For generations, protons have been considered an indivisible part of matter. As particle physics evolved, so did our understanding of matter's inner workings. It turned out that protons weren’t solid spheres at all but dynamic, energetic swirls of tinier particles ruled by a mystical, strong force. Even today, with the power of billion-dollar colliders and decades of theory behind us, the proton remains a great source of puzzles. The results contradict traditional ideas about proton structure that went beyond a simple quark structure.
Rutherford and Bohr made discoveries that provided us with the planetary model of the atom: electrons surround a dense nucleus. This nucleus is made up of protons as well as neutrons. And that was the final answer.
But, in the mid-20th century, we had new tools like particle accelerators. Particle smashing experiments at high energies by scientists saw some unexpected patterns in the debris. Such patterns indicate that even protons and neutrons had inner components. Therefore, in 1964, Murray Gell-Mann and George Zweig launched a bold new model: protons were not elementary particles but made of smaller units called quarks. Quarks were theoretical, then, a mathematical way to solve experimental puzzles. No one had seen one. But the model worked. Finally, the first real proof of such quarklike structures inside protons was given by deep inelastic scattering experiments at the Stanford Linear Accelerator Center (SLAC).
The shift from seeing protons as solid to viewing protons as a cluster of quarks was a great milestone in physics. Through a giant leap the search for what lies inside matter took a giant leap.
Despite our understanding of quarks, one mystery remains: Why has no one ever seen a quark alone? This question of the strong interaction is at the heart of the quark confinement problem. Also unique, the strong force strengthens as the quarks become more distant. Think of stretching a rubber band. The more stretch, the higher the force. However, if you pull hard enough, the rubber band breaks. In the case of quarks, though, rather than breaking and freeing a quark, energy input produces a completely new quark-antiquark pair. This new pair pairs with the existing ones, leading to the confinement of the system.
This then answers why quarks are never found in isolation. Instead, they are always found to be a part of larger particles, known as hadrons, which consist of protons and neutrons. It also explains why the great power was so elusive. The force acts over small distances but is much stronger than electromagnetic or gravity.
Quarks are among the fundamental building blocks of matter. They are alongside leptons (for example, electrons and neutrinos) in the Standard Model of particle physics. Quarks are grouped to form hadrons. The hadrons are, in turn, divided into two main categories according to the model introduced by Gell-Mann and Zweig.
Baryons: Particles made of three quarks. Protons (uud) and neutrons (udd) are examples.
Mesons: Particles made of a quark-antiquark pair, such as pions and kaons.
Gluons, in other words, are particles that mediate the strong interaction. It holds these quarks together. In addition, to bind quarks, gluons have a type of charge called colour charge and interact with one another, adding to strong force dynamics complexities. Each quark has a unique identity, or “flavour,” and exhibits properties like electric charge, mass, and spin. These determine how they are combined to make different particles.
A proton is constituted by three quarks: two up and a down. Even though this picture is foundational, it is incomplete. For example, only up and down quarks are necessary to build everyday matter, but there are more with the rain of cosmic rays or particle collisions. Experiments at the Hadron-Electron Ring Accelerator (HERA) — as well as the Large Hadron Collider (LHC) — have shown that a proton has, in addition to those three "valence" quarks, also a sea of constantly fluctuating particles. The quark sea also includes transient quark-antiquark pairs that come into and out of existence using the energy fluctuations of gluons.
Evidence of pentaquarks was discovered at the LHCb experiment at CERN in 2015. These are exotic particles consisting of four quarks and one antiquark. This finding confirmed the theory that protons and neutrons might momentarily assume more complicated quark forms. These states are short-lived and difficult to detect. However, their existence still casts doubt on the traditional understanding of baryons. This reveals that the proton is not a rigid structure but can, under specific conditions, reorganise internally.
What does this tell us about the matter? In short, it makes the mystery deeper and pushes back the frontiers of particle physics.
Despite decades of study, key questions remain.
One major mystery is the origin of the proton's spin. Although quarks have spin, only the combination of all three spins from the three quarks does not fully explain the total angular momentum of the proton. Still, the precise contributions of gluon spin and orbital angular momentum are being explored.
The upcoming Electron-Ion Collider (EIC), a U.S.-based project, aims to answer this and much more by creating 3D images of the proton's internal structure. The scientists will try to solve puzzles such as where quarks and gluons are and how they move.
The proton exemplifies how a subatomic particle, though incredibly small, can possess remarkable internal complexity. Though very small, it can have immense internal complexity. The nature of it remains unknown even with the most sophisticated scientific tools and ideas, and every atom has it at its core. The Proton will always remind us that science is not something you do to ensure certain answers but in search of new levels of knowledge. With every discovery, there comes another question: how quarks bind together. What decides the mass of the matter? What could be found beyond the Standard Model?
In the end, the proton is not just a particle. It is a frontier in physics. It offers a door to probe the entire composition of matter and the very basic structure of the cosmos.
References
https://openmedscience.com/secrets-of-quark-particles-the-building-blocks-of-the-universe/
https://www.quantamagazine.org/inside-the-proton-the-most-complicated-thing-imaginable-20221019/
https://www.mdpi.com/2218-1997/7/9/330