Do protons and neutrons have internal structure? The answer is yes. With the development of higher and higher energy particle accelerators, physicists have found experimentally that the nucleons are complex objects with their own interesting internal structures.
One of the most significant developments in modern physics is the emergence of the Standard Model of Fundamental Interactions (figure below). This model states that the material world is made up of two categories of particles, quarks and leptons, together with their antiparticle counterparts. The leptons are either neutral (such as the neutrino) or carry one unit of charge, e (such as the electron, muon, and tau ). The quarks are pointlike objects with charge 1/3e or 2/3 e. Quarks are spin-1/2 particles, and therefore are fermions, just as electrons are.
The quarks and leptons can be arranged into three families. The up- and down-quarks with the electron and the electron neutrino form the family that makes up ordinary matter. The other two families produce particles that are very short-lived and do not significantly affect the nucleus. It is a significant fact in the evolution of the universe that only three such families are found in nature–more families would have lead to a quite different world.
One could imagine, then, trying to understand the structure of protons and neutrons in terms of the fundamental particles described in the Standard Model. Because the protons and neutrons of ordinary matter are affected by the strong interaction (i.e., the interaction that binds quarks and that ultimately holds nuclei together), they fall into the category of composite particles known as hadrons. Hadrons that fall into the subcategory known as baryons are made of three quarks. Protons, which consist of two up and one down quark, and neutrons (two down and one up quark) are baryons. There are also hadrons called mesons, which are made of quark-antiquark pairs, an example of which is the pion.
Because baryons and mesons have internal quark structure, they can be put into excited states, just as atoms and nuclei can. This requires that energy be deposited in them. One example is the first excited state of the proton, usually referred to as the Delta-1232 (where 1232 MeV/c2 is the mass of the particle). In the Delta, it is thought that one of the quarks gains energy by flipping its spin with respect to the other two. In an atom, the energy needed to excite an electron to a higher state is on the order of a few to a thousand electron volts. In comparison, in a nucleus, a single nucleon excitation typically costs a MeV (106 eV). In a proton, it takes about 300 MeV to flip the spin of a quark. This kind of additional energy is generally only available by bombarding the proton with energetic particles from an accelerator.
Finding a proper theoretical description of the excited states of baryons and mesons is an active area of research in nuclear and particle physics. Because the excited states are generally very short-lived; they are often hard to identify. Research tools at the newly commissioned Jefferson Lab accelerator have been specially designed to look at the spectrum of mesons and baryons. Such research is also being actively pursued at Brookhaven National Laboratory and at many other laboratories. To study the Standard Model, accelerators that produce much higher energy beams are often needed. Such facilities include Fermilab, near Chicago, SLAC at Stanford, and CERN in Geneva. Accelerators for nuclear physics are described in more detail in Chapter 11.
