Introduction
As we all know, the fundamental building blocks that makeup all the matter in the Universe are atoms. As the size of atoms is very small which can’t be seen with the naked eye. They consist of even smaller subatomic particles called “protons, neutrons, and electrons”. Till the mid-20th century, scientists believed that the smallest unit of matter was atoms which couldn’t be divided further.
Groundbreaking discoveries in particle physics after the mid-20th century revealed an even more fundamental layer of particles. Subatomic particles that make up the nucleus of an atom “Protons and Neutrons” were found to be composed of even smaller particles called quarks.
This discovery of Quarks, raises another question “What are Quarks made of?”. After the discovery now the quarks are the fundamental blocks of the Universe. Do they also have an internal structure like other particles that can be broken down further? Understanding the nature of quarks and what they are made of has been a matter of major concern in particle physics in recent decades.
What are Quarks?
Quarks are the elementary particles that combine to form other subatomic particles (Hadrons) “Protons and Neutrons”. There are 6 different types of quarks:
- Up (u)
- Down (d)
- Strange (s)
- Charm (c)
- Bottom (b)
- Top (t)
Quarks have half-integer spin and they follow the Pauli Exclusion principle. Thus they are classified as “Fermions”.
What is Half-Integer Spin?
In quantum mechanics, all the elementary particles have the intrinsic property of “Spin”. All the known fermions (proton, neutron, electron, and quarks) have a spin of 1/2. How many symmetrical facets a particle has in one full rotation is described by the Spin Number. A Spin of 1/2 means to have the same configuration as when it started, for that the particle has to rotate through 720°.
What is the Pauli Exclusion Principle?
Pauli’s Exclusion principle states that no two identical fermions can occupy the same quantum state simultaneously. In simple words, each electron should be in its unique state or no two electrons will have the same quantum numbers (n, l, ml, and ms).
Quarks use strong nuclear force (one of the four fundamental forces in nature) to interact with each other. Due to the phenomenon of “Confinement”, they are never found in isolation. They are always found in groups of twos (mesons) or threes (baryons).
Are Quarks Truly Fundamental?
The quarks are considered to be the fundamental building blocks of matter that are known today. But here the question arises, that quarks too have an internal structure. In particle physics, some theories suggest that quarks are also composed of even smaller and more fundamental particles. Physicists conduct experiments to check if any other substructure exists inside the quarks. Physicists conduct these experiments using the world’s largest particle accelerator, the Large Hadron Collider (LHC).
So far, physicists have not found any evidence to prove that quarks have an internal structure. Therefore, they consider quarks to be the truly fundamental particles. However, physicists still believe that even more fundamental layers of particles may exist.
Charges and Masses of Different Types of Quarks
Up Quark
Physicists symbolize the up quark with the letter “u” and denote its antiparticle with “u” (with a straight bar above the symbol).
The mass of the Up quark ranges from 1.7 – 3.1 MeV / c2.
Its electronic charge is +2/3 e.
Down Quark
Physicists symbolize the down quark with the letter “d” and denote its antiparticle with “d” (with a straight bar above the symbol).
The mass of the down quark ranges from 4.1 – 5.7 MeV / c2.
Its electric charge is -1/3 e.
The Charm Quark
Physicists symbolize the charm quark with the letter “c” and denote its antiparticle with “c” (with a straight bar above the symbol).
The electric charge is a quark of +2/3 e.
The Strange Quark
Physicists symbolize the strange quark with the letter “s” and denote its antiparticle with “s” (with a straight bar above the symbol).
Its electric charge is -1/3 e.
The Top Quark
Physicists symbolize the top quark with the letter “t” and denote its antiparticle with “t” (with a straight bar above the symbol).
The mass of the top quark is 172.9 – 1.5 GeV/c2.
Its electric charge is +2/3.
The Bottom Quark
Physicists symbolize the bottom quark with the letter “b” and denote its antiparticle with “b” (with a straight bar above the symbol).
The mass of the bottom quark is approximately 4.1 GeV/c2.
Its electric charge is -1/3 e.
History of Quarks
The “Particle Zoo”
In the initial stages of the development of the Quark Model, particle physics faced the “Particle Zoo.” Particle physicists discovered several subatomic particles, with no clear underlying structure or organization. Initially, they only knew of 3 particles (Proton, Neutron, and Electron). But by the 1950s, this quantity had grown which includes a wide variety of “Hadrons”, and some other particles like “Muon and Neutrinos”.
This increase in the number of particles puzzled the physicists, due to which they lack to explain their existence and properties. In 1961, “Eightfold Way” was introduced by the physicist “Murray Gell-Mann”. This way classifies the hadrons based on their symmetrical properties. But the physicists still didn’t get any explanation regarding to fundamental nature of these particles.
Independent proposal of the quark model by Murray Gell-Mann and George Zweig in 1964.
An independent researcher “George Zweig and Gell-Mann” proposed the “Quark Model” while working at the “California Institute of Technology” (Caltech) in 1964. They explain that the hadrons themselves were not fundamental particles. But they are composed of even more components which Gell-Mann named “Quarks”.
Gell-Mann and Zweig’s treating hadrons as they made up of just three quarks. They could elegantly explain the patterns observed in the “Particle Zoo.” The 3 initial quarks were “up, down, and strange” quarks.
The initial role of the quark model as an organizational tool for hadrons.
The Quark model proved to be a useful organizational tool. This model helps physicists to create a mysterious arrangement of hadrons and their properties.
The Quark model was first doubted because there was no experimental evidence for the existence of quarks. This theory gained acceptance with the time when more evidence was collected. In the late 1960s, experiments at Stanford Linear Accelerator Center and many other facilities provided the first direct observations of the quark inside protons and neutrons.
With the discovery of further flavors (types) of quarks “charm, top and bottom”. The Quark model has been expanded and refined further. The real power of the Quark model lies in its deep understanding of quarks as truly fundamental particles bound by strong nuclear forces.
Quarks are not found in isolation (Color Confinement)
An important property of quarks is that they can’t be isolated. This is due to the phenomenon called “Color Confinement which is a part of “Quantum Chromo-Dynamics”. They are always found in hadrons as fundamental particles. Bound by the strong nuclear force, they cannot exist as free particles.
The Search for a Deeper Understanding
Now the observed fundamental particles of matter in the Standard Model of Physics are Quarks. But the research doesn’t stop here, the physicists are always in the research of substructures at even smaller scales. The technological and experimental limitations create challenges in searching for another internal structure inside the quarks.
Limitations of Current Technology
The scale at which quarks exist is 10-19 and even more smaller scales. This extremely small scale makes it difficult to study them directly. Even the world’s most powerful particle accelerators “LHC” can reach enough high energy levels to reveal, if an internal structure exists inside the quarks or not.
To observe quarks even at smaller and smaller scales, we have to push the boundaries of our experimental capabilities. However, there’s no theoretical or experimental proof that quarks too have an internal structure.
String Theory and the Possibility of Substructure
There are only a few theories, according to which quarks also have internal structure. One of them the few theories is “String Theory”. The String Theory is a theoretical framework. In this theory, one-dimensional objects called strings replace the fundamental building blocks of the Universe.
The key idea in string theory is that the various particles we observe, such as quarks, leptons, and even the force carriers like photons and gluons, are not truly fundamental. Instead, particle physicists see quarks as different vibrational modes or excitations of these underlying strings.
In this theory, physicists believe the particles aren’t truly fundamental. Instead, they see the particles as different vibrational modes or excitations of these underlying strings. The interactions and vibrations of these string-like entities can explain quarks’ electric charge, confinement, and other such properties. Yet, the String Theory has not provided a complete and testable model that can fully describe the observed properties of quarks and other particles.
Conclusion
As our experimental and theoretical capabilities are advancing day by day. The possibility of revealing another fundamental layer of reality inside the quarks remains a mysterious perimeter in the unified universe theory.
