Hola! I'm Alexa Guido, a young and curious woman passionate about science. Join me on an exciting journey to explore the wonders of the universe through the lens of physics.
Did you ever stop to wonder what matter is made of? What are the fundamental ingredients of everything we touch and see? Have you heard of quarks? These tiny entities are the essential building blocks of all visible matter in the universe!
Isn't "quark" a lovely name? Surely it's better than its last one, “spinors”... The existence of quarks was first theorized back in 1964 by brilliant minds Murray Gell-Mann and George Zweig, who were both at the California Institute of Technology (CalTech) but who concluded that quarks exist independently of one another.
The odyssey to discover these desired particles began with hyperons, old particles that were notoriously unstable and decayed much quicker than expected, however, not into the particles they were expected to decay into according to probability. This led Gell-Mann to realize there must be an unknown quantum property at work, which he named "strangeness" because of its unexpected behavior.
In quantum mechanics, certain properties known as quantum numbers like strangeness, charge, and spin must be conserved in every system. But to don’t dive into it, the quantum numbers of a given particle have "degrees of freedom", basically the range of values that these numbers can adopt.
These degrees of freedom are called multiplets, and the pattern in which these multiplets could be arranged between different particles led Gell-Mann and Zweig to believe that the particles and their multiplets could be explained if nucleons (protons and neutrons) are composed of two or three smaller particles, excluding the electron!
These quarks were referred to as "up," "down," and "strange" quarks. The up and down don't really refer to anything, while the strange quark has a quantum number of strangeness of –1, thus why it is called "strange." The up-and-down quarks have a strangeness of 0.
Even though scientists believe that isolating quarks might be impossible, in 1968 a series of electron-proton scattering experiments by the MIT-SLAC collaboration at the Stanford Linear Accelerator Center (SLAC) revealed the first signs that nucleons have an inner structure proving the first compelling evidence of quarks' existence. The team fired electrons at protons and observed how the electrons bounced off and combining these results with CERN, it became clear that these constituents have charges of 1/3 and 2/3.
There are actually six types, or flavors, of quarks! Besides the up, down, and strange quarks, we also have "charm", "top" and "bottom" quarks. Each one has its own unique set of quantum numbers, and their masses are extremely different, with the up and down quarks being the least massive, and the top quark being the heaviest with a mass over 61,000 times more massive than the up quark.
Quarks never exist alone, they are always bound together by the strong nuclear force, which allows them to form combined particles known as hadrons. Particles made of two quarks are called mesons, and particles made of three quarks are called baryons. There are even exotic particles like tetraquarks that are made of four quarks, and pentaquarks that have five quarks, and some of these can be nearly stable but will eventually decay over time.
Baryons are particularly fascinating, as they form the backbone of atoms. Every proton is composed of two up and one down quarks, while neutrons have one up and two down quarks. The most enchanting aspect? When we add their fractional charges together, we arrive at the familiar charges of protons and neutrons, which are the building blocks of our visible universe!
Finally, these beautiful particles are also colorful. Quarks exhibit a behavior governed by a model called quantum chromodynamics, being “color” another quantum number that quarks have. It’s similar to the repel-attract game in magnetics, unlike here, colors repel and unlike colors attract, forming stable pairs of quarks. Just like other quantum numbers, color must also be conserved.
Current experiments indicate that something is still missing in our theory, and the study of quarks might help us overcome some of the current gaps in our understanding of the basic building blocks of the universe like gravity in that minimum lengths or if quarks are made up of smaller sub-quarks.
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