Atom Land by Jon Butterworth

Author:Jon Butterworth
Language: eng
Format: epub
Publisher: The Experiment

20

Parity, Helicity, and Chirality

The process by which a neutron converts into a proton is called “beta decay.” It was in beta decay that the weak force sprang its biggest surprise.

The weak force interacts with all the quarks and leptons, but at the same time it only interacts with half of them, in a sense. Put another way, every city on the Isle of Leptons and the Isle of Quarks has an airport, but only half the inhabitants ever fly. Perhaps they are frightened or worried about climate change. Or maybe airport security is very picky. Whatever the reason, it is a puzzling fact. To work out what it means, we need to understand a concept known as chirality.

A chiral object is one that is not identical to its mirror image. It is also sometimes called “handedness.” Some molecules are chiral, having a right-handed version and a left-handed version. This is true of many biological molecules; DNA is chiral, for example: The twist in the double helix goes in a particular direction. So are sugars. Usually only one of the two possible chiralities is actually usable by living organisms.

The particles of our map, too, are chiral, and only one of those chiralities is affected by the weak force.

Spinning particles can be divided into two classes based on whether their spin is pointing in the direction the particle is traveling, or opposite to it. This is called the helicity.40 These are the only two possibilities for the helicity of a quantum particle with a spin of half a unit, such as the quarks and leptons of the Standard Model. If you imagine the particle coming toward you, you can think of the two helicities as corresponding to seeing the spin turning clockwise or counterclockwise, just as the twist of the DNA helix goes in one direction or the other. For a massless particle, these two helicities define the two possible chiralities of the particle.

This is strange behavior. The fact that there are two chiralities is not so surprising. Since we know that most particles have spin, there will be a handedness associated with the sense in which they spin, clockwise or counterclockwise, similar to the handedness of the twist in the double helix of DNA. What is surprising is that the W and Z bosons, which carry the weak force, “see” only one of those chiralities. A massless particle with its spin pointing in the opposite direction to the particle’s direction of motion will feel the weak force; a massless particle that has its spin pointing in the same direction as its motion will be invisible to the weak force. The inverse is true for antiparticles: Those with spin pointing in the same direction as their motion feel the force; those with it pointing against do not.

The fact that in nature, plants and animals produce, and make use of, only one chirality of some molecules is presumably due to random chance—some common ancestor happened to make and use one of them, and the success of that ancestor has meant that the original choice has been “locked in” ever since.

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