LEGO^® Antiquarks and Mesons

Antiquarks and Anti-colour

This page is an excerpt of ideas used in the book Particle Physics Brick by Brick.

Particles made from quarks must be strong charge neutral, this can be achieved if each quark is colour charged with a primary colour of light (red, green, and blue) so that the overall colour charge of the particle is white. There is another way to build particles with a neutral, white colour, overall strong charge but for this we must talk about antiparticles. The three generations of fundamental particle also have mirror versions of themselves; the antiparticles. When you look into a mirror left becomes right but you still look the same size. A similar thing is true in the particle world – mirror antiparticle versions of particles have the same mass but they see the world in opposite ways. They see the world differently in the way they feel and interact through the forces of nature. We say an electron particle has a negative electric charge, then its antimatter version, the positron, will have a positive electric charge. The anti-electron (positron) was first seen in experiment in 1932 (the same year the neutron was discovered), and since then it has been confirmed that antiparticles do indeed exist for all of the three generations of particle.

Antiquarks, the antimatter versions of the quark, also have their electric charges mirrored from positive to negative. As Antiquarks also feel the strong force they must also have their strong colour charges mirrored too – but what is an anti-colour? It turns out that the secondary colours of light can be thought of as anti-colours. Cyan has no red if split by a prism, only green and blue, so is therefore anti-red. Yellow would not contain blue in its spectrum, just red and green, so is therefore anti-blue. Magenta contains no green, only blue and red, and can therefore be thought of a anti-green. We could then say that the opposite, antiquark versions, of the red, green, and blue strong charges of quarks would be either cyan, magenta, or yellow.

Mesons: Quark-Antiquark Particles

Now what happens if we combine a quark with an anti quark? Magenta is made from red and blue, add green to it and you would have white light; yellow made from red and green, add blue and you get white light; cyan is made from blue and green, add red to it and you would get white light. So to create white, strong charge neutral, particles with quarks and antiquarks you would only need to have one quark and one antiquark. A green quark and a magenta antiquark; a red quark and a cyan antiquark; a blue quark and a yellow antiquark would all make particles. These quark-antiquark combinations are a group of particles called Mesons.

Just like the Baryons there is a pattern that Gell-Mann theorised in his Eightfold Way for the possible Meson particles that can be made from up, down and strange quarks; the Meson Octet (below). Mesons do not survive very long because particles and antiparticles are not very stable around one another. Generally when a particle meets its own antiparticle they annihilate one another to produce pure energy. Mesons, as they are constructed by quark and antiquark, use the first opportunity available to either form pure energy or a number of lighter particles. The middle row of the Meson Octet are particles called pions (π) which play a role in keeping protons and neutrons together in the nucleus but also in the production of neutrino particle beams.

 

As we did with the Baryons we can extend this model to include the four lightest quarks to create a Meson multiplet

 

For more on this subject read my blog post covering this subject.

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