What is a quark?

A “quark” is a fundamental building block of nature. There are six known kinds (or “flavors”) of quark. They are cousins to six other known particles, the “leptons,” of which the electron is the most familiar. We do not know why there are six quarks, or six leptons, or whether or not there is a reason there are these two kinds of matter building blocks.

The best way to find answers to questions we can’t yet address is to study these particles like crazy! That is partly what I do: I study quarks and use them in service to studies of other kinds of particles and phenomena. They are both a target of my own work and tools for other research goals.

Quarks were first speculated to exist in the early 1960s. Murray Gell-Mann and George Zweig famously developed the concept of particles more fundamental than protons or neutrons (which they compose), giving them different names but describing the same basic concepts:

  • Quarks would combine to make things like protons and neutrons, the constituents of all atomic nuclei.
  • They would have electric charges that are a fraction of that carried by the electron or the proton. If we denote the degree of electric charge carried by an electron or a proton as a number, e, quarks would carry either +2/3e or -1/3e of electric charge.
  • They would have to also carry a previously unseen kind of charge, later called “color,” that would explain why they combine to make the more complex states of matter known by the 1960s.
Current quark masses for all six flavors in comparison, as balls of proportional volumes. Proton (gray) and electron (red) are shown in bottom left corner for scale. By Incnis Mrsi – Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=22760588

Direct evidence for the quark hypothesis would arrive with a new generation of particle accelerators and detectors in the late 1960s and early 1970s, especially when early successes of the model led to the prediction of a previously unseen quark, the charm quark, that was confirmed in the mid-1970s. A fifth quark, the bottom quark, was found in 1977 and that led to the prediction of a fifth one (since quarks appeared to come as pairs in hierarchical generations, one with a +2/3e and one with a -1/3e charge. Bottom quarks had -1/3e, so a sixth quark, top, was predicted and later discovered in 1995 at the Tevatron facility sited at the Fermi National Accelerator Laboratory.