The quirks of quarks
The LHCb experiment at CERN recently confirmed the existence of a particle that violates the quark model, so it’s a good time to fill you in on just how weird quarks are. LHCb is the name of the Large Hadron Collider beauty collaboration, where “b” refers to the bottom quark, which is sometimes called the beauty quark, as opposed to the top quark, which is sometimes called the truth quark. But forget the sensationalism, that’s for the politicians who write up energy budgets. LHCb announced confirmation of the Z(4430) experiment at the KEK collider in Tsukuba Japan.
Quarks interact primarily through the strong nuclear force. Violating the quark model isn’t a big deal. Violating the quantum theory of the strong force would be a giant, Nobel-quality announcement, but that hasn’t happened.
It’s easiest to describe the strong force by comparing it to the electromagnetic force:
Electric charges come in + and -.
The strong force has three different charges and their opposites. Since we see three unique colors (unless you’re color blind), we call the strong force charges “color charges” and the theory of the strong force, “Quantum Chromodynamics” —“chromo” refers to the colored nature of the force. Strong charges come in red, blue, green, and anti-red, anti-blue, anti-green. Where electrons carry one negative charge, quarks carry one of the three colors.
Just as electric charges radiate electric fields, so do quarks radiate color fields. Electromagnetic radiation is made of photons, strong radiation is made of gluons—called glue because it’s such a strong force.
Here’s where it starts to get bizarre: photons are, of course, neutral objects, but gluons are charged objects. Each gluon carries one of eight different combinations of the six color and anti-colors. For example, a red quark could radiate a red, anti-blue gluon and become a blue quark.
Photons, at least at the energies used in electrical engineering, interfere, but don’t interact. They’re nice linear waves. Since gluons carry strong force charges, they interact with each other. One of the predicted violations of the quark model include bound states of gluons called “glueballs.” Evidence for glueballs has been seen in lots of experiments but never confirmed.
It continues to get weirder: As we separate electric charges by longer distances, the force between them decreases. The opposite is true for quarks. The farther apart, the greater the force between quarks. In other words, when they’re right next to each other, within about a femtometer (1E-15 m), they hardly interact at all. But that force gets big fast as they are separated.
As two quarks are pulled apart, there comes a point where the amount of energy required to separate them exceeds their own mass-energy. With that amount of energy stored in the strong force field, a new pair of quarks literally pops into existence. Out of nowhere, two new quarks appear. They do so in a way that balances nature’s ledgers, of course, the new quarks will be matter-antimatter pairs with opposite charges.
Think of it like this: The work required to pull the quarks apart goes into the space between them, the field. The instant that the field holds enough energy to create another pair of quarks--ping!—there they are.
As soon as the new quark-anti-quark pair appears, they bond with the old pair and we get two pairs of quarks. If the systems still have enough energy, that is, if we keep pulling apart the quark-antiquark pairs, more quark-antiquark pairs will pop out of the vacuum until we run out of energy. By then, we’ll have a bunch of objects composed of bound quarks.
The end result is that it’s impossible to isolate a quark. We can only observe them in bound states with other quarks, like protons, neutrons, pions, and kaons.
Finally, back to the LHCb result.
LHCb confirmed the existence of the Z(4430). The name “Z” means they’re not sure what it is. The number, 4430, means that it has a mass of 4430 million electron volts, roughly the mass of 4.7 protons. The quark model predicts all possible combinations of “mesons,” which are composed primarily of a quark and an anti-quark, and “baryons,” which are states of three quarks. Protons and neutrons are baryons, pions, and kaons are mesons.
The Z(4430) doesn’t fit in that model, and due to the way it decays, the researchers believe that it is composed of two quarks and two anti-quarks bound together, an exotic four-quark state. It would be cooler if they saw a glueball.