Knocking on Heaven's Door - Lisa Randall [53]
The British physicist Paul Dirac first “discovered” antimatter mathematically in 1927 when he tried to find the equation that describes the electron. The only equation he could write down consistent with known symmetry principles implied the existence of a particle with the same mass and opposite charge—a particle that no one had ever seen before.
Dirac racked his brain before capitulating to the equation and admitting this mysterious particle had to exist. The American physicist Carl Anderson discovered the positron in 1932, verifying Dirac’s assertion that “The equation was smarter than I was.” Antiprotons, which are significantly heavier, were not discovered until more than twenty years later.
The discovery of antiprotons was important not only for establishing their existence, but also for demonstrating a matter-antimatter symmetry in the laws of physics essential to the workings of the universe. The world is, after all, made of matter, not antimatter. Most of the mass of ordinary matter is carried by protons and neutrons, not by their antiparticles. This asymmetry in matter and antimatter is critical to the world as we know it. Yet we don’t yet know how it arose.
DISCOVERY OF QUARKS
Between 1967 and 1973, Jerome Friedman, Henry Kendall, and Richard Taylor led a series of experiments that established the existence of quarks inside protons and neutrons. They did their work at a linear accelerator, which—unlike the circular cyclotrons and Bevatrons before it—accelerated electrons along a straight line. The accelerator center was named SLAC, the Stanford Linear Accelerator Center, located in Palo Alto. The electrons that SLAC accelerated radiated photons. These energetic—and hence short-wavelength—photons interacted with quarks inside the nuclei. Friedman, Kendall, and Taylor measured the change in interaction rate as the energy of the collision increased. Without structure, the rate would have gone down. With structure, the rate still decreased, but much more slowly. As with Rutherford’s discovery of the nucleus many years before, the projectile (the photon in this case) scattered differently than if the proton was a blob that lacked structure.
Nonetheless, even with experiments performed at the requisite energy, identifying quarks wasn’t entirely straightforward. Technology and theory both had to progress to the point that the experimental signatures could be anticipated and understood. Insightful experiments and theoretical analyses performed by the theoretical physicists James Bjorken and Richard Feynman showed that the rates agreed with the predictions based on structure inside the nucleus, thereby demonstrating that structure in protons and neutrons—namely, quarks—had been discovered. Friedman, Kendall, and Taylor were awarded the Nobel Prize in 1990 for their discovery.
No one could have hoped to use their eyes to directly observe a quark or its properties. The methods were necessarily indirect. Nonetheless, measurements confirmed quarks’ existence. Agreement between predictions and measured properties, as well as the explanatory nature of the quark hypothesis in the first place, established their existence.
Physicists and engineers have over time developed different and better types of accelerators that operated on increasingly larger scales, accelerating particles to ever higher energy. Bigger and better accelerators produced increasingly energetic particles that were used to probe structure at smaller and smaller distances. The discoveries they made established the Standard Model, as each of its elements was discovered.
FIXED-TARGET EXPERIMENTS VERSUS PARTICLE COLLIDERS
The type of experiment that discovered quarks, in which a beam of accelerated electrons was aimed at stationary matter, is known as a fixed-target experiment. It involves a single beam of electrons that is directed toward matter. The matter target is a sitting duck.
The current highest-energy accelerators are different. They involve