Fifty years ago, in 1964, physicists Murray Gell-Mann at Caltech and George Zweig at CERN came up with the idea of the quark as a response to the bewildering number of elementary particles that were being discovered at the huge "atom smasher" labs sprouting up all over the world. Basically, instead of the dozens of heavy particles like protons and neutrons, or medium-weight particles like pi mesons, you only needed three kinds of elementary quarks, called "up," "down" and "strange." Combining these in threes, you get the heavy particles. Combining them in twos, with one quark and one anti-quark, you get medium-weight mesons. This early idea was extended to include three more types of quarks, dubbed "charmed," "top" and "bottom" (or on the other side of the pond, "charmed," "truth" and "beauty"). These six quarks form three generations -- (U, D), (S, C), (T, B) -- in what physicists call the Standard Model of particle physics. Also included are the eight gluon particles that transmit the strong nuclear force between quarks and cause quarks always to be bound together and never found isolated in nature.
Particle tracks at CERN/CMS experiment (credit: CERN/CMS)
At first the quark model (or parton model, if you were an experimenter) only accounted for the then-known particles. A proton would consist of two up quarks and one down quark (U, U, D), and a neutron would be (D, D, U). A pi-plus meson would be (U, anti-D), and a pi-minus meson would be (D, anti-U), and so on. It's a bit confusing to combine quarks and anti-quarks in all the possible combinations, but when you do this in twos and threes for U, D and S quarks, you get the entire family of the nine known mesons, which forms one geometric pattern in the figure below, called the Meson Nonet.
The basic Meson Nonet (credit: Wikimedia Commons)
If you take the three quarks U, D and S and combine them in all possible unique threes, you get two patterns of particles shown below: the Baryon Octet (left) and the Baryon Decuplet (right).
Normal baryons made from three-quark triplets
The problem was that before 1964, there was a single missing particle in the simple geometric patterns. The Omega-minus at the bottom of the Baryon Decuplet was nowhere to be found. This slot was empty until Brookhaven National Laboratory discovered it in early 1964. It was the first indication that the quark model was on the right track and could predict something that no one had seen before. Once the other three quarks (C, T and B) were discovered in 1970, 1974 and 1977, respectively, it was clear that there were many more slots to fill in the geometric patterns that emerged from a six-quark system. The first particles predicted and then discovered in these patterns were the J/Psi "charmonium" meson (C, anti-C) in 1974, and the Upsilon "bottomonium" meson (B, anti-B) in 1977. Apparently there are no possible top mesons because the top quark decays so quickly that it is gone before it can bind together with an anti-top quark to make even the lightest stable toponium meson!
The number of possible particles that result by simply combining the six quarks and six anti-quarks in patterns of twos (mesons) is exactly 18 charmonium mesons and 21 bottomonium mesons. Of these, only 11 of the possible charmonium mesons and 15 of the bottomonium mesons have been detected as of 2014. These particles have masses between 3.6 and 4.2 GeV for the charmonium particles, and 9.5 to 10.2 GeV for the bottomonium mesons. (Masses are actually given in GeV/c2, but it is convenient to set c=1 for printing! By the way, a proton has a mass of 0.9 GeV.)
For the still-heavier three-quark baryons, the quark patterns predict 40 baryons containing combinations of all six quarks. Of these, the proton and neutron are the least massive! But there are 16 combinations that have not been detected yet. These include the lightest missing particle, the bottom Sigma (U, D, B) with a mass of about 5.8 GeV, and the most massive particle, called the charmed double-bottom Omega (C, B, B) with a mass higher than 6 GeV.
There is a second family of three-quark baryons that include 35 particles with masses between 1.2 GeV and more than 5.8 GeV. Of these possibilities, 18 remain to be found, including the lightweight bottom Sigma (U, D, B) with a mass close to 5.8 GeV, and the heavyweight triple-bottom Omega (B, B, B) with a mass greater than about 6 GeV. This week CERN/LHC announced the discovery of two of these missing particles, called the bottom Xi baryons (B, S, D), with masses near 5.8 GeV.
Other combinations of more than three quarks are also possible. A pentaquark baryon particle can contain four quarks and one anti-quark. The first of these, called the Theta-plus baryon, was predicted in 1997 and consists of (U, U, D, D, anti-S). This kind of quark package seems to be pretty rare and hard to create. There have been several claims for a detection of such a particle near 1.5 GeV, but experimental verification remains controversial. Two other possibilities called the Phi double-minus (D, D, S, S, anti-U) and the charmed neutral Theta (U, U, D, D, anti-C) have been searched for but not found.
Comparing normal and exotic baryons (credit: Quantum Diaries)
There are also the simpler tetraquark mesons, which consist of four quarks. The Z-meson (C, D, anti-C, anti-U) was discovered by the Japanese Bell Experiment in 2007 and confirmed in 2014 by the Large Hadron Collider at 4.43 GeV, hence the proper name Z(4430). The Y(4140) was discovered at Fermilab in 2009 and confirmed at the LHC in 2012 and has a mass 4.4 times the proton. It could be a combination of charmed quarks and charmed anti-quarks (C, anti-C, C, anti-C). The X(3830) particle was also discovered by the Japanese Bell Experiment and confirmed by other investigators and could be yet another tetraquark consisting of a pair of quarks and anti-quarks (q, anti-q, q, anti-q).
So the Standard Model and the six-quark model it contains make specific predictions for new baryon and meson states to be discovered. All totaled, there are 45 particles predicted that remain to be discovered! At the current pace of a few particles per year or so, we may finally wrap up all the predictions of the quark model in the next few decades. Then we really get to wonder what lies beyond the Standard Model once all the predicted particles slots have been filled. It is actually a win-win situation, because we either completely verify the quark model, which is very cool, or we discover anomalous particles that the quark model can't explain, which may show us the way to the Standard Model v.2.0!