Warped Passages - Lisa Randall [82]
Figure 49. The interaction with a W-gauge boson changes a neutron into a proton (and a down quark contained in the neutron into an up quark contained in the proton).
Figure 50. The Feynman diagram (on the right) representation of a photon-electron interaction. The squiggly line is the photon. It interacts with the electron that comes in and leaves the interaction vertex, as illustrated schematically on the left.
However, because the neutron and proton have different masses and carry different charges, the neutron must decay into a proton plus other particles, so as to conserve charge, energy, and momentum. And it turns out that when a neutron decays, it produces not only a proton, but also an electron and a particle called a neutrino.* This is the process known as beta decay, illustrated in Figure 51.
When beta decay was first observed, no one knew about the neutrino, which interacts only through the weak force and not through the electromagnetic force. Particle detectors can find only charged particles or those that deliver energy. Because the neutrino has no electric charge and does not decay, it was invisible to detectors and no one knew it existed.
Figure 51. In beta decay, a neutron decays via the weak force into a proton, an electron, and an antineutrino. A Feynman diagram representation of this process is shown on the right. A neutron turns into a proton and a virtual W- gauge boson, which then turns into an electron and an electron antineutrino.
But without the neutrino, beta decay looked as if it wouldn’t conserve energy. The conservation of energy is a fundamental principle in physics, and says that energy can be neither created nor destroyed—it can only be transferred from one place to another. The assumption that beta decay failed to conserve energy was outrageous, yet many respected physicists, unaware of the neutrino’s existence, were willing to make this radical (and erroneous) claim.
In 1930, Wolfgang Pauli paved the way to the doubters’ scientific salvation by proposing what he called “a desperate way out”: a new electrically neutral particle.* His idea was that the neutrino spirits away some energy when a neutron decays. Three years later, Enrico Fermi gave the “little” neutral particle, which he named the neutrino, a firm theoretical foundation. Yet the neutrino seemed such a shaky proposition at the time that the leading scientific journal Nature rejected Fermi’s paper because “it contained speculations too remote to be of interest to the reader.”
But Pauli’s and Fermi’s ideas were correct, and physicists today universally agree on the existence of the neutrino.† In fact, we now know that neutrinos constantly stream through us, released along with photons from the nuclear processes in the Sun. Trillions of solar neutrinos pass through you each second, but interact so weakly that you never notice. The only neutrinos that we know for sure exist are left-handed; right-handed neutrinos either don’t exist or are very heavy—too heavy to be produced—or interact very weakly. No matter which is true, right-handed neutrinos have never been produced at colliders, and we have never seen them. Because we are much more certain about left-handed neutrinos than right-handed ones, I’ve included only left-handed neutrinos in Figure 52, where I list left-and right-handed particles separately.
Figure 52. The three generations of the Standard Model. Left-and right-handed quarks and leptons are listed separately. Each column contains particles with the same charge (different flavors of the particle type). The weak force can change elements of the first column into elements of the second, and elements of the fifth column