The Particle at the End of the Universe: How the Hunt for the Higgs Boson Leads Us to the Edge of a New World (7 page)

Cartoon image of an atom; in this case, helium. A nucleus consisting of two protons and two neutrons sit at the center, while two electrons “orbit” on the outskirts.

All that being granted, the basic cartoon we have in mind of what an atom looks like isn’t that bad, if what we’re looking for is some intuitive grasp of what’s going on. Nucleus in the middle, electrons on the outskirts. The electrons are relatively light; more than 99.9 percent of the mass of an atom is located in the nucleus. That nucleus is made of a combination of protons and neutrons. A neutron is a bit heavier than a proton—a neutron is about 1,842 times as heavy as an electron, while a proton is about 1,836 times as heavy. Protons and neutrons are both called “nucleons,” as they are the particles that make up nuclei (plural of “nucleus”). Aside from the fact that the proton has an electric charge and the neutron is a bit heavier, the two nucleons are remarkably similar particles.

Like many things in life, the nature of an atom is one of exquisite balance. The electrons are attracted to the nucleus by the force of electromagnetism, which is enormously stronger than the force of gravity. The electromagnetic attraction between an electron and a proton is about 10
³⁹
times stronger than the gravitational attraction between them. But while gravity is simple—everything attracts everything else—electromagnetism is more subtle. Neutrons get their name from the fact that they are neutral, having no electric charge at all. So the electromagnetic force between an electron and a neutron is zero.

Particles with the same kind of electric charge repel one another, while opposites live up to the romantic cliché and attract. Electrons are attracted to the protons inside a nucleus because electrons carry a negative charge and protons carry a positive one. So—you may be asking yourself—why don’t the protons packed so closely inside a nucleus push one another apart? The answer is that their mutual electromagnetic repulsion does indeed push them apart, but it is overwhelmed by the strong nuclear force. Electrons don’t feel the strong force (just like neutrons don’t feel electromagnetism), but protons and neutrons do, which is why they can combine to make atomic nuclei. Only up to a point, however. If the nucleus gets too big, the electric repulsion just becomes too much to take, and the nucleus becomes radioactive; it may survive for a while, but eventually it will decay into smaller nuclei.

Antimatter

Everything you see around you right now, and everything you have ever seen with your own eyes, and everything you have ever heard with your ears and experienced with any of your senses, is some combination of electrons, protons, and neutrons, along with the three forces of gravity, electromagnetism, and the nuclear force that holds protons and neutrons together. The story of electrons, protons, and neutrons had come together by the early 1930s. At that time, it must have been irresistible to imagine that these three fermions were really the fundamental ingredients of the universe, the basic Lego blocks out of which everything is constructed. But nature had some more twists in store.

The first person to understand the basic way fermions work was British physicist Paul Dirac, who in the late 1920s wrote down an equation describing the electron. An immediate consequence of the Dirac equation, although one that took physicists a long time to accept, is that every fermion is associated with an opposite type of particle, called its “antiparticle.” The antimatter particles have exactly the same mass as their matter counterparts, but an opposite electric charge. When a particle and an antiparticle come together, they typically annihilate into energetic radiation. A collection of antimatter is therefore a great way (in theory) to store energy, and has fueled much speculation about advanced rocket propulsion in science-fiction stories.

Dirac’s theory became a reality in 1932, when American physicist Carl Anderson discovered the positron, the antiparticle of the electron. There is a tight symmetry between matter and antimatter; a person made of antimatter would undoubtedly call the particles of which they were made “matter,” and accuse us of being made of antimatter. Nevertheless, the universe we observe is full of matter and contains very little antimatter. Exactly why that should be so remains a mystery to physicists, although we have a number of promising ideas.

Anderson was studying cosmic rays, high-energy particles from space that crash into the earth’s atmosphere, producing other particles that eventually reach us on the ground. It’s like you’re using the air above you as a giant particle detector.

To create images of the tracks of charged particles, Anderson used an amazing technology known as the “cloud chamber.” It’s an apt name, as the basic principle is similar to that of the actual clouds we see in the sky. You fill a chamber with gas that is supersaturated with water vapor. “Supersaturated” means that the water vapor really wants to form into droplets of liquid water, but it won’t do it without some external nudge. In a regular cloud, the nudge typically comes in the form of some speck of impurity, such as dust or salt. In a physicist’s cloud chamber, the nudge comes when a charged particle passes through. The particle bumps into the atoms inside the chamber, shaking loose electrons and creating ions. Those ions serve as nucleation sites for tiny droplets of water. So a passing charged particle will leave a trail of droplets in its wake, much like the contrail created by an airplane, lingering evidence of its passage.

Anderson took his cloud chamber, wrapped in a powerful magnet, up to the roof of the aeronautics building at the California Institute of Technology, or Caltech, and watched for cosmic rays. Obtaining the properly supersaturated vapor inside required a rapid decrease in pressure, caused by a piston that would cause a loud bang each time it was released. The chamber was only operated at night due to its massive electricity consumption. Bangs would reverberate through the Pasadena air every evening, noisy testimony that secrets of the universe were being discovered.

The pictures Anderson took showed an equal number of particles curving clockwise and counterclockwise. The obvious explanation was that there were just as many protons as electrons contained in the radiation; indeed, you might expect exactly that, since negatively charged particles can’t be created without also creating a balancing positive charge. But Anderson had another piece of data he could use: the thickness of the ion trail left in his cloud chamber. He recognized that, given the curvature of the tracks, any protons that would produce them would have to be relatively slow-moving. (In this context, that means “slower than 95 percent the speed of light.”) In that case, they would leave thicker ion trails than what was observed. It seemed that the mysterious particles passing through the chamber were positively charged, like a proton, but relatively light, like an electron.

There was one other logical possibility: Maybe the tracks were simply electrons moving backward. To test this idea, Anderson introduced a plate of lead bisecting the chamber. A particle moving from one side of the lead to the other would slow down just a bit, clearly indicating the direction of its trajectory. In an iconic image from the history of particle physics, we see a counterclockwise-curving particle moving through the cloud chamber, passing through the lead, and slowing down afterward—the discovery of the positron. Giants of the field, such as Ernest Rutherford, Wolfgang Pauli, and Niels Bohr, were incredulous at first, but a beautiful experiment will always win out over theoretical intuition, no matter how brilliant. The idea of antimatter had entered the world of particle physics for good.

The cloud-chamber image from the discovery of the positron by Carl Anderson. The path of the positron is the curved line that starts near the bottom, hits the lead plate in the middle, and curves more sharply as it continues toward the top.

Neutrinos

So instead of just three fermions (proton, neutron, electron), we have three more (antiproton, antineutron, positron) for a total of six—still fairly parsimonious. But nagging problems remained. For example, when neutrons decay, they turn into protons by emitting electrons. Careful measurements of this process seemed to indicate that energy was not conserved—the total energy of the proton and electron was always a bit less than that of the neutron from which they came.

The answer to this puzzle was suggested in 1930 by Wolfgang Pauli, who realized that the extra energy could be carried off by a tiny neutral particle that was hard to detect. He called his idea the “neutron,” but that was before the name was attached to the heavy neutral particle we find in nuclei. After that happened, to stave off confusion Enrico Fermi dubbed Pauli’s particle the “neutrino,” from the Italian for “little neutral one.”

In fact the decay of a neutron emits what we now recognize as an antineutrino, but the principle was absolutely right. Pauli was quite embarrassed at the time for suggesting a particle that didn’t seem detectable, but these days neutrinos are bread and butter for particle physicists (as is proposing hard-to-observe hypothetical particles).

There was still the question of the exact process by which neutrons decay. When particles interact with one another, that implies some kind of force, but the decay of a neutron wasn’t what we would expect from gravity, electromagnetism, or the nuclear force. So physicists started attributing neutron decay to the “weak nuclear force,” because it obviously had something to do with nucleons but also obviously wasn’t the force holding nuclei together, which was dubbed the “strong nuclear force.”

Decay of the neutron into a proton, electron, and antineutrino.

The existence of the neutrino established a nice little symmetry among the elementary particles. There were two light particles, the electron and neutrino, which were eventually dubbed “leptons” from the ancient Greek word for “small.” And there were two heavy particles, the proton and neutron, which were (somewhat later on) dubbed “hadrons” from the ancient Greek for “large.” The hadrons feel the strong nuclear force, while the leptons do not. Each category contained one charged particle and one neutral one. You could be forgiven for thinking that we had it nailed down.

Generations

Then in 1936, a visitor dropped in from the sky—the muon. Carl Anderson, discoverer of the positron, and Seth Neddermeyer were again studying cosmic rays. They found a particle that is negatively charged like the electron but heavier, although lighter than an antiproton would be. It was dubbed the “mu meson,” but physicists later realized that it wasn’t a meson (which is a boson made of a quark and an antiquark) at all, so the name was shorted to “muon.” For a time in the 1930s, fully half of the known elementary particles (electron, positron, proton, neutron, muon, and antimuon) had been discovered in Carl Anderson’s lab at Caltech. Who knows? Maybe a decade or two from now, half of the by-then-known elementary particles will have been discovered at the LHC.