What is a subatomic particle ?.
Even among particle physicists, the exact definition of a particle has diverse descriptions. These professional attempts at the definition of a particle include:
What is a subatomic particle ?
Wikipedia: List of unsolved problems in physics
The “Measurement” Problem: In the strange world of electrons, photons and other fundamental particles quantum mechanics is law. Particles do not behave like little bullets, but as waves spread over a large region. Each particle is described by a wave function that tells what its location, speed and other characteristics are more likely to be, but not what these properties are. The particle instead has countless opportunities for each, until one experimentally measures one of them - location, for example - then the particle wave function “collapses” and, apparently at random, a single well-defined position is observed. But how and why does a measurement on a particle make its wave function collapse, which in turn produces the concrete reality we perceive? This issue, the Measurement Problem in quantum physics, may seem esoteric, but our understanding of what reality is, or if it even exists, depends on the answer. Even worse: according to quantum physics it should be impossible to ever get a certain value for anything. It is characteristic of quantum physics that many different states coexist. The problem is that quantum mechanics is supposed to be universal, that is, should apply regardless of the size of the things we describe. Why then do we not see ghostly superpositions of objects even at our level? This problem is still unsolved. When can something be said to have happened at all? Without additional assumptions beyond quantum physics, nothing can ever happen! This is because the wave function mathematically is described by socalled linear equations, where states that have ever coexisted will do so forever. Despite this, we know that specific outcomes are entirely possible, and moreover happen all the time. Another strange thing is that the uncertainty in quantum physics arises only in the measurement. Before that, quantum mechanics is just as deterministic as classical physics, or even more so, because it is exactly linear and thus “simple”. Only when we understand how our objective macroscopic world arises from the ghostly microscopic world, where everything that is not strictly forbidden is compulsory, can we say that we truly know how nature really works.
The Matter-Antimatter Asymmetry: The question of why there is so much more matter than its oppositely charged mirror image, antimatter, is actually the crucial question of why anything exists at all. It is assumed that the universe when it is born treats matter and antimatter symmetrically. Thus, the Big Bang should have produced equal parts matter and antimatter. This should then have resulted in a total annihilation of the two: protons would have annihilated with antiprotons, electrons with antielectrons (positrons), neutrons with antineutrons, and so on, which would have left behind a structure-less sea of photons in a matterless void. For some reason there remained a tiny excess of matter that was not annihilated. And here we are. This has not yet any explanation. Because what we mean by matter is only a definition, we see that we could just as well have obtained a universe dominated by antimatter. But that in turn means that the answer to this riddle must contain a fundamental time direction, the universe cannot be run backwards because two completely different final states (matter or antimatter) would have arisen from the same initial state. A more complicated way of saying the same thing is that today’s most fundamental theory is CPT-invariant, i.e. the same when simultaneously changing the particle charges (C), mirrorreflecting their state (P) and re-versing the direction of time (T). And because we know that CP is broken for some reactions, this means that also T is broken, i.e., that the theory (or nature itself) is asymmetrical in the time direction. The problem is that the known CP-violation is far too weak and insignificant to explain why we today have only matter.
Ethan Siegel The Four Different Meanings Of 'Nothing' To A Scientist May 1, 2020
It's also one of the biggest puzzles in physics: if the laws of physics are such that we can only create matter and antimatter in equal amounts, how did we wind up with a Universe where every structure we see is made of matter and not antimatter? Every planet, star, and galaxy we've ever seen is known to be made of matter and not antimatter. So how, then, did we create an excess of these necessary raw ingredients if the Universe wasn't born with one? This is what is meant when you hear that the matter in our Universe arose from nothing. The origin of the matter-antimatter asymmetry — a puzzle known in the physics community as baryogenesis — is one of the greatest unsolved problems in physics today. Many ideas and mechanisms have been proposed and are theoretically plausible, but we do not yet know the answer. We don't know why there's something (more matter than antimatter) instead of nothing (equal amounts) at all.
The “Arrow of Time” and low entropy at the beginning of the universe: It is sometimes argued that time moves forward due to the fact that a property of the universe called entropy, defined as the degree of disorder, never decreases for a macroscopic system. There is thus no way to reverse an increase in the total entropy after it has occurred. The fact that the entropy increases is because there are many more disordered ways of arranging something than there are ordered ones, so when things change this tends to increase the disorder. But the underlying and unresolved question then becomes: why was the entropy so low in the past? In other words, why was the universe so ordered in the beginning, when a huge amount of energy was contained in a very small space? We have merely replaced one mystery with an at least equally great. As mentioned above, it seems that even microscopically there is a very small asymmetry between time forwards and backwards, because of the measured CP-violation in the weak nuclear interaction. But this symmetry breaking is far too weak to explain the time arrow and also only operates on extremely short length scales, mainly inside atomic nuclei. Maybe even time, as we so far have described it in our theories, is really just an illusion
https://www.diva-portal.org/smash/get/diva2%3A979253/FULLTEXT01.pdf
Gordon J. Aubrecht, II The newest Standard Model Chart from the Contemporary Physics Education Project 2 de octobre de 2016
Is there just a single Higgs particle? How will we see the physics beyond the Standard Model? Why is the universe accelerating? What happened to the original (assumed equal) amount of antimatter in the universe? What is the origin of mass? How will quantum mechanics and gravitation finally be reconciled? How can we be certain of the origin of dark matter? Why does the range of masses in the universe span from 10-31 kg to 1035 kg? Are the additional dimensions in Kaluza-Klein-type theories physically realizable? Are there really no magnetic monopoles in the universe? How does neutrino mass arise?
http://www.lajpe.org/dec16/4303_Aubrecht_2016.pdf
A large number of subatomic particles exist in nature. These particles can be classified in two ways: the property of spin and participation in the four fundamental forces. Recall that the spin of a particle is analogous to the rotation of a macroscopic object about its own axis. Particles of matter can be divided into fermions and bosons. Fermions have half-integral spin ⎛ ⎝ 1 2 ℏ, 3 2 ℏ, … ⎞ ⎠ and bosons have integral spin (0ℏ, 1ℏ, 2ℏ, …). Familiar examples of fermions are electrons, protons, and neutrons. A familiar example of a boson is a photon. Fermions and bosons behave very differently in groups. For example, when electrons are confined to a small region of space, Pauli’s exclusion principle states that no two electrons can occupy the same quantum-mechanical state. However, when photons are confined to a small region of space, there is no such limitation. The behavior of fermions and bosons in groups can be understood in terms of the property of indistinguishability. Particles are said to be “indistinguishable” if they are identical to one another. For example, electrons are indistinguishable because every electron in the universe has exactly the same mass and spin as all other electrons—“when you’ve seen one electron, you’ve seen them all.” If you switch two indistinguishable particles in the same small region of space, the square of the wave function that describes this system and can be measured ⎛ ⎝|ψ| 2⎞ ⎠ is unchanged. If this were not the case, we could tell whether or not the particles had been switched and the particle would not be truly indistinguishable. Fermions and bosons differ by whether the sign of the wave function ( ψ )— not directly observable—flips
Cody Cottier Bosons, Fermions and Anyons: What Are the Three Particle Kingdoms in the Quantum World? May 12, 2021
The totality of existence can be divided into these categories, each with a vital role in the structure of the universe. Every single particle fits into one of just three classes, or kingdoms: bosons, fermions and anyons, the latter just discovered in the past year. You can think of these groups as akin to the taxonomic tiers of organic life, each as different from the others as plants are from animals and bacteria.
All of observable reality arises from this trio of building blocks and their peculiarities — the standoffish fermions, the gregarious bosons and the eccentric anyons come with enormous implications for the ordering of the cosmos, and for human technology. But just how do these basic ingredients produce the stunning diversity of substance and phenomenon we see around us, not to mention the exotic behavior most of us never see?
Two Traditional Kingdoms
For centuries scientists were puzzled by an apparent dualism in nature, between matter and light. In the early 20th century, quantum mechanics finally consolidated the two realms by showing that both an electron and a photon are subject to the same mathematical equations. Under certain circumstances, each can behave like either a particle or a light wave.
“It was a great advance to realize that different kinds of particles … actually have a unified description,” says Frank Wilczek, a Nobel prize-winning physicist at the Massachusetts Institute of Technology. But this unification applies only to individual particles — as soon as you let two or more of them mingle, their collective conduct reveals a different subatomic division, between bosons and fermions.
Bosons — named for Indian physicist Satyendra Nath Bose — are the conformists of the particle world, and photons their poster child. Technically speaking, they “show an enhanced probability to be in the same quantum state,” Wilczek says. “More colloquially,” he adds, “you might say they like to do the same thing.” Think of a laser beam: It’s made of countless photons all moving in the same direction, exhibiting the same color. They cooperate, in a sense.
Fermions — named for Italian physicist Enrico Fermi — on the other hand, are antisocial. They refuse to occupy the same quantum state, or, to extend the analogy, they don’t like to do the same thing. This is the essence of the Pauli exclusion principle, which finds its epitome in electrons. Because no two can exist in the same state, they are forced into the various shells around their atoms. This restricted arrangement produces all the elements in the periodic table, along with their dazzling chemical properties.
In fact, this repulsion is the reason atoms don’t collapse, the reason matter is hard. Each time we take a step, it prevents us falling through the Earth. Similarly, at the macroscopic level, it’s what keeps white dwarves — the burnt-out, shrunken cores of stars like our sun — from crashing in on themselves. This inherent resistance of electrons “plays a very important role in the universe, both at the atomic scale and in these astrophysical contexts,” Wilczek says. Without it, everything would rapidly devolve into “a structureless goo.”
The Particle Family Tree
Fermions are generally thought of as particles of matter: the quarks, which combine to form protons and neutrons; and the leptons, which include electrons, the lesser-known muons and taus, and the nearly massless neutrinos, which only rarely interact with other matter.
Bosons, on the other hand, are the “force-carrying” particles: the photons, the gluons, and the Z and W bosons. Fermions exchange these between each other to generate, respectively, the electromagnetic, strong and weak forces. (There is presumably a graviton for gravitation, but it has yet to be detected). Lastly, the Higgs boson does not represent a force, per se, but it is what imbues the fermions with mass.
But the above distinction — between matter and force — is a bit of an oversimplification. Under certain circumstances, fermions can even become bosons (this is how superfluids form). The crucial difference between the two is actually their spin, or angular momentum. Bosons all have an integer spin, of 0, 1 or 2, while fermions have half-integer spins, like 1/2 or 3/2.
'Anything Goes'
Until recently, bosons and fermions were the only proven classes of particle, both inhabiting the three-dimensional space we know and love. But mathematically, there was always another possibility: particles confined to two dimensions. They’re called anyons, and strictly speaking they are quasiparticles, or emergent particles — excitations that emerge from the activity of other particles in a material. You’ll never find them outside the material, on their own. They don’t fit the mould of electrons and photons.
“But what is a particle, after all?” Wilczek muses. “It’s a concentration of energy that has a certain integrity to it, that can move around, that has reproducible properties,” and anyons meet those criteria as well as any other particle. “They are how energy organizes itself into units. If you lived inside the material and didn’t know any better, you would call them particles.” It’s hardly stranger than to call an atom or proton by that name, since both are composed of more elementary units.
Though the existence of anyons wasn’t confirmed until last year, Wilczek and his colleagues predicted it in the 1980s. He named them as a cheeky reference to the fact that, when it comes to these bizarre denizens of the quantum world, “anything goes.” They neither fall in line nor avoid each other completely, like particles in the other kingdoms. In technical terms, this is because of their respective wave functions, the mathematical formulas used to describe a particle’s quantum state. When two bosons loop around each other, their initial and final wave functions are identical — nothing has changed. Repeating the exercise with fermions, the wave functions will be off by a factor of -1. With anyons, Wilczek says, “You can get, roughly speaking, anything.” The behavior of particles in the traditional kingdoms is rigid by comparison. “Whereas bosons and fermions are just one thing, in a sense,” Wilczek says, “anyons is a much bigger family, conceptually.”
Coupling constant
A coupling constant (or an interaction constant) is a parameter in the field theory, which determines the relative strength of interaction between particles or fields.
https://en.wikiversity.org/wiki/Coupling_constant
Bosons and Fermions
https://www.theguardian.com/science/life-and-physics/2011/aug/13/1
Particles, Patterns, and Conservation Laws
https://courses.lumenlearning.com/physics/chapter/33-4-particles-patterns-and-conservation-laws/#footnote-6796-1
Even among particle physicists, the exact definition of a particle has diverse descriptions. These professional attempts at the definition of a particle include:
- A particle is a collapsed wave function
- A particle is a quantum excitation of a field
- A particle is an irreducible representation of the Poincaré group
- A particle might be a vibrating string
- A particle is a thing measured in a detector
What is a subatomic particle ?
Wikipedia: List of unsolved problems in physics
The “Measurement” Problem: In the strange world of electrons, photons and other fundamental particles quantum mechanics is law. Particles do not behave like little bullets, but as waves spread over a large region. Each particle is described by a wave function that tells what its location, speed and other characteristics are more likely to be, but not what these properties are. The particle instead has countless opportunities for each, until one experimentally measures one of them - location, for example - then the particle wave function “collapses” and, apparently at random, a single well-defined position is observed. But how and why does a measurement on a particle make its wave function collapse, which in turn produces the concrete reality we perceive? This issue, the Measurement Problem in quantum physics, may seem esoteric, but our understanding of what reality is, or if it even exists, depends on the answer. Even worse: according to quantum physics it should be impossible to ever get a certain value for anything. It is characteristic of quantum physics that many different states coexist. The problem is that quantum mechanics is supposed to be universal, that is, should apply regardless of the size of the things we describe. Why then do we not see ghostly superpositions of objects even at our level? This problem is still unsolved. When can something be said to have happened at all? Without additional assumptions beyond quantum physics, nothing can ever happen! This is because the wave function mathematically is described by socalled linear equations, where states that have ever coexisted will do so forever. Despite this, we know that specific outcomes are entirely possible, and moreover happen all the time. Another strange thing is that the uncertainty in quantum physics arises only in the measurement. Before that, quantum mechanics is just as deterministic as classical physics, or even more so, because it is exactly linear and thus “simple”. Only when we understand how our objective macroscopic world arises from the ghostly microscopic world, where everything that is not strictly forbidden is compulsory, can we say that we truly know how nature really works.
The Matter-Antimatter Asymmetry: The question of why there is so much more matter than its oppositely charged mirror image, antimatter, is actually the crucial question of why anything exists at all. It is assumed that the universe when it is born treats matter and antimatter symmetrically. Thus, the Big Bang should have produced equal parts matter and antimatter. This should then have resulted in a total annihilation of the two: protons would have annihilated with antiprotons, electrons with antielectrons (positrons), neutrons with antineutrons, and so on, which would have left behind a structure-less sea of photons in a matterless void. For some reason there remained a tiny excess of matter that was not annihilated. And here we are. This has not yet any explanation. Because what we mean by matter is only a definition, we see that we could just as well have obtained a universe dominated by antimatter. But that in turn means that the answer to this riddle must contain a fundamental time direction, the universe cannot be run backwards because two completely different final states (matter or antimatter) would have arisen from the same initial state. A more complicated way of saying the same thing is that today’s most fundamental theory is CPT-invariant, i.e. the same when simultaneously changing the particle charges (C), mirrorreflecting their state (P) and re-versing the direction of time (T). And because we know that CP is broken for some reactions, this means that also T is broken, i.e., that the theory (or nature itself) is asymmetrical in the time direction. The problem is that the known CP-violation is far too weak and insignificant to explain why we today have only matter.
Ethan Siegel The Four Different Meanings Of 'Nothing' To A Scientist May 1, 2020
It's also one of the biggest puzzles in physics: if the laws of physics are such that we can only create matter and antimatter in equal amounts, how did we wind up with a Universe where every structure we see is made of matter and not antimatter? Every planet, star, and galaxy we've ever seen is known to be made of matter and not antimatter. So how, then, did we create an excess of these necessary raw ingredients if the Universe wasn't born with one? This is what is meant when you hear that the matter in our Universe arose from nothing. The origin of the matter-antimatter asymmetry — a puzzle known in the physics community as baryogenesis — is one of the greatest unsolved problems in physics today. Many ideas and mechanisms have been proposed and are theoretically plausible, but we do not yet know the answer. We don't know why there's something (more matter than antimatter) instead of nothing (equal amounts) at all.
The “Arrow of Time” and low entropy at the beginning of the universe: It is sometimes argued that time moves forward due to the fact that a property of the universe called entropy, defined as the degree of disorder, never decreases for a macroscopic system. There is thus no way to reverse an increase in the total entropy after it has occurred. The fact that the entropy increases is because there are many more disordered ways of arranging something than there are ordered ones, so when things change this tends to increase the disorder. But the underlying and unresolved question then becomes: why was the entropy so low in the past? In other words, why was the universe so ordered in the beginning, when a huge amount of energy was contained in a very small space? We have merely replaced one mystery with an at least equally great. As mentioned above, it seems that even microscopically there is a very small asymmetry between time forwards and backwards, because of the measured CP-violation in the weak nuclear interaction. But this symmetry breaking is far too weak to explain the time arrow and also only operates on extremely short length scales, mainly inside atomic nuclei. Maybe even time, as we so far have described it in our theories, is really just an illusion
https://www.diva-portal.org/smash/get/diva2%3A979253/FULLTEXT01.pdf
Gordon J. Aubrecht, II The newest Standard Model Chart from the Contemporary Physics Education Project 2 de octobre de 2016
Is there just a single Higgs particle? How will we see the physics beyond the Standard Model? Why is the universe accelerating? What happened to the original (assumed equal) amount of antimatter in the universe? What is the origin of mass? How will quantum mechanics and gravitation finally be reconciled? How can we be certain of the origin of dark matter? Why does the range of masses in the universe span from 10-31 kg to 1035 kg? Are the additional dimensions in Kaluza-Klein-type theories physically realizable? Are there really no magnetic monopoles in the universe? How does neutrino mass arise?
http://www.lajpe.org/dec16/4303_Aubrecht_2016.pdf
A large number of subatomic particles exist in nature. These particles can be classified in two ways: the property of spin and participation in the four fundamental forces. Recall that the spin of a particle is analogous to the rotation of a macroscopic object about its own axis. Particles of matter can be divided into fermions and bosons. Fermions have half-integral spin ⎛ ⎝ 1 2 ℏ, 3 2 ℏ, … ⎞ ⎠ and bosons have integral spin (0ℏ, 1ℏ, 2ℏ, …). Familiar examples of fermions are electrons, protons, and neutrons. A familiar example of a boson is a photon. Fermions and bosons behave very differently in groups. For example, when electrons are confined to a small region of space, Pauli’s exclusion principle states that no two electrons can occupy the same quantum-mechanical state. However, when photons are confined to a small region of space, there is no such limitation. The behavior of fermions and bosons in groups can be understood in terms of the property of indistinguishability. Particles are said to be “indistinguishable” if they are identical to one another. For example, electrons are indistinguishable because every electron in the universe has exactly the same mass and spin as all other electrons—“when you’ve seen one electron, you’ve seen them all.” If you switch two indistinguishable particles in the same small region of space, the square of the wave function that describes this system and can be measured ⎛ ⎝|ψ| 2⎞ ⎠ is unchanged. If this were not the case, we could tell whether or not the particles had been switched and the particle would not be truly indistinguishable. Fermions and bosons differ by whether the sign of the wave function ( ψ )— not directly observable—flips
Cody Cottier Bosons, Fermions and Anyons: What Are the Three Particle Kingdoms in the Quantum World? May 12, 2021
The totality of existence can be divided into these categories, each with a vital role in the structure of the universe. Every single particle fits into one of just three classes, or kingdoms: bosons, fermions and anyons, the latter just discovered in the past year. You can think of these groups as akin to the taxonomic tiers of organic life, each as different from the others as plants are from animals and bacteria.
All of observable reality arises from this trio of building blocks and their peculiarities — the standoffish fermions, the gregarious bosons and the eccentric anyons come with enormous implications for the ordering of the cosmos, and for human technology. But just how do these basic ingredients produce the stunning diversity of substance and phenomenon we see around us, not to mention the exotic behavior most of us never see?
Two Traditional Kingdoms
For centuries scientists were puzzled by an apparent dualism in nature, between matter and light. In the early 20th century, quantum mechanics finally consolidated the two realms by showing that both an electron and a photon are subject to the same mathematical equations. Under certain circumstances, each can behave like either a particle or a light wave.
“It was a great advance to realize that different kinds of particles … actually have a unified description,” says Frank Wilczek, a Nobel prize-winning physicist at the Massachusetts Institute of Technology. But this unification applies only to individual particles — as soon as you let two or more of them mingle, their collective conduct reveals a different subatomic division, between bosons and fermions.
Bosons — named for Indian physicist Satyendra Nath Bose — are the conformists of the particle world, and photons their poster child. Technically speaking, they “show an enhanced probability to be in the same quantum state,” Wilczek says. “More colloquially,” he adds, “you might say they like to do the same thing.” Think of a laser beam: It’s made of countless photons all moving in the same direction, exhibiting the same color. They cooperate, in a sense.
Fermions — named for Italian physicist Enrico Fermi — on the other hand, are antisocial. They refuse to occupy the same quantum state, or, to extend the analogy, they don’t like to do the same thing. This is the essence of the Pauli exclusion principle, which finds its epitome in electrons. Because no two can exist in the same state, they are forced into the various shells around their atoms. This restricted arrangement produces all the elements in the periodic table, along with their dazzling chemical properties.
In fact, this repulsion is the reason atoms don’t collapse, the reason matter is hard. Each time we take a step, it prevents us falling through the Earth. Similarly, at the macroscopic level, it’s what keeps white dwarves — the burnt-out, shrunken cores of stars like our sun — from crashing in on themselves. This inherent resistance of electrons “plays a very important role in the universe, both at the atomic scale and in these astrophysical contexts,” Wilczek says. Without it, everything would rapidly devolve into “a structureless goo.”
The Particle Family Tree
Fermions are generally thought of as particles of matter: the quarks, which combine to form protons and neutrons; and the leptons, which include electrons, the lesser-known muons and taus, and the nearly massless neutrinos, which only rarely interact with other matter.
Bosons, on the other hand, are the “force-carrying” particles: the photons, the gluons, and the Z and W bosons. Fermions exchange these between each other to generate, respectively, the electromagnetic, strong and weak forces. (There is presumably a graviton for gravitation, but it has yet to be detected). Lastly, the Higgs boson does not represent a force, per se, but it is what imbues the fermions with mass.
But the above distinction — between matter and force — is a bit of an oversimplification. Under certain circumstances, fermions can even become bosons (this is how superfluids form). The crucial difference between the two is actually their spin, or angular momentum. Bosons all have an integer spin, of 0, 1 or 2, while fermions have half-integer spins, like 1/2 or 3/2.
'Anything Goes'
Until recently, bosons and fermions were the only proven classes of particle, both inhabiting the three-dimensional space we know and love. But mathematically, there was always another possibility: particles confined to two dimensions. They’re called anyons, and strictly speaking they are quasiparticles, or emergent particles — excitations that emerge from the activity of other particles in a material. You’ll never find them outside the material, on their own. They don’t fit the mould of electrons and photons.
“But what is a particle, after all?” Wilczek muses. “It’s a concentration of energy that has a certain integrity to it, that can move around, that has reproducible properties,” and anyons meet those criteria as well as any other particle. “They are how energy organizes itself into units. If you lived inside the material and didn’t know any better, you would call them particles.” It’s hardly stranger than to call an atom or proton by that name, since both are composed of more elementary units.
Though the existence of anyons wasn’t confirmed until last year, Wilczek and his colleagues predicted it in the 1980s. He named them as a cheeky reference to the fact that, when it comes to these bizarre denizens of the quantum world, “anything goes.” They neither fall in line nor avoid each other completely, like particles in the other kingdoms. In technical terms, this is because of their respective wave functions, the mathematical formulas used to describe a particle’s quantum state. When two bosons loop around each other, their initial and final wave functions are identical — nothing has changed. Repeating the exercise with fermions, the wave functions will be off by a factor of -1. With anyons, Wilczek says, “You can get, roughly speaking, anything.” The behavior of particles in the traditional kingdoms is rigid by comparison. “Whereas bosons and fermions are just one thing, in a sense,” Wilczek says, “anyons is a much bigger family, conceptually.”
Coupling constant
A coupling constant (or an interaction constant) is a parameter in the field theory, which determines the relative strength of interaction between particles or fields.
https://en.wikiversity.org/wiki/Coupling_constant
Bosons and Fermions
https://www.theguardian.com/science/life-and-physics/2011/aug/13/1
Particles, Patterns, and Conservation Laws
https://courses.lumenlearning.com/physics/chapter/33-4-particles-patterns-and-conservation-laws/#footnote-6796-1
