# How does quark color affect the identity of a hadron?

I've read about colors relating to quarks and hadrons and I know that they can change colors because of the exchange with gluons, but does changing color change the type of hadron? Does a proton become a neutron because the quark color changes?

• A proton and a neutron differ by more than quark color (as any look on their Wikipedia articles should tell you). I'm not sure what the question is here. Jun 11, 2016 at 10:04
• Well, all hadrons and mesons are color-singlets. Jun 11, 2016 at 22:13

Hadrons come in 2 families: baryons and mesons. Both of them consist from colourless combinations of quarks. Mesons contain pairs of quarks of colour-anticolour and hadrons contain 3 quarks of different colours making them white in analogy with regular colour perception.

You are right that quark can change its colour by interaction with gluons, but the colour is conserved — it is just that gluons carry a pair of different colour-anticolours such that colour lines are always unbroken.

Although this seems cumbersome, this picture is well motivated by the group symmetry found in hadrons. Additionally, it explains why only baryons and mesons are observed by themselves and not quarks and gluons (which are not colourless or white).

Best illustration I've seen so far is here:

(from https://physics.stackexchange.com/a/2237/119172, look there for more technical explanations)

Be, however, cautious as in this picture gluons actually have colour and anticolour, but to decide which is which, you need to assign it a direction of movement.

Here it is better seen, as times goes from left to right:

(could not find the source)

• Thanks for the explanation, but what exactly do you mean when you say "The color lines are always unbroken"? Jun 11, 2016 at 16:11
• Colour as many other charges has to be conserved - it cannot disappear or appear from the vacuum. Diagrammatically we describe this by continuous lines of charge flow. You can see on the diagrams that each colour line is either continuous or ends in a point where it meets respective anticolour and they disappear. This is similar to electrodynamics where electron emits chargeless photon and has continuous line or meets positron and their lines end, again emitting photons. In this case we just have 3 colours that also can "interact" in a sense that white combinations have specific properties Jun 11, 2016 at 16:35

No, not at all! The color of the quarks has no effect whatsoever.

If you're studied intro physics, you know that a potential $V(x)$ is identical in every way to a potential $V(x) + V_0$ for some constant $V_0$.

Now consider two hydrogen atoms, where I've set the potential at infinity to be $3 \text{ V}$ for one of them and $4 \text{ V}$ for the other. Then their potentials are slightly different, mathematically, but in every conceivable physical way the two systems are identical. We just have a redundancy in our description of the system.

This redundancy is called a gauge symmetry, and the color of quarks is built on a more complicated gauge symmetry called $SU(3)$. However, the same point stands: picking out "the" colors of the quarks in a hadron is as meaningless as pinning down the potential for a hydrogen atom. Specific colors exist in the math, but not in reality.

• (For those keeping score, yes, I lied a little.) Jun 11, 2016 at 17:52
• I believe this is an oversimplification. Colour is a quantum number that lifts the problem of Pauli exclusion of quarks in hadrons. It also gives a nice and intuitive explanation of what can be free and what is confined. I think the question was precisely in misunderstanding what quarks are and how strong interaction operates, not about gauge symmetry. Jun 13, 2016 at 15:47

I think the above picture with captioning below answers the question best, at least for me. Basically the green quark emits a green-antiblue gluon, turning it blue. This gluon is absorbed by the blue gluon, and it changes from blue to green, restoring the color symmetry and keeping the Baryon overall colorless. And it happens so fast that no overall Baryon color can be observed.

Source: "Gauge Theories of the Forces Between Elementary Particles", Gerard 't Hooft, Scientific American, June 1980