An Antimatter of Fact

When I asserted that everything in the universe consisted of seventeen fundamental particles, I wasn’t entirely telling the truth. The Standard Model only tells half the story, because, as I’m sure you’re aware, each particle also has a corresponding… antiparticle.

In 1928 Paul Dirac developed an equation that agreed with both quantum mechanics and special relativity, and could describe the motion of an electron at very high speeds, close to that of light. There was one issue, however; his equation gave two solutions for the energy of a particle – a positive value and a negative value. If this were a few decades earlier, there would be no guilt in discarding the negative value for energy since classical physics dictated that energy cannot be negative. But after the earlier developments in quantum mechanics, those rules kind of went out the window.

Dirac interpreted the second solution as the existence of ‘antielectrons’ which were identical to electrons in every way except for a few properties. Whereas an electron has a negative charge, this antielectron would have a positive charge, and thus the term ‘positron’ was coined. Other properties such as spin and quantum numbers are also opposite.

It turns out not only electrons have antiparticles, but also every other particle in the Standard Model. There are antiquarks – if you replaced each quark in a proton with its corresponding antiquark, you would have an antiproton. And if you took the antiparticle instead of each normal particle in an atom, you would get an antiatom.

In theory bosons also have antiparticles, but this is more controversial. For bosons without charge (photons and gluons and so on), they are considered to be antiparticles of themselves. The bosons with charge, W+ and W, are considered to be antiparticles of each other.

The concept of antimatter arises from the law of conservation of energy. According to Einstein’s acclaimed equation mc2, anything with mass can be considered to have a particular amount of energy. So for matter to exist, there has to also exist ‘negative matter’ which has negative energy, in order conserve energy.

This leads to the idea of pair production. When a single photon, which has no mass and is simply pure energy, is disturbed (for example by approaching a nucleus), it may convert into a particle with mass, such as an electron. However, conservation laws demand that it also produces a corresponding antiparticle. This means that mass can only be produced as a particle-antiparticle pair.

The reverse is also possible. A particle and its antiparticle can annihilate to produce pure energy (usually as high-energy photons). As a result energy, momentum, charge and a whole bunch of other things that need to be conserved are conserved. The energy released by matter-antimatter annihilations is ridiculous – one gram of antimatter has the same power as a nuclear bomb.

Luckily antimatter is also difficult to synthesise – all the particle accelerators in the world don’t have much more than tens of nanograms of antimatter. It is also claimed to be the costliest material to exist at $25 billion per gram of positrons.

The theory of the Big Bang insists that an equal amount of matter and antimatter were created during the birth of our universe. The obvious question looming over us then is: why do we detect an imbalance of matter over antimatter? This matter-antimatter asymmetry problem is one that CERN is investigating over in LHCb. It might just be because the missing antimatter happens to be far away from us. There is also a sneaky possibility that CP violations have something to do with it.

Yanhao

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