Up, down, strange, charm, beauty, truth: six particles, each with its own “flavour”, divided into three generations, interacting through all four fundamental forces of nature, “coloured” in three different colours (and three anti-colours). They are called quarks and together with six other particles (leptons) are considered the building blocks from which the matter of the entire universe is made.
The first two generation quarks, up and down, together form protons and neutrons. A proton is made of two up quarks and one down quark, a neutron is made of two down and one up. Protons and neutrons, in turn, are the nuclei around which electrons rotate, forming the atoms of the periodic table. The other quarks, discovered later and named second and third generation, form unstable particles, which decay rapidly.
From an etymological point of view, saying that an atom is made of electrons orbiting around a nucleus of protons and neutrons is a contradiction. The word atom literally means “indivisible”. The first to think of matter as constituted by small indivisible entities were the Greek philosophers of Atomism, already in the VII century b.C.
Hence, when John Dalton gave life to his chemical theory, at the beginning of the nineteenth century, it was natural to call the smallest possible portion of an element with the name: “atom”.
Up, down, strange, charm, beauty, truth: six particles, each with its own “flavour”, divided into three generations, interacting through all four fundamental forces of nature, “coloured” in three different colours (and three anti-colours).
A century later, when the idea that an atom was actually divisible began to make its way, and various models of its internal structure were theorized. The Rutherford model prevailed, where the atom’s mass is almost totally concentrated in a positively charged nucleus (made of protons) around which electrons rotate. This model was referred to as “planetary”, for its resemblance to the Sun around which the planets rotate. Later it was understood that the nucleus could not be formed only by protons, and neutrons were discovered in 1932.
In the sixties it was time to look at protons and neutrons through a magnifying glass. While electrons were (and are still today) considered as punctiform, physicists Murray Gell-Mann and George Zweig advanced separately the first theories about proton and neutron sub-structure. Their theories were formulated on a completely mathematical basis, rather than an experimental one. If anything, the scientific community was rather skeptical about the real existence of sub-particles that could compose protons and neutrons. It was only a few years later that the existence of these particles was confirmed experimentally.
It was Murray Gell-Mann who coined the name “quark”. In his book The quark and the jaguar, he explains the name’s origin. At the beginning he only thought of the sound effect of the word, something similar to “kwork”. Then, reading Finnegans Wake by James Joyce, he read the sentence “Three quarks for Muster Mark!”, in which quark has no specific meaning. The fact that there were exactly three quarks mentioned by Joyce was a fortuitous coincidence: to form a proton or a neutron there are exactly three quarks needed. And so kwork became quark. (In this way, taking a meaningless word, if we will discover that even quarks are divisible, at least we would no longer have the problem of having called them with a wrong word as it had happened with atoms).
Quarks, therefore, are considered today as indivisible particles that, combined together, form matter around us. They are, however, rather enigmatic objects.
It often happens that the common language is not able to support what in mathematics is instead well defined. Quarks are a prime example. These particles differ from each other for characteristics that, while finding a clear mathematical definition, are completely inexplicable in words. Therefore, physicists are sometimes forced to call these characteristics with ambiguous names, name that can make us think of something known, associated with qualities we know, even if it is not so.
The flavour quark, for example. Quarks are divided into six different flavours (up, down, strange, charm, beauty or bottom, truth or top), which have nothing to do with the taste we perceive with our taste buds. They also have different colours (red, green and blue), but these colours have nothing to do with real chromatic tones. They have fractional electric charges, that is not -1 (like electrons) or +1 (like protons), but rather +2/3 or -1/3. For this reason, quarks can never be separated from each other: fractional charges isolated in nature do not exist. So even if a proton (charge +1) is composed by two up quarks (each of charge +2/3) and one down quark (charge -1/3), it is not possible to separate the quarks and therefore it is not possible to see isolated fractional charges. Moreover, quarks have extremely different weights: the quark truth (or top) has a mass 35 thousand times higher than up and down quarks. The fact that it is called truth, however, has nothing to do with some kind of specific truth. The same is goes for the other third generation quark, beauty, which is not more beautiful or uglier than the others.
The question now is whether, thanks to these quarks, we have reached the final goal. Is there a sub-structure of the quarks or have we really reached the most elementary particles? Historically we have learnt to be wary of this belief and in fact, there are mathematical theories that foresee the further decomposition of the quarks, but they are still far from being verified experimentally. Currently the largest particle accelerator in the world, the LHC at Geneva CERN (responsible for the discovery of the Higgs boson), is not able to produce such energies to see smaller particles. However, the LHC also plans to build a new accelerator, which is larger (100 km in circumference) and costs 21 billion euros. Will this new accelerator be able to open the box of nature again, and find a new matryoshka inside it?