But here’s the kicker: so long as you make a combination that’s color-neutral, it ought to be able to stably exist (at least, temporarily) in this Universe. You can make something color neutral either with a combination of a color charge and its anti-color charge (like a quark-antiquark pair), or a combination of three colors (or three anti-colors), like a proton, which is made up of three quarks.

We call this color-neutral combination “white,” and so long as something’s white, it can exist if the other conditions are right in nature. In all cases, these quarks (or antiquarks) change their individual colors over time by the emission and absorption of (colored) gluons, but the total combination always remains color-neutral.

Image credit: Brooks/Cole - Thomson, via http://slideplayer.com/slide/2812151/.

For the quark-antiquark combinations, those are known as mesons. If you have only two quarks available (such as up and down), you have limited combinations of the particles you can make, depending on how other quantum properties (such as spin) are available for configuration. If you have more quarks (strange, strange and charm, etc.), you can make more combinations. What you wind up with is an entire spectrum of possible particles, with everything predicted so far — within the reach of experiment — having been successfully confirmed.

Image credit: Fermi National Accelerator Laboratory, via http://www.fnal.gov/pub/presspass/images/sigma-b-baryon-images.html.

For the three quark (or three antiquark) combinations, you can create baryons (or anti-baryons). Again, as you go to higher and higher energies, and incorporate not only up and down quarks, but also strange, charm, and bottom (and so on) quarks into the mix, you wind up predicting an entire spectrum of baryons. And as with the mesons, the better our experimental detectors (and collider energies) have gotten, the more of these particles we’ve discovered.

But as you may have already figured out, quark-antiquark pairs and combinations of three quarks (or antiquarks) aren’t the only stable possibilities out there.

Image credit: Julich, via http://www.fz-juelich.de/SharedDocs/Bilder/PORTAL/DE/pressebilder/PM2014/14-05-23-infografik_dibaryon_b_EN.jpg?__blob=poster.

For example, here are some colorless objects of interest:

You could have two quarks and two antiquarks: a tetraquark state.

You could have four quarks and an antiquark: a pentaquark state.

You could have six quarks (or three quarks and three antiquarks) all bound in a single object: a hexaquark state.

Or you could even have a quasi-stable configuration make exclusively of gluons, all adding up to a colorless combination: a glueball.

Image credit: K. Peters, via http://slideplayer.com/slide/3387472/.

For a long time, these objects were theoretical only. And yet, the theory of the strong interactions — Quantum Chromodynamics (QCD) — demands that they must exist. If they don’t then QCD is wrong!

Pentaquarks were first claimed discovered back in the mid-2000s, a discovery which turned out to be spurious. But over the past few years, the first tetraquarks were discovered, and just this past week, the first verified pentaquark state was announced.