A quark is a fundamental constituent of matter, representing one of the smallest known building blocks of the universe. Protons and neutrons, which make up the atomic nucleus, were long considered elementary, but experiments in the late 1960s confirmed they were composite. The concept of the quark was independently proposed in 1964 by physicists Murray Gell-Mann and George Zweig to explain the complex array of subatomic particles being discovered at the time. Quarks are now categorized within the Standard Model of particle physics, which describes the most basic particles and the forces that govern their interactions.
Defining the Fundamental Particle
Quarks possess several intrinsic properties that define their existence as truly fundamental, indivisible particles. Like electrons, they are classified as fermions, meaning they have a half-integer spin, which restricts how they occupy quantum states within a composite particle. This spin is an internal form of angular momentum.
A distinguishing feature of quarks is their fractional electric charge. Quarks come in two types of fractional charge: positive two-thirds (+2/3) or negative one-third (-1/3) of the elementary charge (e). This is a unique trait in the Standard Model, as all other observable particles, such as electrons and protons, have charges that are integer multiples of e. Additionally, quarks are the only elementary particles that experience all four known fundamental interactions: gravitation, electromagnetism, the weak nuclear force, and the strong nuclear force.
The Six Quark Flavors
Quarks are categorized into six distinct types, or “flavors,” based on their mass and electric charge. These six flavors are grouped into three generations of increasing mass. The first generation includes the Up and Down quarks, which are the lightest and most stable types.
The second generation consists of the Strange and Charm quarks, while the third generation comprises the Bottom and Top quarks. Each generation includes one quark with a positive charge of +2/3e (Up, Charm, Top) and one with a negative charge of -1/3e (Down, Strange, Bottom). Up and Down quarks form the matter of the everyday world, such as protons and neutrons. The heavier flavors are highly unstable, decaying quickly into the lighter Up and Down quarks through the weak nuclear force. They are only found in nature during high-energy events like cosmic ray collisions or created artificially in particle accelerators.
How Quarks Build Hadrons
Quarks are never observed in isolation; instead, they combine to form composite particles known as Hadrons. The rule for combination is that the resulting hadron must have an overall electric charge that is an integer, such as +1, 0, or -1. Hadrons are divided into two main categories: Baryons and Mesons.
Baryons are particles composed of three quarks, including the proton and the neutron. A proton is made of two Up quarks and one Down quark, resulting in a net charge of +1e. Conversely, a neutron is composed of one Up quark and two Down quarks, which yields a net charge of 0.
Mesons are the second class of hadrons, formed from a quark and an antiquark pair. An antiquark is the antimatter equivalent of a quark, possessing the same mass but opposite charge. The combination results in a short-lived particle. These quark combinations explain the existence and properties of hundreds of subatomic particles beyond protons and neutrons.
The Force That Binds Them
The strong nuclear force holds quarks together within hadrons, which is mediated by exchange particles called gluons. Gluons constantly flow between quarks, binding them with a powerful attraction. This force is governed by a property of quarks known as “color charge,” a concept analogous to electric charge but with three types: red, green, and blue.
For a hadron to exist as a stable particle, it must be “color neutral.” This is achieved by combining quarks in a way that the colors cancel out, similar to mixing red, green, and blue light to create white. Baryons achieve this neutrality by containing one quark of each color. Mesons achieve neutrality by pairing a quark of one color with an antiquark of the corresponding anticolor.
This mechanism leads to color confinement, which explains why a quark can never be found in isolation. If a scientist attempts to pull a quark away from its companions, the strong force does not weaken with distance; instead, it grows stronger. The energy required to separate the quarks creates a new quark-antiquark pair from the available energy, forming new hadrons instead of freeing a single quark.