# Particle Physics

## Connection for AP® Courses

Continuing to use ideas that would be familiar to the ancient Greeks, we look for smaller and smaller structures in nature, hoping ultimately to find and understand the most fundamental building blocks. Atomic physics deals with the smallest units of elements and compounds. Through the study of atomic physics, we have found a relatively small number of atoms with systematic properties that explain a tremendous range of phenomena.
Nuclear physics is concerned with the nuclei of atoms and their substructures, supporting Big Idea 1, that systems have internal structure. Furthermore, the internal structure of a system determines many properties of the system (Enduring Understanding 1.A). Here, a smaller number of components—the proton and neutron—make up all nuclei. Neutrons and protons are composed of quarks. Electrons, neutrinos, photons, and quarks are examples of fundamental particles. The positive electric charge on protons and neutral charge on neutrons result from their quark compositions (Essential Knowledge 1.A.2).
This chapter divides elementary particles into fundamental particles as objects that do not have internal structure and composed particles whose properties are defined by their substructures (Essential Knowledge 1.A.2). The magnetic dipole moment, related to the properties of spin (angular momentum) and charge, is an intrinsic property of some fundamental particles such as the electron (Essential Knowledge 1.E.6). This property is the fundamental source of magnetic behavior in matter (Enduring Understanding 1.E).
Exploring the systematic behavior of interactions among particles has revealed even more about matter, forces, and energy. Mass and electric charge are properties of matter that are conserved (Enduring Understanding 1.C). The total energy of the system is also conserved (Enduring Understanding 5.B). In quantum mechanical systems, mass is actually part of the internal energy of an object or system (Essential Knowledge 5.B.11). It has been discovered experimentally that, due to certain interactions between systems, mass can be converted to energy and energy can be converted to mass (Essential Knowledge 1.C.4, Essential Knowledge 4.C.4), supporting Big Idea 4. These process can also lead to changes in the total energy of the system (Enduring Understanding 4.C).
Particle physics deals with the substructures of atoms and nuclei and is particularly aimed at finding those truly fundamental particles that have no further substructure. In general, any system can be viewed as a collection of objects, where objects do not have internal structure (Essential Knowledge 1.A.1). Just as in atomic and nuclear physics, we have found a complex array of particles and properties with systematic characteristics analogous to the periodic table and the chart of nuclides. We have discovered that changes in the systems are constrained by the conservation laws, supporting Big Idea 5. In the case of elementary particles, these conservation laws include mass-energy conservation and conservation of electric charge (Enduring Understanding 5.C). Electric charge is conserved in elementary particle reactions, even when elementary particles are produced or destroyed (Essential Knowledge 5.C.1).
The chapter revisits the ideas of fundamental forces (Enduring Understanding 3.G) and their fields in connection to elementary particles. This supports Big Ideas 2 and 3, because these particles are carriers of a specific force that provides existence of the field in space (Enduring Understanding 2.A). The field is simply the macroscopic outcome of all these force-carrying particles. The approximate relative strength and range of the gravitational force (Essential Knowledge 3.G.1), electromagnetic force (Essential Knowledge 3.G.2), strong force (Essential Knowledge 3.G.3) and weak force are considered in relation to the properties of their carrier particles. The details of these considerations go beyond AP® expectations.
An underlying structure is apparent, and there is some reason to think that we are finding particles that have no substructure. Of course, we have been in similar situations before. For example, atoms were once thought to be the ultimate substructure. Perhaps we will find deeper and deeper structures and never come to an ultimate substructure. We may never really know, as indicated in .
This chapter covers the basics of particle physics as we know it today. An amazing convergence of topics is evolving in modern particle physics. We find that some particles are intimately related to forces, and that nature on the smallest scale may have a defining influence on the large-scale character of the universe. The study of particle physics is an adventure beyond even the best science fiction, because it is not only fantastic, it is real. 
Big Idea 1 Objects and systems have properties such as mass and charge. Systems may have internal structure.
Enduring Understanding 1.A The internal structure of a system determines many properties of the system. 
Essential Knowledge 1.A.1 A system is an object or a collection of objects. Objects are treated as having no internal structure.
Essential Knowledge 1.A.2 Fundamental particles have no internal structure.
Enduring Understanding 1.C Objects and systems have properties of inertial mass and gravitational mass that are experimentally verified to be the same and that satisfy conservation principles. 
Essential Knowledge 1.C.4 In certain processes, mass can be converted to energy and energy can be converted to mass according to E = mc2, the equation derived from the theory of special relativity.
Enduring Understanding 1.E Materials have many macroscopic properties that result from the arrangement and interactions of the atoms and molecules that make up the material.
Essential Knowledge 1.E.6 Matter has a property called magnetic dipole moment.
a. Magnetic dipole moment is a fundamental source of magnetic behavior of matter and an intrinsic property of some fundamental particles such as the electron.
Big Idea 2 Fields existing in space can be used to explain interactions.
Enduring Understanding 2.A A field associates a value of some physical quantity with every point in space. Field models are useful for describing interactions that occur at a distance (long-range forces) as well as a variety of other physical phenomena.
Big Idea 3 The interactions of an object with other objects can be described by forces.
Enduring Understanding 3.G Certain types of forces are considered fundamental. 
Essential Knowledge 3.G.1: Gravitational forces are exerted at all scales and dominate at the largest distance and mass scales.
Essential Knowledge 3.G.2 Electromagnetic forces are exerted at all scales and can dominate at the human scale.
Essential Knowledge 3.G.3 The strong force is exerted at nuclear scales and dominates the interactions of nucleons.
Big Idea 4 Interactions between systems can result in changes in those systems.
Enduring Understanding 4.C Interactions with other objects or systems can change the total energy of a system. 
Essential Knowledge 4.C.4 Mass can be converted into energy and energy can be converted into mass.
Big Idea 5. Changes that occur as a result of interactions are constrained by conservation laws.
Enduring Understanding 5.B The energy of a system is conserved.
Essential Knowledge 5.B.11 Beyond the classical approximation, mass is actually part of the internal energy of an object or system with E = mc2.
Enduring Understanding 5.C The electric charge of a system is conserved.
Essential Knowledge 5.C.1 Electric charge is conserved in nuclear and elementary particle reactions, even when elementary particles are produced or destroyed. Examples should include equations representing nuclear decay.