What Are Quarks? – World of Subatomic Particles

“In the world of the very small, where particle and wave aspects of reality are equally significant, things do not behave in any way that we can understand from our experience of the everyday world…all pictures are false, and there is no physical analogy we can make to understand what goes on inside atoms. Atoms behave like atoms, nothing else.”

John Gribbin, British Science Writer

Throughout our existence, humans have had an insatiable desire to understand the universe.

From the stone age to the very present, mankind has evolved because of curiosity alone.

From stars that are billions of light-years away to the tiniest particles that make up everything, we have always tried to unmask the cosmos to its finest detail.

This journey, however, still seems infinitely long, and so does our hunger.

Today, we know that most of the answers to the questions we ask are found in the tiniest particles – the subatomic particles.

As of now, scientists have compiled the Standard Model of Elementary Particles, which does a great job of describing all elementary particles currently known, along with the fundamental forces associated with them.

the standard model of physics
The classification of elementary particles is as per the Standard Model of Elementary Particles. Credits: CERN

We have all heard about electrons, protons, and neutrons for sure.

They are subatomic particles indeed, but not all of them are elementary particles (the building blocks of matter). 

Note that “subatomic” and “elementary” are not supposed to be used synonymously. 

Is that to say, these subatomic particles are composed of even smaller particles?

Yes; electrons are indeed elementary particles, but it turns out that protons & neutrons are composed of even smaller particles.

The Standard Model of Elementary Particles classifies all known elementary particles into two fundamental classes – Fermions and Bosons

Fermions are further divided into quarks and leptons.

Quarks and leptons (both of which are fermions) are the “most” elementary particles. 

So, the protons and neutrons that we know of are not fundamental but instead fall under the category of what we call hadrons. 

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In the Standard Model of Elementary Particles, electrons are classified under leptons, while protons and neutrons are hadrons.

So what are hadrons, you might ask? Well, hadrons are particles that are formed by the combination of different quarks in varying compositions.

venn diagram composing hadrons, fermions, and bosons
A Venn diagram comprising Hadrons, Fermions, and Bosons. Credits: Wikipedia

Yes, we understand that the hierarchy of particles is quite confusing.

Apparently, it’s the quarks that are the heart of our discussion (not to mention that they’re also the heart of all particles) today.

So let us begin our quirky quest into the quantum quiver that holds arrows more bizarre than we can imagine.    

Discovery of Quarks

Although quarks first appeared just picoseconds (1/1000000000000th of a second) after the big bang, the world didn’t come to know about them until the past century.

The mid-20th century saw quite a few proposals and theories leading to progress in the understanding of elementary particles.

Quarks were first theorized independently by physicists, unassociated with any observations.

In the late 1960s, hadrons (protons and neutrons) were observed in particle accelerator experiments, and their results hinted at a possible discovery that was waiting to be unraveled.

It was speculated that there must exist more fundamental particles (inside hadrons), within the framework of particle physics, in order to better explain the important properties and the behavior of hadrons.  

Earlier, In 1961, Murray Gell Mann formulated a method for classifying all known hadronic particles, known as the Eightfold way.

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Fact: The Eightfold way was inspired by the Eightfold Path in Buddhism – a representation of the 8 noble practices in the Buddhist culture.

the noble eightfold path buddhism
The Noble Eightfold Path describes Buddhist teachings by binding eight noble practices into a Dharmachakra – wheel of dharma (righteousness). Credits: Balanced Achievement

The eightfold way (of hadrons; not Buddhism) revealed the behavioral pattern in hadrons.

It indicated that hadrons were not fundamental at all but made up of more fundamental constituents.

The physics community questioned whether quarks were a physical entity or mere abstract to explain concepts that weren’t entirely understood at that time.

Three years later i.e. in 1964, American physicists Murray Gell-Mann and George Zweig proposed the quark model independently.

While Gell-Mann established the grounds for theorizing the particle with his years of research, George Zweig made a similar contribution through a paper published on 17th January 1964.

Therein, he theorized that both mesons and baryons (the two classes of hadrons) are composed of three even smaller constituent particles, which he called aces.

Although the two physicists did not comply with the names that they gave to the elementary particle, the idea behind both “quarks” and “aces” was the same.

Both Gell-Mann and Zweig suggested that the quarks (or aces, as Zweig called them) had fractional electrical charges.

Only experiments could tell if such particles (with absurd-sounding fractional charges) existed or not.

Spoiler alert…they do.

In 1968, protons were (literally) made to collide with electrons (at extremely high speed) inelastically, and their scatterings were observed in huge particle accelerators.

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These scattering experiments caused the protons and electrons to (collide and) burst open into finer particles

The observations showed that protons were made up of not one, not two, but three tinier particles. 

Physicists did not favor the quark model (then), so they referred to those particles as ‘partons’.

However, it was later verified (and identified) that the particles observed already existed on paper as up and down quarks. 

This was how the scattering experiments solidified the experimental discovery of the elementary particles that make up all matter around us – quarks.

What Types of Quarks Are There?

Gell-Mann and Zweig proposed that the hadrons we know of are made up of quarks and antiquarks (antiparticles of quarks) in different combinations. 

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Initially, there were three flavors of quarks – up, down, and strange.

A year later, a fourth quark was theorized as well and was named the charm quark.

Quarks existed only on paper until the late 1960s.

After some time, the ‘strange’ and ‘charm’ quarks were also discovered experimentally

In 1973, two more quarks were theorized to explain the violation of Charge-Parity Symmetry.

Charge-Parity Symmetry says that the physical parameters for a particle should remain the same if it is exchanged with its antiparticle and its spatial coordinates are inverted.

These two new quarks were named ‘top’ and ‘bottom’.

Hence, quarks were finally classified into six typesup, down, charm, strange, top, and bottom quarks.

How were Quarks Named?

You might say that the discovery of quarks was revolutionarily cool, but their names, specifically, seem rather absurd (it might also be funny to some).

Honestly, scientists are quite weird when it comes to nomenclature, aren’t they (can you think of another example to support this accusation)?

Why Are Quarks Called ‘Quarks’?

For the naming of the elementary particle, Murray Gell-Mann initially came up with the sound “quork”.

He later landed upon a line from James Joyce’s novel “Finnegans Wake”, which was – “Three quarks for Muster Mark”. 

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Gell-Mann liked the word used by Joyce, as not only did it sound like Gell-Mann’s “quorks” but came with the word “three” (not to mention, quarks also come in three, at least in a proton). 

“Quarks” (as used by James Joyce in his writing) did not have any special meaning and was possibly a variation of the word “quarts”.

How were Quarks Named? 

As for the types of quarks, scientists had actually used the (English) letters ‘u’ and ‘d’ for the first two quarks. 

Since both of the quarks were opposite in charge, they named them up and down quarks.

Similarly, the letter ‘s’ was assigned to the third quark.

It “strangely” had a longer lifespan than up and down quark, and was hence named the strange quark.

The fourth quark i.e. the charm quark was named on a whim. 

It was because its mathematics, when applied, complemented the theory like a charm.

The two quarks, following the discovery of the other four, shared similar properties to that of up and down quarks and were hence named the top and bottom quarks.

Not much different than up-and-down quarks, they were just relatively heavy.

You know, as they say, inspirations come from anywhere, scientists have well implemented the saying as they named the types of quarks.

What Holds Quarks Together?

So far, we know that quarks (and antiquarks), in the Standard Model, combine to form hadronic particles.

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We also know that these quarks experience mutual repulsive forces (“like” charges repel), pushing one away from the other. 

So what is it that holds these quarks together?

Another way to phrase this question is – what holds the like-charged protons in place (as protons are, after all, made-up quarks)?   

Quarks, inside a hadron, are bound together by the strong nuclear force (also known as the strong force) – one of the four fundamental forces of nature.

Every fundamental force is the result of carrier particles called Bosons.

Therefore, the force-carrying particle responsible for carrying the strong nuclear force is the gluon (essentially a boson).

You might notice that the name ‘gluon’ is quite similar to ‘glue’.

Well, you are right if you do! 

The term was coined by Murray Gell-Mann, in 1962, as the gluons carry or mediate the strong nuclear force, which in turn holds the rumbling quarks together – like a glue

Why are Quarks Strange?

The world of tiny subatomic particles can be just as bizarre as it is colossal.

For a world that was quite adamant about the established elementariness of protons and neutrons in the first half of the 20th century, the discovery of quarks and the strangeness that it brought along was puzzling, to say the least.

One of many such bizarre facts about quarks was the quirkiness in the charge that they held.

Quarks, which held fractional charges, fascinated physicists because a charge less than that of an electron had never been observed before.

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Charge on Quarks

Each quark has a spin of 1/2 and a fractional electric charge, both of which are its intrinsic properties.

The up, top, and charm quarks are positively charged quarks that have a charge of 2e/3 each.

On the other hand, the down, bottom, and strange quarks are negatively charged quarks that (each) have a charge of -1e/3.

Remember, ‘e’ represents the magnitude of the charge on an electron, also called the elementary charge.

Electrons are transferred when something gets charged or drained (of charge).

The object being charged gains the electrons, while that being drained loses the electrons.

Since the transfer of charge is due to electrons, charges gained or lost by an object are integral multiples of the charge on electrons.

Thus, ‘e’ is called the elementary charge

You go through the same process when you buy a banana.

You might purchase one, two, or three bananas, but never half or three halves.

Similarly, one, two, or any other whole number of electrons might get transferred, but never in fractions.

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In simpler words, you can imagine that a charge can only exist as whole multiples of ‘e’ (like 1e, 2e, 3e, and so on), but never fractions (like 1e/2 or 3e/4).

But that is certainly not the rule that quarks obey, is it? 

This means that quarks do not follow the rule of charge quantization, which says that a charge can only exist as an integral multiple ofe’. 

Initially, it was weird because a charge less than that of an electron (or simply, a fractional charge) was never observed before; so physicists had a tough time wrapping their heads around it.

This contradiction to charge quantization is one of the most bizarre behaviors of quarks.

Well, since it seems to be a matter of mere designation, we can state the charge on a quark as the new discrete value or the elementary charge, right? 

No, this comes with two problems:

First, we would have to rework the entire physics and change various constants.

Second, we haven’t really observed the fractional charges yet.

Since we haven’t observed an isolated quark (Quark Confinement), we can’t exactly say that the charge on a quark is the elementary unit for the charge. 

Quarks can only exist in combined form, and their combined charge does account for an integral multiple of ‘e’.

So, if you have a proton, which is made up of two up and one down quarks, the total charge you’d have will be the sum of the charge on two up quarks and a down quark i.e. (4e/3) + (-1e/3) = 3e/3 = e (which is the charge on a proton).

Eventually, it all comes down to the net charge being an integral multiple of ‘e’, owing to the combination of quarks (that have fractional charges).

Why is Charge on a Quark an Integral Multiple of 1e/3?

The charge of a quark is perfectly an integral multiple of 1e/3.

Experiments suggest that quarks have a fractional charge of either -1e/3 or +2e/3, but why?

Well, we don’t yet have a perfect explanation for this but it is speculated to be due to the trade-off between electric charge and color charge.

Color Charge & Its Importance

Color charge is a property of quarks (and other elementary particles) that describes the strong nuclear interactions i.e. how gluons (particles that mediate the strong force) and quarks interact with each other, inside a hadron.

Note that it has nothing to do with the actual colors or charges.

Just like the electric charge, mass, and spin, color charge is also an intrinsic property of a particle. 

Quantum Chromodynamics (QCD) describes how quarks in hadron particles (like protons & neutrons) are held together by gluons.

Similar to electrical charges in quantum electrodynamics (QED), the theory of quantum chromodynamics consists of three color charges – red, green, and blue.

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It also consists of an anticolor for each color charge (similar to an antiparticle for every particle), which are – antired, antigreen, and antiblue, represented by the colors cyan, magenta, and yellow respectively.

Quarks (or antiquarks) combine in such a way that the resultant color charge is neutral (just as we add opposite electric charges, say +1 and -1, and get a neutral electrical charge).

For instance, a proton is made up of three quarks, which are red, green, and blue in “color” – combining to give a neutral-colored (or colorless) proton.

Again, it does not imply that the quarks are actually colored, it’s just that their (color) charges.

Similarly, a meson is made up of a quark and an antiquark, where the quark is of one of the three colors (let’s say, red), and the antiquark is of the matching anticolor (here, antired), thus giving a colorless meson. 

According to the Grand Unified Theory (or the Theory of Everything, as some might call it), all fundamental forces were unified during the big bang.

However, these unified forces began to disunite (or separate) and that was when quarks and leptons were created.

Because of the color charge, three quarks were created for each lepton, hence their charge is in multiples of 1/3.

Since a charge less than ‘e’ (i.e. 1e/3) does in fact exist, some even consider that 1e/3 is the quantum charge and that the elementary charge ‘e’ is three times the quantum charge. 

Nonetheless, there are still many gaps in the explanation of fractional charge on quarks, so we cannot entirely utilize this (or anyone) explanation.

The simplest non-scientific answer to this puzzle could be – it is what it is!

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Can Quarks be Separated Out? – Color Confinement

Elementary particles, such as protons and neutrons, are held together by a strong force.

So, to explore the particles hidden beyond the bounds of hadrons, we make these subatomic particles (like electrons, protons, and neutrons) collide with each other in large particle accelerators.

To get you an idea below is a picture of a typical lead-ion collision, where lead nuclei (containing protons and neutrons) are smashed into one another at colossal energies and speed:

Lead ion collision at Compact Muon Solenoid CMS particle detector at LHC CERN
A lead-ion collision at the Compact Muon Solenoid (CMS) particle detector at LHC, CERN. Credits: CERN Document Server

Some of the electrons scatter off sideways or often backward, which is only possible if there are much smaller particles present inside the proton.

If the proton was just an elementary blob, the electrons would never rebound like that.

So, it seemed possible to be able to knock a quark out of a proton, by firing electrons at it. 

However, none of the experiments, in various laboratories around the world, ever saw an isolated quark.

We could figure out the direction that the quark went in, by observing the scattered electrons. 

But instead of a quark, we see sprinkling jets of newer particles that seem to contain more quarks themselves.

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The reason behind this is that the energy required to knock off quarks out of protons (under the influence of the strong force) in turn creates more quarks (or even antiquarks), thus producing mesons (particles made up of a quark-antiquark pair), or even baryons (heavy hadrons made up of an odd number of quarks).

Hence, before we can observe an isolated quark, the new quarks created instantly bind up with the isolated (or existing) one to create new particles, which disallows us to observe an isolated quark.

It also makes it difficult to study or measure the properties (like mass) of quarks directly.

This idea that a free quark can never be isolated is called quark confinement (or just confinement), in quantum chromodynamics.

What Makes Up Quarks? Is There Anything Smaller?

Pictorial representation of size of quarks
Pictorial representation of the size of a quark. Credits: Fermilab

After all, how tiny can the matter be? Have we hit the rock bottom of the abyss of particles?

Yes(for now at least).

However, even after the discovery of quarks, point particles (hypothetical particles that are dimensionless, and take no space) called preons were conceived to be the subcomponents of quarks and leptons, in the 1980s.

The motivation behind the preon (or parton) model was to find a more fundamental explanation underlying the fascinating world of particle physics.

It was a ridiculously interesting attempt to surpass the achievements of the periodic table and the standard model. 

The interest in preons gradually faded away as no relevant experimental evidence was found to support their existence in the universe.

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For a long time, we have been satisfied with the Standard Model, which continues to describe particle physics most successfully.

After all these years of rigorous research in the field, we seem to have stopped at quarks; or have we?

Although physicists are constantly working on a handful of theories that have a promising future, we (as of today) know of nothing more fundamental, or smaller, than quarks.

Mind you, a quark is already too small to imagine.

Do We Know Everything About Quarks?

Quarks have prevalently been a hot topic of debate and have been well understood over the past 50 years, but physicists still don’t agree on a few explanations.

Something that they do agree on, though, is that there are no more (types of) quarks left to be discovered.

However, just like any other branch of physics, we don’t know what we don’t know about quarks.

They are obliged to behave in their own way and it doesn’t depend upon how much we know about them; that’s just how nature works after all.

Nonetheless, our advancements in the understanding of quarks and elementary particles have been astounding, and we’re open to a lot of surprises that are yet to come, for sure. 

Because as the saying goes – “The absence of evidence is not evidence of absence.”

We have had an extremely fascinating history of particle physics.

Call it strange, bizarre, or confusing, but finding the answers to these mysteries, for all these years, is what has brought us all together.

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Despite the absence of answers to many of the questions posed by the universe, we have an insatiable appetite for knowledge, which is constantly driving us ahead.

“Not only is the universe stranger than we think, it is stranger than we can think.”

Werner Heisenberg, German Theoretical Physicist

Recommendations

Devesh Sharma, ‘History of Particle Physics & the Standard Model‘, Evincism

Isolating quarks, Fermilab

Devesh Sharma, ‘New Exotic Particles Discovered at CERN by the LHCb‘, Evincism

Devesh Sharma, ‘Odd Muon G-2 Experiment Setbacks the Standard Model of Physics‘, Evincism

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Vikrant Singh, Amay Tripathi, Pranav Mahapatra