Warped Passages (27 page)

As with photon exchange, weak gauge boson exchange produces forces that can be attractive or repulsive, depending on the particles’
weak charges
. Weak charges are numbers that play the same role for the weak force that electric charge plays for the electromagnetic force. Only particles that carry weak charge experience the weak force, and their particular charge determines the strength and type of interactions they will experience.

However, there are several important distinctions between the electromagnetic force and the weak force. One of the most surprising is that the weak force distinguishes left from right, or, as physicists
would say,
violates parity symmetry
. Parity violation means that the mirror image of particles would behave differently to each other. The Chinese-American physicists C.N. Yang and T.D. Lee formulated the theory of parity violation in the 1950s, and another Chinese-American physicist, C.S. Wu, confirmed it experimentally in 1957. Yang and Lee received the Nobel Prize for Physics that year. Curiously, Wu, the only woman who played a role in the Standard Model developments I’m discussing, didn’t receive a Nobel prize for her momentous discovery.

Some violations of parity invariance should be familiar. For example, your heart is on the left side of your body. But if evolution had proceeded differently, and people had ended up with the heart on the right, you would expect that all its properties would be the same as the ones we now see. That the heart is on one side and not the other shouldn’t matter for any fundamental biological processes.

For many years prior to Wu’s 1957 measurement, it had been “obvious” that physical laws (though not necessarily physical objects) couldn’t have a preferred handedness. After all, why should they? Certainly gravity and electromagnetism and many other interactions make no such distinction. Nonetheless, the weak force, a fundamental force of nature, distinguishes left from right. Although it’s very surprising, the weak force violates parity symmetry.

How could a force prefer one handedness over the other? The answer lies in fermionic intrinsic spin. Just as a screw is threaded so that you screw it in by twisting it clockwise, but not counterclockwise, particles can also have a handedness, which indicates the direction in which they spin (see Figure 48). Many particles, such as the electron and the proton, can spin in one of two directions: either to the left or the right. The word
chirality
, derived from the Greek word
cheir
, which means hand, refers to the two possible directions of spin. Particles can be left-or right-handed, just like the fingers of your hands, one set of which curls to the left and the other set to the right.

The weak force violates parity symmetry by acting differently on left-handed and right-handed particles. It turns out that only left-handed particles experience the weak force. For example, a left-handed electron would experience the weak force, whereas one spinning to the right would not. Experiments show this clearly—
it’s the way the world works—but there is no intuitive, mechanical explanation for why this should be so.

Figure 48.
Quarks and leptons can be either right-or left-handed.

Imagine a force that could act on your left hand but not on your right! All I can say is that parity violation is a startling but well-measured property of weak interactions; it is one of the Standard Model’s most intriguing features. For example, the electrons that emerge when a neutron decays are always left-handed. Weak interactions violate parity symmetry, so when I list the full set of elementary particles and the forces that act on them (in Figure 52, Chapter 7) I’ll need to list separately the left-and right-handed particles.

The violation of parity symmetry, strange as it seems, is not the only novel property of the weak force. A second, equally important property is that the weak force can actually convert one particle type into another (while nonetheless preserving the total amount of electromagnetic charge). For example, when a neutron interacts with a weak gauge boson, a proton might emerge (see Figure 49). This is very different from a photon interaction, which would never change the net number of charged particles of any particular type (that is, the number of particles minus the number of antiparticles), such as the number of electrons minus the number of positrons. (For comparison,
a photon interacting with an electron that enters and emerges is illustrated in Figure 50, along with the schematic figure type we used before.) The interaction of a charged weak gauge boson with the neutron and the proton is what allows an isolated neutron to decay and turn into an entirely different particle.

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 into elements of the sixth. The quarks experience the strong force, whereas the leptons do not.

So we now know that weak interactions act only on left-handed particles, and can change particle type. But to truly understand the weak force we need a theory that predicts the interactions of the weak gauge bosons that communicate the force. Physicists initially found that constructing that theory was not simple. They needed to make a major theoretical advance before they could truly understand the weak force and its consequences.

The problem was the final bizarre feature of the weak force: it falls away precipitously over a very short distance, one ten thousand trillionth (10
^-16
) of a centimeter. That makes it quite unlike gravity and electromagnetism, for both of which, as we saw in Chapter 2, strength decreases with distance in proportion to the inverse square of the separation. Although gravity and electromagnetism become weaker as you go further out, they don’t drop off nearly as quickly as the weak force. The photon conveys the electromagnetic force to large distances. Why does the weak force behave so differently?