Weak Nuclear Field
Shape-Shifter of Particles
The Force That Changes What Things Are
Gravity, electricity and the strong force all push or pull. The weak force does something none of them can: it turns one kind of matter particle into another, a down quark into an up quark or a muon into a neutrino. It is how every heavier kind of matter, from the muon to the top quark, ends up as the everyday kinds. Leave a neutron on its own and, after about fifteen minutes on average, it becomes a proton. Half of all free neutrons have changed in about ten minutes.
Inside, a neutron is built on three quarks, one up and two down. One of the down quarks turns into an up quark, and the three now make a proton. The change stirs the W field, one of the weak force’s fields, for an instant, and the stir at once becomes an electron and an antineutrino. The charges balance: the neutron had none, and the proton’s plus cancels the electron’s minus. The antineutrino has no charge and almost no mass, and it passes through nearly anything.
The antineutrino was guessed at long before anyone saw one. In this kind of decay the electron comes out with a spread of energies, up to what the change releases but nearly always less, and the spins before and after do not add up either. So physicists proposed that an unseen particle carries off the rest, and built a theory of the decay around it. It was caught decades later, beside a nuclear reactor. Inside a stable nucleus a neutron does not decay at all. The proton it would become has no low place left to sit, because the low places for protons are already taken, so the change would cost energy instead of releasing it.
Not Weak, Just Short
The weak force’s fields have three kinds of ripple: two W bosons, one with each sign of charge, and a neutral Z boson. Unlike the photon, the massless ripple of the electromagnetic field, they are heavy. A W weighs about 86 times as much as a proton, and a Z about 97 times. A field whose ripples are heavy dies away within a short distance of whatever stirs it, and the heavier the ripple, the shorter the distance. For the W it is about a seven-hundredth of a proton’s width. That is also how a W that weighs 86 times as much as a neutron can take part in its decay. The W there is not a particle set free but a brief stir in the W field, which hardly ever gets much farther than that before it becomes an electron and an antineutrino.
That short reach is the whole of its weakness. Close in, the weak force is no weaker than the electric force; in itself it is somewhat stronger. But two particles almost never come that close, so the weak force almost never gets to act. One collider showed this directly. When electrons hit protons hard enough to get within the weak force’s reach, the weak force scattered them about as often as the electric force did.
That likeness is no accident. The weak and electric forces are two sides of one electroweak force. The theory that joins them predicted something new: a neutral particle, the Z, whose field would let a neutrino bounce off matter without changing into anything. The W carries electric charge, so whatever stirs the W field must change its charge, and with it its kind; the Z carries none. A bubble chamber caught such bounces, and later the W and the Z themselves turned up. How strongly the two sides mix is a number the theory takes from measurement. Theories that join all the forces into one predict it, and some come close, but none is confirmed.
Why are the W and Z heavy when the photon is not? They feel the Higgs field, which fills all of space, and the photon does not. If universe was ever hotter than about two million billion degrees, as most models of its first moments assume, then in its first ten trillionths of a second the Higgs field had not yet settled. The W and Z got no mass from it, and electromagnetism and the weak force had not yet split apart. As universe cooled past that point, they smoothly parted.
The theory predicts the W’s mass from other measurements, so weighing the W tests it. A few years ago one measurement found the W heavier than predicted, by about seven times the uncertainty of the measurement and the prediction together. Three detectors at the Large Hadron Collider have also measured it, two of them since, and all three found the predicted mass. The latest combination of every result leaves that one number standing alone.
Left, Not Right
Particles carry spin, a built-in amount of turning about an axis, as real as a top’s, though nothing inside them goes round. A particle in flight can turn like a right-handed screw, its spin pointing the way it goes, or like a left-handed one, its spin pointing back. Physicists long took it for granted that the laws of physics have no favorite. Any process seen in a mirror, they assumed, is also a process the laws allow.
Yet no one had ever tested this for the weak force. Once that was noticed, the test took only months. Physicists cooled cobalt nuclei to a few thousandths of a degree above absolute zero, lined up their spins with a magnet, and counted the electrons their decay threw out. Most came out opposite to the spins. In a mirror they would come out along them, so the mirror image is a process the laws do not allow.
The W field, the part of the weak force that changes particles, acts only on left-handed particles and right-handed antiparticles. The neutrino feels nothing but the weak force and gravity, so it shows this most plainly. Its spin has been measured, from the light given off by the nucleus it leaves behind, and every neutrino caught so far is left-handed. Strictly, the W picks out a deeper property that matches the screw sense only for particles moving close to the speed of light, as every neutrino caught so far does. Because neutrinos have a little mass, a different neutrino, right-handed in that deeper sense, could also exist and simply ignore the weak force. Experiments have looked for one and have not found it.
Physicists then hoped that a mirror combined with a second swap, every particle for its antiparticle, would leave the laws unchanged. Particles called kaons break that too. Differences between matter and antimatter from the same source have since turned up in heavier particles, and recently in protons’ heavier cousins. It matters because universe is made of matter, with almost no antimatter left. But the difference the weak force makes is about ten billion times too small to explain that by itself.
Why the Sun Burns Slowly
The Sun shines by fusing hydrogen into helium. A hydrogen nucleus is a single proton, and helium holds two neutrons as well as two protons. So on the way, protons must turn into neutrons, and only the weak force can do that.
The first step is the hard one. In the core, at about 15 million degrees, protons rush about and their like charges push them apart. Now and then two slip through that push, a quantum effect called tunneling, and come within the strong force’s reach. But two protons cannot stay together, and they fly apart at once. They hold only if, in that brief meeting, one of them turns into a neutron, throwing off a positron, the electron’s antiparticle, and a neutrino. The pair is then a deuteron, the nucleus of heavy hydrogen. A neutron weighs more than a proton, so the change takes energy; it can happen only because the deuteron’s binding gives back more than the change costs.
That change is so unlikely that a proton in the Sun’s core waits billions of years for it. The next step needs no weak force: a deuteron catches another proton within seconds. Pair for pair, two protons fuse more than a hundred million billion times more slowly than a proton and a deuteron. This is why the core, volume for volume, gives off less energy than your body: a few hundred watts in each cubic meter, where a person at rest makes more than a thousand. The Sun is bright only because its core is vast. After 4.6 billion years, half the hydrogen at its very center is gone.
Every one of those changes sends a neutrino straight out of the Sun, a trip of about two seconds. The energy that becomes sunlight takes something like a hundred thousand years to work its way out, as light through the inner Sun and then in rising gas. About 60 billion neutrinos from the Sun pass through each square centimeter of you every second. A detector under a mountain in Italy has caught the neutrinos from this very first step.
Would a stronger or weaker weak force have made the Sun burn out fast, or never light? Less than it seems. A star sets its own pace. If its fusion runs fast, its core swells, cools, and slows the fusion down, and its brightness is set mainly by how quickly light leaks out. Models of universes whose weak force is a hundred times weaker, or far stronger, still make stars that burn for billions of years. One model went further: it took the weak force away entirely, and changed the early mix of matter and light so that deuterium survived the Big Bang. Its stars burned that deuterium and still shone for billions of years. Two doubts remain: exploding stars need neutrinos to blow themselves apart, and such universes might make little oxygen. No experiment can settle this; only better models of those other universes can.
