Warped Passages - Lisa Randall [81]
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),