SIDEBAR 2.1 THE NEUTRINO
Neutrinos, they are very small.
They have no charge and have no mass
And do not interact at all.
The earth is just a silly ball
To them, through which they simply pass,
Like dustmaids down a drafty hall
Or photons through a sheet of glass
The mysterious neutrino entered the popular culture in John Updike’s 1959 poem “Cosmic Gall.” He stated neutrinos’ two most important and puzzling features—masslessness and elusiveness. Today, we know that neutrinos are almost, but not quite, massless. The interaction between neutrinos and other forms of matter is extremely rare because they interact only through the weak nuclear force: It would take a wall of ordinary matter more than 100 light-years thick to stop a beam of neutrinos like those produced by the Sun. Precisely because they are so elusive, neutrinos produced at the center of the Sun traverse the entire mass of the Sun without being absorbed, allowing us to see deep into the Sun’s center (see Figure 4.4).
There are three types of neutrinos: electron, mu, and tau neutrinos—so named because they are associated with the electron, muon, and tau particles. These six “leptons,” together with the six types of quarks—up, down, charm, strange, top, and bottom—are the basic building blocks of matter (see Figure 2.1.1). The three neutrinos differ from the nine other building blocks of matter because they are so light and interact so weakly. These two differences are at the root of their importance to modern astrophysics and physics.
Said simply, the unique role of neutrinos is “seeing deep.” By detecting neutrinos from astrophysical objects we can see deep into the Sun, into exploding stars (supernovae), and someday, one can hope, into the mighty explosions that power the mysterious gamma-ray bursts seen across the universe.
Neutrinos also allow us to study the forces of nature at the shortest distances by observing rare processes in which they participate. For instance, neutrinos permit us to “see deep” into the nuclei of atoms through the process of neutrino scattering from the quarks within the proton. Their tiny masses and the transformations of one neutrino type to another (see Figure 2.1.2) have even revealed physics beyond the Standard Model of particle physics. Studying and understanding neutrino mass and oscillation provide a unique view into how the forces and particles are unified.
Because neutrinos are uncharged, it is possible that, like photons, they are their own antiparticles. If this is so, it may explain the existence of the kind of matter we are made of. Shortly after the big bang, there were equal amounts of matter and antimatter. Were it not for the fact that a slight excess of matter over antimatter developed later, all matter and antimatter would have been annihilated long ago. If neutrinos (of nonzero mass) are their own antiparticles (unlike quarks), additional pathways for matter-antimatter differences become possible, and thus neutrinos are likely to have played a role in how the slight excess of matter arose in today’s universe.