacts only with charged particles, and the interaction is only via the electromagnetic force. No other particle behaves in this manner.

Table 32-3 Some Particles and Their Properties

Family | Particle | Particle Symbol | Antiparticle Symbol | Rest Energy (MeV) | Lifetime (s)
Photon | Photon |  | Self* | 0 | Stable
Lepton | Electron | e− or | e+ or | 0.511 | Stable
| Muon |  |  | 105.7 | 2.2 × 10−6
| Tau |  |  | 1777 | 2.9 × 10−13
| Electron neutrino | ve |  | <3 × 10−6 | Stable
| Muon neutrino |  |  | <0.19 | Stable
| Tau neutrino |  |  | <18.2 | Stable
Hadron |  |  |  |  |
Mesons |  |  |  |  |
| Pion |  |  | 139.6 | 2.6 × 10−8
|  |  | Self* | 135.0 | 8.4 × 10−17
| Kaon | K+ | K− | 493.7 | 1.2 × 10−8
|  |  |  | 497.7 | 8.9 × 10−11
|  |  |  | 497.7 | 5.2 × 10−8
| Eta |  | Self* | 547.3 | <10−18
Baryons |  |  |  |  |
| Proton | p |  | 938.3 | Stable
| Neutron | n |  | 939.6 | 886
| Lambda |  |  | 1116 | 2.6 × 10−10
| Sigma |  |  | 1189 | 8.0 × 10−11
|  |  |  | 1193 | 7.4 × 10−20
|  |  |  | 1197 | 1.5 × 10−10
| Omega |  |  | 1672 | 8.2 × 10−11

a) |  | The particle is its own antiparticle.

The lepton family consists of particles that interact by means of the weak nuclear force. Leptons can also exert gravitational and (if the leptons are charged) electromagnetic forces on other particles. The four better-known leptons are the electron, the muon, the electron neutrino ve, and the muon neutrino . Table 32-3 lists these particles together with their antiparticles. Two other leptons have also been discovered, the tau particle () and its neutrino (), bringing the number of particles in the lepton family to six.
The hadron family contains the particles that interact by means of both the strong nuclear force and the weak nuclear force. Hadrons can also interact by gravitational and electromagnetic forces, but at short distances () the strong nuclear force dominates. Among the hadrons are the proton, the neutron, and the pions. As Table 32-3 indicates, most hadrons are short-lived. The hadrons are subdivided into two groups, the mesons and the baryons, for a reason that will be discussed in connection with the idea of quarks.

| Quarks

As more and more hadrons were discovered, it became clear that they were not all elementary particles. The suggestion was made that the hadrons are made up of smaller, more elementary particles called quarks. In 1963, a quark theory was advanced independently by M. Gell–Mann (1929– ) and G. Zweig (1937– ). The theory proposed that there are three quarks and three corresponding antiquarks, and that hadrons are constructed from combinations of these. Thus, the quarks are elevated to the status of elementary particles for the hadron family. The particles in the photon and lepton families are considered to be elementary, and as such they are not composed of quarks.
The three quarks were named up (u), down (d), and strange (s), and were assumed to have, respectively, fractional charges of +e, −e, and −e. In other words, a quark possesses a charge magnitude smaller than that of an electron, which has a charge of −e. Table 32-4 lists the symbols and electric charges of these quarks and the corresponding antiquarks. Experimentally, quarks should be recognizable by their fractional charges, but in spite of an extensive search for them, free quarks have never been found.

Table 32-4 Quarks and Antiquarks

| Quarks | Antiquarks
Name | Symbol | Charge | Symbol | Charge
Up | u | +e |  | −e
Down | d | −e |  | +e
Strang | s | −e |  | +e
Charmed | c | +e |  | −e
Top | t | +e |  | −e
Bottom | b | −e |  | +e

According to the original quark theory, the mesons are different from the baryons, because each meson consists of only two quarks—a quark and an antiquark—whereas a baryon contains three quarks. For instance, the pion (a meson) is composed of a d quark and a antiquark, , as Figure 32-14 shows. These two quarks combine to give the pion a net charge of −e. Similarly, the pion is a combination of the and u quarks, . In contrast, protons and neutrons, being baryons, consist of three quarks. A proton contains the combination d + u + u, and a neutron contains the combination d + d + u (see Figure 32-14). These groups of three quarks give the correct charges for the proton and neutron.

Figure 32-14 According to the original quark model of hadrons, all mesons consist of a quark and an antiquark, whereas baryons contain three quarks.

The original quark model was extremely successful in predicting not only the correct charges for the hadrons, but other properties as well. However, in 1974 a new particle, the meson, was discovered. This meson has a rest energy of 3097 MeV, much larger than the rest energies of other known mesons. The existence of the meson could be explained only if a new quark–antiquark pair existed; this new quark was called charmed (c). With the discovery of more and more particles, it has been necessary to postulate a fifth and a sixth quark; their names are top (t) and bottom (b), although some scientists prefer to call these quarks truth and beauty. Today, there is firm evidence for all six quarks, each with its corresponding antiquark. All of the hundreds of the known hadrons can be accounted for in terms of these six quarks and their antiquarks.
In addition to electric charge, quarks also have other properties. For example, each quark possesses a characteristic called color, for which there are three possibilities: blue, green, or red. The corresponding possibilities for the antiquarks are antiblue, antigreen, and antired. The use of the term “color” and the specific choices of blue, green, and red are arbitrary, for the visible colors of the electromagnetic spectrum have nothing to do with quark properties. The quark property of color, however, is important, because it brings the quark model into agreement with the Pauli exclusion principle and enables the model to account for experimental observations that are otherwise difficult to explain.

| The Standard Model

The various elementary particles that have been discovered can interact via one or more of the following four forces: the gravitational force, the strong nuclear force, the weak nuclear force, and the electromagnetic force. In particle physics, the phrase “the standard model” refers to the currently accepted explanation for the strong nuclear force, the weak nuclear force, and the electromagnetic force. In this model, the strong nuclear force between quarks is described in terms of the concept of color, the theory being referred to as quantum chromodynamics. According to the standard model, the weak nuclear force and the electromagnetic force are separate manifestations of a single, even more fundamental force, referred to as the electroweak force, as we have seen in Section 31.5.
In the standard model, our understanding of the building blocks of matter follows the hierarchical pattern illustrated in Figure 32-15. Molecules, such as water (H2O) and glucose (C6H12O6), are composed of atoms. Each atom consists of a nucleus that is surrounded by a cloud of electrons. The nucleus, in turn, is made up of protons and neutrons, which are composed of quarks.

Figure 32-15 The current view of how matter is composed of basic units, starting with a molecule and ending with a quark. The approximate sizes of each unit are also listed.

Check Your Understanding 4

Of the following particles, which ones are not composed of quarks and antiquarks: (a) a proton, (b) an electron, (c) a neutron, (d) a neutrino? [The answer(s) is (are) given at the end of the book.] Background: Of the hundreds of particles that have been discovered, those in the hadron family are composed of quarks, and those in the lepton family are not.
For similar questions (including calculational counterparts), consult Self-Assessment Test 32.1, which is described at the end of Section 32.7.

Copyright © 2007 John Wiley & Sons, Inc. All rights reserved.