What are Quarks? Discovery, Types and Properties

Quarks: These cryptic entities lie at the heart of matter, embodying a world of mystery, complexity, and elegant symmetry. They are the smallest, most fundamental constituents of matter as we understand it, forming the building blocks of protons and neutrons—components of atomic nuclei.  As a testament to their elusive nature, quarks have never been directly observed; yet, their existence has been illuminated through a panoply of high-energy physics experiments, giving birth to a new understanding of the universe’s underlying structure.

Comprising six distinct ‘flavors’—up, down, charm, strange, top, and bottom—quarks interact via all four fundamental forces of nature: gravity, electromagnetism, weak nuclear force, and, most uniquely, the strong nuclear force. This interaction is governed by a principle known as ‘color confinement,’ binding quarks together in such a profound way that they can never be found in isolation—a concept as perplexing as it is fascinating.

What are quarks?

Quarks represent the most fundamental constituents of matter as proposed by our current understanding of physics, specifically the quantum field theory known as the Standard Model. They are elementary particles that combine in specific ways to form protons, neutrons, and other hadrons, particles that constitute the atomic nucleus and significantly contribute to the mass of atoms.

Named by Murray Gell-Mann in 1964—inspired by a line in James Joyce’s “Finnegans Wake”—the term “quark” is now integral to the lexicon of particle physics.  Gell-Mann’s proposition, developed concurrently with George Zweig’s “aces”, formed the basis for what would later become the quark model. For their pivotal contributions, Gell-Mann was awarded the Nobel Prize in Physics in 1969.

Quarks are categorized into six distinct types, or “flavors”: up, down, charm, strange, top, and bottom. The up and down quarks are the lightest and most common, forming protons and neutrons—key components of atomic nuclei. The other flavors, while less common in the natural world, are routinely produced and studied in high-energy particle accelerators.

A crucial aspect of quark behavior is their adherence to the principle of color charge, a fundamental property entailing the strong nuclear force, one of the four fundamental forces of nature. This charge is unrelated to the concept of visual color, but utilizes the color terminology (red, blue, green) as a convenient metaphor for the property of quarks that enables their binding into larger particles. The strong force operates under a principle called color confinement, which stipulates that observable particles must be “color-neutral”. This results in quarks appearing in combinations where their color charges cancel out, such as triplets forming baryons (e.g., protons and neutrons), and quark-antiquark pairs forming mesons.

Quarks are point-like particles and, as far as our current knowledge extends, do not have physical size or structure, making them unlike most familiar particles. They carry fractional electric charges, which is an anomaly in the world of subatomic particles, where most have integer charges. For instance, an up quark has a charge of +2/3, while a down quark has a charge of -1/3.

Quarks also exhibit a property known as “spin”, a type of intrinsic angular momentum that is a fundamental characteristic of quantum particles. Unlike classical angular momentum, spin does not correspond to physical rotation, but is an abstract, mathematical property that contributes to the quantum behavior of particles.

Despite their fundamental nature, quarks cannot be isolated due to the principle of color confinement mentioned above. Consequently, our knowledge of these entities comes indirectly, primarily from high-energy scattering experiments and the study of particle decays.

The discovery of quarks

The discovery of quarks is a significant milestone in the realm of particle physics, an arduous journey of the 20th century that has culminated in our current understanding of matter’s fundamental constituents. The story of their discovery is intertwined with the development of the quark model, which intricately delineates the behavior and properties of quarks.

In the 1960s, physicists were grappling with a burgeoning “particle zoo,” an influx of newly discovered particles detected in high-energy physics experiments.  Many of these particles were unstable and decayed rapidly into other particles, exhibiting a bewildering array of properties. The challenge was to discern an underlying order within this seeming chaos, a theory that could satisfactorily account for the myriad particles and their interactions.

Murray Gell-Mann and George Zweig, working independently, provided the solution. In 1964, they proposed that a classification scheme based on certain mathematical symmetry principles could successfully organize the “particle zoo”. According to their model, many of the particles could be viewed as combinations of three types of more fundamental particles. Gell-Mann called these fundamental particles “quarks,” a term borrowed from James Joyce’s novel, “Finnegans Wake.” Concurrently, Zweig developed a similar model and referred to the fundamental entities as “aces”.

Empirical support for the existence of quarks came later in the decade through a series of high-energy scattering experiments at the Stanford Linear Accelerator Center (SLAC). By firing electrons at protons and neutrons and observing how the electrons scattered, researchers inferred that these particles were not indivisible, but contained smaller, point-like entities. The data from these experiments matched the predictions of the quark model with startling accuracy, bolstering the case for the physical existence of quarks.

The 1970s and 1980s saw the discovery of the charm, bottom, and top quarks through high-energy accelerator experiments, bringing the total number of quark flavors to six. These findings confirmed the existence of a third generation of quarks, solidifying the six-quark framework within the Standard Model of particle physics.

The discovery of quarks transformed our understanding of matter’s fundamental nature, effectively reducing the “particle zoo” to a manageable number of building blocks.  Quarks are now established as the key components of protons, neutrons, and other hadrons, their properties and interactions governed by the strong nuclear force. The story of their discovery underscores the interplay between theoretical innovation and experimental verification in advancing our understanding of the universe.

Types of quarks

Quarks, come in six distinct types or “flavors”: up, down, charm, strange, top, and bottom. Each of these quark flavors possesses unique properties, such as mass and electric charge, contributing to the diversity of particles that can be constructed from quarks. They form a crucial part of the Standard Model of particle physics, which serves as our best description of the subatomic world.

1. Up Quark: The up quark is the lightest of all quark flavors, with a mass only a few MeV/c^2, or roughly 0.005 times the mass of a proton. It carries an electric charge of +2/3. Up quarks are one of the two quark flavors, alongside down quarks, that form the protons and neutrons in atomic nuclei.
2. Down Quark: The down quark has a slightly higher mass than the up quark, approximately 0.01 times the mass of a proton. Its electric charge is -1/3. Protons contain one down quark, while neutrons contain two.
3. Charm Quark: The charm quark is considerably heavier than the up and down quarks, with a mass about 1.27 GeV/c^2 or roughly 1.3 times the mass of a proton. Like the up quark, the charm quark has an electric charge of +2/3. It was discovered in 1974 through the J/Psi particle, a bound state of a charm quark and an anti-charm quark.
4. Strange Quark: The strange quark has a mass of about 0.1 GeV/c^2 or approximately 0.1 times the mass of a proton. Its electric charge is -1/3, the same as the down quark. Despite its name, the strange quark is not particularly “strange” in any physical sense—the name was chosen because particles containing strange quarks were observed to have unusually long lifetimes.
5. Top Quark: The top quark is the heaviest of all quark flavors, with a mass of approximately 173 GeV/c^2, or about 180 times the mass of a proton. It was the last of the quark flavors to be discovered, in 1995, due to its high mass making it difficult to produce in particle accelerators. Like the up and charm quarks, the top quark has an electric charge of +2/3.
6. Bottom Quark: The bottom quark, also known as the beauty quark, has a mass about 4.2 GeV/c^2, or roughly 4 times the mass of a proton. It carries an electric charge of -1/3. The discovery of the bottom quark in 1977 was significant because it provided the first evidence for a third generation of quarks.

These six flavors of quarks combine in specific ways to form a plethora of composite particles, known as hadrons. The most familiar hadrons are the proton (composed of two up quarks and one down quark) and the neutron (composed of one up quark and two down quarks). However,  quarks can also combine in other ways to form a variety of other particles, such as mesons (composed of a quark and an antiquark) and other baryons (composed of three quarks or three antiquarks).

Properties of quarks

Quarks, as fundamental constituents of matter, possess a range of unique and intriguing properties that set them apart from other elementary particles. Understanding these characteristics is essential for comprehending the structure of matter as well as the interactions between its most basic components.

Quarks are grouped into six “flavors”: up, down, charm, strange, top and bottom.  Each flavor is distinguished by a unique set of properties, including mass, electric charge and spin.

1. Mass: The mass of quarks varies substantially between different flavors. Up and down quarks, the lightest, contribute to the bulk of the ordinary matter around us. In contrast, top quarks are the heaviest, with a mass approximately 180,000 times greater than the up quark. The masses of quarks are not absolute values but are given within a range because of the complexities in their calculation, primarily due to the effects of quantum chromodynamics (QCD).
2. Electric Charge: Unlike most particles, quarks carry a fractional electric charge. Up, charm, and top quarks have a charge of +2/3, while down, strange, and bottom quarks carry a -1/3 charge. These fractional charges combine in such a way that hadrons (composite particles made of quarks) always have integer electric charges.
3. Spin: Quarks, like all elementary particles, possess a property called spin, which is a form of intrinsic angular momentum. The spin of a quark is always 1/2, making it a fermion, a category of particles that follow the Pauli Exclusion Principle, which states that no two fermions can occupy the same quantum state simultaneously.
4. Color Charge: Quarks also carry a property called color charge, which is completely unrelated to the concept of color as we perceive it visually. This property is fundamental to the strong nuclear force—the force that binds quarks together within hadrons. The color charge of a quark can be red, green, or blue, and the anti-colors for antiquarks are anti-red, anti-green, and anti-blue. The strong force operates under a principle known as color confinement, which results in observable particles always being color-neutral.
5. Handedness: Quarks can also be characterized by a property called handedness or chirality, which relates to their spin direction relative to their motion. Quarks can be left-handed or right-handed, and this property plays a crucial role in weak interactions, one of the four fundamental forces.

The characteristics described above are not independent but are intertwined through the laws of quantum mechanics and the theory of quantum chromodynamics.  Despite the term “color” and the use of fractions, these properties do not have a direct analogue in our macroscopic world but are mathematical constructs that describe the behavior and interactions of quarks remarkably well.

The role of quarks in particle physics

Quarks play a pivotal role in particle physics as they are the most fundamental constituents of matter. According to the Standard Model, the current theoretical framework that describes the elementary particles and their interactions, quarks, along with leptons, form the building blocks of matter. Quarks, in particular, combine to form composite particles known as hadrons, including the protons and neutrons that comprise atomic nuclei. Therefore, understanding quarks and their properties is essential to comprehend the nature of matter and the universe.

In addition to their crucial role in forming matter, quarks also provide the means to study and test fundamental theories in particle physics. For instance,  the discovery of the top quark, the heaviest known elementary particle, has allowed precise tests of the Standard Model and helped refine estimates of the mass of the as-yet-undetected Higgs boson before its discovery in 2012. The study of quark interactions has also shed light on the asymmetry between matter and antimatter in the universe, a significant unsolved question in cosmology.

Moreover, quarks are central to the study of quantum chromodynamics (QCD), the theory that describes the strong nuclear force. QCD allows predictions of quark behavior and their interactions, including phenomena such as asymptotic freedom (quarks behaving as free particles at high energies) and confinement (quarks permanently bound together at low energies). Studying these properties provides insights into the dynamics of the early universe and neutron stars, where extreme conditions may create a state of matter called a quark-gluon plasma.

Are quarks the smallest particles?

According to the research of LotusBuddhas, quarks, along with leptons (which include electrons and neutrinos), are considered the smallest known particles, or more accurately, fundamental particles in the sense that they are not composed of any smaller constituents.

However, the term “smallest” can be somewhat misleading when applied to particles at the quantum scale. In quantum mechanics, particles like quarks and leptons are treated as point-like entities, implying they have no size or structure. Instead, they are defined by their properties, such as mass, charge, and spin, rather than physical dimensions. When physicists refer to these particles as the “smallest,” they mean that they cannot be divided into more fundamental constituents.

Furthermore,  it is important to note that our understanding of fundamental particles is not absolute but based on the limits of current scientific knowledge and technology. The Standard Model, while incredibly successful in explaining a vast range of experimental results, has several known limitations and leaves some phenomena unexplained. For example, it does not incorporate gravity, nor does it account for the mysterious dark matter and dark energy that appear to dominate the universe.

One active area of research that might alter our view of fundamental particles is string theory, a theoretical framework in which the point-like particles of particle physics are replaced by one-dimensional objects called strings. If string theory is correct, then quarks are not point-like but composed of extremely tiny, vibrating strings. However, as of 2021, string theory has not been empirically confirmed and remains a speculative, albeit promising, area of theoretical physics.

The difference between quarks and lepton

Quarks and leptons are the most fundamental constituents of matter, as posited by the Standard Model of particle physics. Although they share some common properties, such as having half-integer spin and thus being classified as fermions, they exhibit significant differences that distinguish them from each other.  These differences chiefly arise from their interactions with the four fundamental forces of nature: gravity, electromagnetism, the weak nuclear force, and the strong nuclear force.

1. Interactions with fundamental forces: The most salient difference between quarks and leptons is their interaction with the strong nuclear force. Quarks carry a property known as color charge, making them subject to the strong force, which binds quarks together to form hadrons (composite particles such as protons and neutrons). This interaction is described by the theory of Quantum Chromodynamics (QCD). On the other hand, leptons, which include electrons, muons, taus, and neutrinos, do not carry color charge and hence do not interact via the strong nuclear force.
2. Composite vs. Elementary particles: As a consequence of their interaction with the strong force, quarks combine to form composite particles, or hadrons. These can be baryons, composed of three quarks (like protons and neutrons), or mesons, composed of a quark and an antiquark pair. Leptons, in contrast, are truly elementary particles—they are not composed of any smaller constituents.
3. Electric charge: Quarks possess fractional electric charges of +2/3 or -1/3, depending on the type of quark. This is a unique property among the known elementary particles. Leptons, in contrast, carry integer electric charges. The electron, muon, and tau particles have a charge of -1, while their associated neutrinos are electrically neutral.
4. Generation and Mass: Both quarks and leptons exist in three generations, or “families,” with each subsequent generation having greater mass. The up and down quarks form the first generation, the charm and strange the second, and the top and bottom the third. Similarly, the electron and electron neutrino comprise the first lepton generation, the muon and muon neutrino the second, and the tau and tau neutrino the third. However, the absolute masses of the quarks and leptons differ greatly, with quarks generally being much heavier than their lepton counterparts.

Experiments or discoveries related to quarks

The discovery and subsequent study of quarks represent a remarkable journey of scientific inquiry and experimentation in the field of particle physics. The very existence of quarks, initially proposed theoretically, has been corroborated through numerous high-energy experiments over the past half-century.

1. Deep Inelastic Scattering Experiments: The first experimental evidence for the existence of quarks came from deep inelastic scattering experiments conducted in the late 1960s and early 1970s at the Stanford Linear Accelerator Center (SLAC).  In these experiments, high-energy electrons were fired at protons and neutrons. The manner in which the electrons scattered off these particles provided the first indirect evidence of quarks’ existence, as the scattering patterns could be best explained if the protons and neutrons had substructure, i.e., they were composed of point-like particles, later identified as quarks.
2. Discovery of the Charm Quark: The discovery of the charm quark in 1974 was a significant milestone. Until then, only the up, down, and strange quarks were known. The J/Psi particle, discovered almost simultaneously by teams at SLAC and Brookhaven National Laboratory, was found to consist of a charm quark and an anti-charm quark. This discovery not only expanded the quark family but also provided a vital confirmation of the Standard Model, which had predicted the existence of the charm quark to explain the absence of certain flavor-changing weak decays.
3. Discovery of the Bottom and Top Quarks: The discovery of the bottom (or beauty) quark in 1977 at the Fermilab, evidenced by the production of the Upsilon particle (a bottom quark and anti-bottom quark pair), provided the first evidence for a third generation of quarks. The top quark, the last to be discovered, was observed at Fermilab in 1995. Its large mass, approximately 180 times that of a proton, had made it elusive in earlier experiments.
4. High-precision measurements at LEP: The Large Electron-Positron Collider (LEP) at CERN, operational from 1989 to 2000, performed precise measurements of the Z boson’s properties, an electrically neutral particle mediating the weak interaction. The number of types of light neutrinos was determined to be three, in excellent agreement with the three generations of quarks and leptons posited by the Standard Model.
5. Quark-Gluon Plasma at RHIC and LHC: Experiments at the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory and the Large Hadron Collider (LHC) at CERN have created a state of matter known as quark-gluon plasma, in which quarks and gluons, the carriers of the strong force, are not confined within hadrons but move relatively freely. This state of matter is thought to have existed just microseconds after the Big Bang and offers insights into the fundamental properties of the strong force and quarks.

These experiments and discoveries represent milestones in the understanding and validation of the existence and properties of quarks. However, It is worth noting that quarks have never been directly observed in isolation due to color confinement, a feature of the strong force that prevents quarks from being separated from their companion particles in observable states.  Nonetheless, the accumulated experimental evidence for quarks’ existence is robust and continues to grow, playing a foundational role in the continually evolving understanding of particle physics.

Reference from:

• Quark models Latest Research Papers: https://www.sciencegate.app/keyword/842435
• Study of quark speeds finds a solution for a 35-year physics mystery: https://news.mit.edu/2019/quark-speed-proton-neutron-pairs-0220