Mass from the strong force

Quite some time ago, I have discussed the Higgs effect, and how it gives the matter particles in the standard model their mass. However, if we look around us, it turns out that most mass we see is actually not due to the Higgs effect.

If we knock on a table, or look at us, then most of this is made up out of atoms. As you might remember, atoms are made up out of nuclei and electrons. The electrons actually get their mass from the Higgs, so that is alright. But they make up less than 0.05% of the mass of the atoms. Thus, one can forget about them for this purpose. Then there are the nuclei. They are made up out of protons and neutrons. These in turn consist out of quarks. But the quarks are rather light, and make up not more than one percent of the mass of the protons and neutrons, and thus of the nuclei. So where does all the remaining mass comes from?

Well, this comes this time from the strong nuclear force, QCD, which has been presented here and here. I have already indicated there that it is a pretty strong force. It is this strength which, indirectly, creates all the mass we are yet missing.

How it works is actually quite complicated in detail, but when being a bit fuzzy about the details, it can be illustrated quite nicely. Looking through such fuzzy glasses, it actually looks like a repetition of the Higgs effect. Remember, the Higgs effect worked by letting the Higgs particles condense. The interaction with this condensate slows particle down, and therefore they behave as having a mass.

Now, how does this proceed in the case of the strong force? The first observation is that the strong force is attractive between quarks, i.e. quarks are attracted to each other. As a consequence, the quarks can form all these nice things like protons. However the force is also attractive between the quarks and their so-called anti-particles. What an anti-particle is I will discuss next time. This time, it is just sufficient to say that it behaves like a quark, but has opposite charges and the same mass.

The strong force now pulls also the quarks and the antiquarks together. The combination of two such particles is then neutral, as all the charges are opposite. It behaves therefore very similar to a Higgs particle. In very much the same way, but this time because of gluons rather than due to the Higgs interacting with itself, this creates also a condensate. That is just like for the Higgs particles. Thus, the universe is filled with quarks and anti-quarks, bound together, and condensed by the strong force. The strong interaction of quarks with this condensate, and the corresponding slowing down, is what provides the remainder mass for the protons and neutrons, and thus for the nuclei. Again, because all the charges cancel, we do not see this condensate, as photons due not directly interact with it.

Putting in numbers, the contribution of the strong force to the mass of the nuclei is much larger than the one due to the Higgs effect. However, the calculation of this was rather challenging. Hence, most of the mass stored in the atoms in the universe is due to the strong force.

It is also said that the strong force provides all the luminous mass in the universe. Here, luminous means actually not only all stars which emit light by themselves, but also everything which reflects light, like planets, interstellar gas clouds, and asteroids. Actually, the latter also emit a kind of light by themselves, but our eyes are not sensitive for the wave-lengths they emit, and therefore we do not see it. The distinction is necessary, because we have indirect evidence that there is also more than just this type of mass in the universe. In fact, we expect that there is about five times more non-luminous, or dark, matter in the universe, than luminous matter. What the origin of mass is in this case is not known.

There is another thing you may wonder about. The quarks have all very different masses due to the Higgs effect. Is the contribution due to the strong interaction also very different for the different quarks? The answer to this is actually no, the contribution from the strong force is about the same for all quarks. Thus, it makes up about 99% of the mass of the light quarks, but less than half a percent for the heaviest one. Thus, while the Higgs makes a difference between the different quark (and lepton) species, the strong force does not. Why this is the case is also yet unknown, and one of the bigger mysteries. Since the different quarks and leptons are also called different flavors of quarks and leptons, it is said that the strong force is flavor-blind, it makes no difference between different flavors. On the other hand, the Higgs makes a different between different flavors.

Finally, it should be noted that the generation of mass can be traced back to a symmetry, though I will not detail this now. This is the so-called chiral symmetry. A thing is called chiral, if it makes a difference between left and right. The associated symmetry in the standard model is a local symmetry. The Higgs effect breaks, loosely spoken, this symmetry to a global one. The strong force then breaks this global symmetry. It is possible, but mathematically involved, to show that these breakings correspond to the existence of mass. Hence, both the Higgs effect and the strong force produce mass. But the above explanations are somewhat more illuminating, I think, though mathematically both views are essentially equivalent. Thus, it is said that mass is created by chiral symmetry breaking, a notion I will right now not dwell on anymore.