11.1 Introduction to particle physics  (Page 2/22)

The fundamental forces may not be truly “fundamental” but may actually be different aspects of the same force. Just as the electric and magnetic forces were unified into an electromagnetic force, physicists in the 1970s unified the electromagnetic force with the weak nuclear force into an electroweak force    . Any scientific theory that attempts to unify the electroweak force and strong nuclear force is called a grand unified theory    , and any theory that attempts to unify all four forces is called a theory of everything    . We will return to the concept of unification later in this chapter.

Classifications of elementary particles

A large number of subatomic particles exist in nature. These particles can be classified in two ways: the property of spin and participation in the four fundamental forces. Recall that the spin of a particle is analogous to the rotation of a macroscopic object about its own axis. These types of classification are described separately below.

Classification by spin

Particles of matter can be divided into fermion     s and boson     s . Fermions have half-integral spin $\left(\frac{1}{2}\hslash ,\frac{3}{2}\hslash ,\text{…}\right)$ and bosons have integral spin $(0\hslash ,1\hslash ,2\hslash ,\text{…}).$ Familiar examples of fermions are electrons, protons, and neutrons. A familiar example of a boson is a photon. Fermions and bosons behave very differently in groups. For example, when electrons are confined to a small region of space, Pauli’s exclusion principle    states that no two electrons can occupy the same quantum-mechanical state. However, when photons are confined to a small region of space, there is no such limitation.

The behavior of fermions and bosons in groups can be understood in terms of the property of indistinguishability. Particles are said to be “indistinguishable” if they are identical to one another. For example, electrons are indistinguishable because every electron in the universe has exactly the same mass and spin as all other electrons—“when you’ve seen one electron, you’ve seen them all.” If you switch two indistinguishable particles in the same small region of space, the square of the wave function that describes this system and can be measured $\left(|\psi {|}^2\right)$ is unchanged. If this were not the case, we could tell whether or not the particles had been switched and the particle would not be truly indistinguishable. Fermions and bosons differ by whether the sign of the wave function ( $\psi$ )— not directly observable—flips:

Fermions are said to be “antisymmetric on exchange” and bosons are “symmetric on exchange.” Pauli’s exclusion principle is a consequence of exchange symmetry    of fermions—a connection developed in a more advanced course in modern physics. The electronic structure of atoms is predicated on Pauli’s exclusion principle and is therefore directly related to the indistinguishability of electrons.

Classification by force interactions

Fermions can be further divided into quark     s and lepton     s . The primary difference between these two types of particles is that quarks interact via the strong force and leptons do not. Quarks and leptons (as well as bosons to be discussed later) are organized in [link] . The upper two rows (first three columns in purple) contain six quarks. These quarks are arranged into two particle families: up, charm, and top ( u , c , t ), and down, strange, and bottom ( d , s , b ). Members of the same particle family share the same properties but differ in mass (given in $\text{MeV/}{c}^2$ ). For example, the mass of the top quark is much greater than the charm quark, and the mass of the charm quark is much greater than the up quark. All quarks interact with one another through the strong nuclear force.