Elementary-Particle Physics (1986) / Chapter Skim
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2 What is Elementary-Particle Physics?
2 What is Elementary-Particle Physics?
Pages 18-47
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From page 18... ...
Some of the first answers came with the discovery of certain of the basic forces in nature: the gravitational force, the electrical force, and the magnetic force. It was not until the middle of the nineteenth century that it was discovered that the electric and magnetic forces are in fact two different aspects of the force that we now call electromagnetism.
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From page 19... ...
Nor is the nucleus elementary: by bombarding nuclei with high-energy particles or with high-energy light rays called gamma rays, the nucleus can also be broken up into protons and neutrons. For about 50 years physicists considered the neutron and proton to be elementary, but in the last two decades we have found that these particles themselves are made up of yet simpler particles called quarks.
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From page 20... ...
In this section we present a historical perspective. Figure 2.2 sketches the history of our progress in understanding the number of kinds of elementary particles.
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From page 21... ...
At this point, a modern particle physicist might have questioned whether these elements were truly elementary. But the list grew steadily, doubling before Mendeleev found a convincing way to classify them into smaller related families in 1868.
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From page 22... ...
In 1976, more charmed particles were discovered, and in 1977 a fifth quark, the b or bottom quark, was discovered. Thus the number of fundamental constituents of matter has now grown to 11, and if the expected t or top quark is found it will be 12.
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From page 23... ...
Elementary-particle physics in its search for the simplest forms of matter has become the physics of the very small. Elementary Particles and High Energy At first it seems puzzling that elementary-particle physics, the physics of the very small, is also called high-energy physics.
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From page 24... ...
An electron volt is the energy acquired by an electron or proton passing through an electric potential with a total voltage of 1 volt. As we shall see, the electron volt is a rather small unit of energy or mass, so the elementaryparticle physicist uses larger units: MeV - 1046 eV = 1 million electron volts GeV = 10~9 eV = 1 billion electron volts TeV = 10+'2 eV = 1 trillion electron volts The significance of these energy units can be appreciated by looking at some particle masses expressed in electron volts: 1.
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From page 25... ...
But the gravitational force exerted by one elementary particle is very small compared with the three other forces that can be exerted by that particle. The electromagnetic forces between elementary particles follow the same laws as the electromagnetic forces that are used in modern technology, such as in motors, generators, and electronic equipment.
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From page 26... ...
Assumed O strong force but are affected by the weak force. The radioactive decay of the neutron and of nuclei, as well as the decays of many of the elementary particles, occur through the weak force.
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From page 27... ...
THE LEPTONS ties: The lepton family of elementary particles is defined by two proper1. Leptons are affected by the gravitational, electromagnetic, and weak forces but not by the strong force.
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From page 28... ...
is also defined by two properties: Quarks are affected by all four basic forces. Because they are affected by the strong force, quarks act very differently from the leptons in many situations.
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From page 29... ...
The up and down quarks form one pair; the charm and strange quark form a second pair. There are strong theoretical reasons for assuming the existence of a sixth quark, called the top quark, to be paired with the bottom quark.
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From page 30... ...
PARTICLES AND ANTIPARTICLES In Figures 2.3 and 2.4 we listed the six known leptons and five known quarks. For each of the particles listed there exists a related particle, of the same mass but opposite electric charge, called its antiparticle.
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From page 31... ...
Some examples of these processes are discussed in the next section. COLLISIONS AND DECAYS Collisions of Particles Elementary particles and hadronic particles are too small to be studied directly.
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32 ELEMENTARY-PARTICLE PHYSICS 4-~ ~ -A (a) Protons about to collide head-on.
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From page 33... ...
Collisions and Interactions The concentration of mass and energy in Figures 2.5 and 2.6 represents the crux of how particles interact through the basic forces. Often we know enough about that concentration region to explain it in simple terms.
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As time advances (i.e., moving to the right in the figure) , the electron and positron collide; the collision annihilates the electron and positron and produces a highly excited photon, which contains all the collision energy.
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CONSERVATION LAWS AND SYMMETRY IDEAS What Are Conservation Laws? As particles interact and decay, they present a picture of a world dominated by change.
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From page 36... ...
We have seen that to the best of our knowledge single leptons and quarks cannot be either created or destroyed, but that particleantiparticle pairs can be produced and can annihilate. These observations are expressed in the form of two new and important conservation laws: the law of lepton number conservation and the law of quark number conservation.
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From page 37... ...
Then without knowing anything about the laws of gravitational force, we can make two statements from just the arguments that a sphere is symmetric about its center for any rotation and that the gravitational force must be invariant to any such rotation. First, the size of the force must be the same, no matter where the person walks
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From page 38... ...
When the sphere is simply rotated about its axis the shapes of the lines are not changed; that is called a global symmetry transformation. If the surface of the sphere is distorted as one might do with a sphere made out of rubber, such that the lines of longitude and latitude are twisted, that is a local symmetry transformation.
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From page 39... ...
Each of the fundamental forces is now thought to arise from a similar requirement that a law of Nature be invariant under local symmetry transformation. Because the earliest attempts to construct interactions from symmetries dealt with invariance under a change of scale or gauge, the resulting theories are called gauge theories.
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From page 40... ...
A final example is that the search for new particles usually requires that other particles collide together at high energies to produce the new particles. The primary experimental method in elementary-particle physics involves the collision of two particles at high energy and the subsequent study of the particles that come out of such a collision.
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From page 41... ...
To ~ '~ / arget' A-/ Particle Detectors Proton in To rget A_ Path of High Energy Proton ~ n Beam 4 Particles Produced In Collision FIGURE 2.1 1 In a fixed-target experiment a beam of high-energy particles, for example protons' is produced by an accelerator. The beam of particles interacts with the target producing new particles.
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From page 42... ...
Particle Detectors for Charged Particles Not only charged particles, such as protons or charged plans or electrons, but also neutral particles, such as neutrons and photons, can come out of a collision. Charged means that the particle has positive or negative electrical charge, as opposed to a neutron or photon, which have no electrical charge.
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From page 43... ...
Chapter 6 describes particle detectors in detail, including a discussion of how neutral particles are detected. Secondary Particle Beams The primary beam produced in an accelerator is always either protons or electrons, because stable and charged particles must be used for the acceleration process.
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From page 44... ...
The upper vertex is the point at which a neutral charmed meson decayed into four charged particles: Do ~ K+7~+~r-~-. The decay distance was 9 millimeters, which corresponds to an unusually long lifetime for this particle of 5.5 x 10-'~ second.
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From page 45... ...
In particle colliders both beams must consist of stable, charged particles; the choice in practice has been restricted to protons and electrons and to their antiparticles antiprotons and positrons. The most common form of collider uses opposing beams of electrons and positrons.
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From page 46... ...
Experiments at Particle Colliders Since there is no fixed target in a particle collider, the particle detector must look directly at the region where the opposing beams of particles collide. Figure 2.15 shows how this is done in a circular collider where the beams of particles move in opposite directions ~ BEaMS ~ r or cO~LlDE ~ HERE ~ ~ -_ ~ .~.
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From page 47... ...
Others look for new particles, such as free quarks or magnetic monopoles, in ordinary matter. Still others study with great precision the properties of the stable or almost stable particles, testing, for example, the equality of the size of the electric charge of the electron and the proton.
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