The Particle at the End of the Universe: How the Hunt for the Higgs Boson Leads Us to the Edge of a New World by Sean Carroll Page B
Authors: Sean Carroll
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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
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
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