“In any case, the number three fitted perfectly the way quarks occur in nature.” –Murray Gell-Mann
You might think that we know it all, at least as far as knowing-it-all is possible. After all, we know that matter is made up of atoms, which are made up of electrons and nuclei, and the nuclei are made up of protons and neutrons, and then the protons and neutrons are made up of quarks and gluons.
Along with the electrons, the quarks and gluons are — as far as we know — indivisible, which places them among the fundamental particles of the Universe.
Every proton and neutron is an example of a more general type of particle known as a baryon, which is a particle made up of three quarks, as well as the gluons that hold them together. Each quark has two types of charge: an electric charge, just like an electron has, as well as a color charge!
Unlike the electric charge, which is fixed for particles and has its force carried by a chargeless particle (the photon), a quark always carries a color — either red, green or blue — but that color always changes over time, since the gluons that carry the force are also colored!
Last year, I wrote up a brief introduction called The Strong Force For Beginners, which I encourage you to look at if you want more details about how this works. With six quarks and eight different gluons mediating the strong force, the number of different baryons, or stable/quasi-stable combinations of three quarks, are tremendous. The key is that, to exist, the combinations of quarks needs to be completely colorless when taken all together.
But baryons are not the only possibility for satisfying this.
Each quark has a color charge, and each anti-quark has an anti-color charge, but these are not independent! For example:
- Anti-red is the same as blue+green, since red+anti-red or red+blue+green both = white.
- Anti-blue is the same as red+green, since blue+anti-blue or blue+red+green both = white.
- Anti-green is the same as blue+red, since green+anti-green or green+blue+red both = white.
So you can have a quark-antiquark combination, which is known as a meson. Or you could have three quarks, a baryon, or three antiquarks, an anti-baryon.
All of these particles, with the sole exception of the proton and antiproton, are unstable, and will decay over time.
If a particle is unstable under the strong interactions, it will decay the most quickly, as the strong interaction is (duh) the strongest! A particle that undergoes a strong decay lives only some ~10-22 to 10-24 seconds.
If a particle is stable to the strong interactions but unstable to the electromagnetic interaction, it decays very quickly, but not quite as quickly as the strong interactions. A particle (like the neutral pion) that undergoes electromagnetic decay lives ~10-17 to 10-21 seconds.
And if a particle is both stable to the strong and electromagnetic interactions but not to the weak interactions — which pretty means it needs to change its quark-type — it lives the longest: ~10-8 to 10-13 seconds. (The lone exception is the free neutron, which for a variety of reasons lives about 10 minutes.)
But we can make other combinations of quarks and gluons than just mesons, baryons and antibaryons that are allowed by our governing theory: quantum chromodynamics, or QCD.
While they’ll be unstable to strong decays, and hence will have incredibly short lifetimes, we can theoretically also have combinations such as tetraquarks (two quarks and two antiquarks), pentaquarks (four quarks and one antiquark, or four antiquarks and one quark), glueballs (bound states of gluons-only), or hybrid particles (a quasi-stable quark configuration with one or more extra gluons inside).
For a long time, these were only theoretical predictions of our theory of the strong interactions, but about a decade ago, claims started rolling in from particle accelerators that a pentaquark state had been discovered!
Now, it turns out that — like many borderline discoveries in particle physics — this one went away with more data, although there is at least one group that still claims to have found a pentaquark. (If you remember how long it took for physicists to confidently announce the discovery of the Higgs Boson, it’s because they waited until they had enough data so that they could make sure they weren’t announcing something that could possibly have gone away!)
And yet, states like tetraquark, pentaquarks and glueballs must exist if QCD is correct, so long as there are at least three colors.
In a surprising announcement earlier this week, two independent teams have just found overwhelming evidence for a tetraquark state: the Zc at a mass/energy of 3900 MeV! [Known, at least for now, as the Zc(3900).] Made up of two quarks, an up and a charm, and two anti-quarks, an anti-down and an anti-charm, this is the first confirmed particle made up of quarks-and-gluons that doesn’t fit into our standard picture of either meson, baryon, anti-baryon, or a multi-baryon combination (which is what atomic nuclei are).
Yes, it’s true that with a lifetime of less than 10-20 seconds, it’s not like these particles have much effect on the Universe today. But back when the Universe was very young — less than 100 picoseconds after the Big Bang — these particles were just as abundant as any baryon or meson that existed, and provide the first real confirmation of one of the most novel predictions of our theory of the strong force! Here’s hoping it continues to hold up, and that many more of these exotic particles await us in the future. (I’m particularly excited for the first glueball.) The search, and the journey, continues.