Leptons: The elementary particles explained

 Neutrinos are leptons and are created during a supernova, here a graphic illustration of a supernova. Bright yellow and white streaks spread out from a central bright white point. Swirls of purple and blue fill the right side of the image and on the left a black backdrop with a handful of bright stars.
Neutrinos are a form of lepton. Most of the energy of a collapsing supernova is radiated in the form of neutrinos, produced when protons and electrons in the nucleus combine to form neutrons. (Image credit: Naeblys via Getty Images )

Leptons are elementary particles, which means that they are not made from any smaller particles. 

There are six known types of lepton (12 if you count their anti-particles). Three of these are negatively charged particles: electrons, muons and tau particles. The other three are neutrinos, which are electrically neutral. There is a corresponding neutrino for each type of charged lepton, so we have the electron neutrino, the muon neutrino and the tau neutrino.

Leptons are a crucial part of the Standard Model of particle physics. Electrons are important components of atoms, while neutrinos flood the universe and are produced by nuclear fusion reactions in stars as well as by particle decay.

Related: 10 mind-boggling things you should know about quantum physics

What is the Lepton Number?

An example of leptons involved in particle decay is the decay of a neutron. Neutrons are stable when bonded to other neutrons and protons in atomic nuclei, but when they are on their own outside of atomic nuclei they are unstable and decay after about 15 minutes (opens in new tab) into a proton, an electron and an anti-electron neutrino.

This decay reaction demonstrates a couple of the fundamental properties of leptons. First of all, it conserves a property known as the Lepton Number, which is defined by physicists at Georgia State University (opens in new tab) as the number of leptons minus the number of anti-leptons. A neutron is a baryon, not a lepton, so its Lepton Number is 0. Therefore its decay products must also add up to a Lepton Number of 0. The Lepton Number of an electron is 1, and the Lepton Number of an anti-neutrino is –1, hence they cancel and conserve the overall Lepton Number of the reaction.

Complicating things are the three families of leptons (electron and electron neutrinos, muons and muon neutrinos, and tau particles and tau neutrinos) and the rules of Lepton Numbers say they can't be mixed and matched. So, a neutron could never decay and produce an electron and an anti-muon neutrino, because they belong to different families of leptons. 

However, once an anti-electron neutrino is produced from the decay of a neutron, the neutrino itself can change flavor into a muon or tau neutrino. This is referred to as neutrino oscillation and is described by physicists in Stanford University's Neutrino Group (opens in new tab). Neutrino oscillations are the solution to the mystery of the solar neutrino problem (opens in new tab), where it seemed that only a third of the expected number of electron neutrinos from the sun were reaching Earth. It turned out that they weren't vanishing, but oscillating into muon and tau neutrinos on their way here.

Properties of leptons

The electron was the first lepton to be discovered, in 1897 by the British physicist Joseph John Thomson (opens in new tab). An electron has a rest mass energy of 0.511 MeV (opens in new tab) (Mega electron-Volt) (which equates to 9.1 x 10^–31 kilograms). Electrons are important components of atoms, orbiting the nucleus of an atom composed of protons and neutrons. An atom will have the same number of electrons as it does protons, ensuring that the

positive charges on the protons and the negative charges on the electrons cancel out to create an electrically neutral atom. Many chemical processes are related to the presence of these electrons in atoms.

British physicist Joseph John Thomson discovered the electron in 1897. He is photographed here in the Cavendish Laboratory, Cambridge, UK.  (Image credit: Photo12/Universal Images Group via Getty Images)
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Muons were discovered in 1936 (opens in new tab) by Carl Anderson and Seth Neddermeyer, who were performing experiments with cosmic rays from deep space (Anderson had already discovered the electron's anti-particle, the positron (opens in new tab), four years earlier). A cosmic ray is something of a misnomer — it's not a 'ray' but a particle of immensely high energy produced by violent processes in the universe such as quasars, supernovae and highly-magnetized supernova remnants. When cosmic rays enter Earth's atmosphere, they collide with atmospheric molecules and smash apart, producing a shower of daughter particles created in the collision. Muons are among these daughter particles, but they are short-lived, decaying back into an electron after just 2.2 millionths of a second. Fortunately, because they are traveling at almost the speed of light, they can reach the surface of the Earth before they decay, allowing scientists to detect them.

Muons are more massive than electrons; 207 times more massive to be precise, with a rest mass-energy of 105.7 MeV (opens in new tab) (equivalent to 1.9 x 10^–28 kg).

Tau particles were discovered by Martin Perl particle-accelerator experiments in 1975 (opens in new tab) and, like muons, are also only created in violent particle collisions. Tau particles are even more massive than muons, with a rest mass energy of 1,777 MeV (opens in new tab) (equating to 3.1x 10^–27 kg) meaning they are about 3,700 times more massive than an electron (opens in new tab). Like muons, tau particles are incredibly short-lived, decaying after just 29 trillionths of a second (opens in new tab). Literally, blink and you'll miss them, which is why they took much longer to be detected.

The name 'lepton' was coined in 1948 by physicist Léon Rosenfeld (opens in new tab) and the final lepton to be discovered, the tau neutrino was found as recently as the year 2000. No further leptons are predicted to exist in the Standard Model, although there has been some suggestion that there could be a hypothetical fourth type of neutrino called a sterile neutrino. The sterile neutrino is one possible explanation for the identity of dark matter. If sterile neutrinos really do exist, then they would be an indication of physics beyond the Standard Model.

The important difference between leptons and quarks

Leptons are fermions, which means that they have a spin of 1/2 (fermions have half-integer quantum spins, i.e. 1/2, or 3/2). Quarks — which are the building blocks of protons and neutrons that form the basis of atomic nuclei — are also fermions and elementary particles. So, is there any difference between leptons and quarks?

Yes, there is. Crucially, quarks are the only particles to experience all four fundamental forces: the strong nuclear force, the weak interaction, the electromagnetic force and gravity. On the other hand, leptons only experience three of them: the weak interaction, the electromagnetic force and gravity. 

The strong force is the glue that binds quarks together to form atomic nuclei. Because of this, no quark can exist in isolation. Because leptons don't feel the strong force, they are free to exist alone, outside of atoms, floating through space. Although muons and tau particles don't exist long enough before decaying via the weak interaction to make the most of their freedom, free electrons and neutrinos are key components of the particle universe. 

The four fundamental forces of nature are illustrated here from top-left clockwise, gravity, electromagnetic, strong and weak.  (Image credit: MARK GARLICK/SCIENCE PHOTO LIBRARY via Getty Images)
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Free electrons, for example, scatter photons. When the universe was very young and hot, space was swamped by free electrons that scattered light, meaning that photons couldn't travel any appreciable distance and the universe remained pretty dark. About 379,000 years after the Big Bang, the universe had cooled enough that atomic nuclei were able to join with electrons to form complete atoms of hydrogen and helium. With most of the free electrons being swept up that path was cleared to allow photons to finally travel through space unhindered. These first photons are what we see today as the cosmic microwave background (CMB) radiation that tells us so much about the very early universe and the Big Bang.

Of course, there are still many free electrons today; the energy of a photon impacting an atom can be enough to liberate an electron and 'ionize' the atom. Inside Earth's sun, where temperatures can reach 27 million degrees Fahrenheit (15 million degrees Celsius) in the core, such collisions happen all the time. Photons of energy generated in the sun's core by nuclear fusion reactions continually scatter off free electrons inside the sun's inner 'radiative zone' meaning that depending upon what assumptions you use in your calculations, they can take anywhere between 4,000 years (opens in new tab) and a million years (opens in new tab) to reach the surface of the sun and be emitted as light. As a result, the sunlight that we see is very old indeed!

Additional resources

Explore the Standard Model of particle physics in more detail with these resources from the Department of Energy (opens in new tab). Learn more about leptons with chemeurope.com (opens in new tab), a specialist portal for the chemical sector. Take a deeper dive into particle physics with this free online learning course by The Open University (opens in new tab).  

Bibliography

Particle Physics, by Brian R. Martin (2011, One-World Publications) 

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Keith Cooper
Contributing writer

Keith Cooper is a freelance science journalist and editor in the United Kingdom, and has a degree in physics and astrophysics from the University of Manchester. He's the author of "The Contact Paradox: Challenging Our Assumptions in the Search for Extraterrestrial Intelligence" (Bloomsbury Sigma, 2020) and has written articles on astronomy, space, physics and astrobiology for a multitude of magazines and websites.

  • rod
    I note this from the article.

    "This decay reaction demonstrates a couple of the fundamental properties of leptons. First of all, it conserves a property known as the Lepton Number, which is defined by physicists at Georgia State University(opens in new tab) as the number of leptons minus the number of anti-leptons. A neutron is a baryon, not a lepton, so its Lepton Number is 0. Therefore its decay products must also add up to a Lepton Number of 0. The Lepton Number of an electron is 1, and the Lepton Number of an anti-neutrino is –1, hence they cancel and conserve the overall Lepton Number of the reaction."

    It would be good to show in the BB model, when did the leptons appear.

    https://www.sciencedaily.com/releases/2022/09/220908172401.htm, "Summary: Early in its history, shortly after the Big Bang, the universe was filled with equal amounts of matter and 'antimatter' -- particles that are matter counterparts but with opposite charge. But then, as space expanded, the universe cooled. Today's universe is full of galaxies and stars which are made of matter. Where did the antimatter go, and how did matter come to dominate the universe? This cosmic origin of matter continues to puzzle scientists..."
    Reply
  • Classical Motion
    The quantum steps of the electron and proton are the same steps. Normally the electron occupies the low steps. The proton is at home in the high states. However these personalities can be inverted. An electron can be charged up to the high energy proton state, and the proton can be relaxed to the electron state.

    Science calls it antimatter. It's inverted matter. And that's why you don't see much of it. Antimatter is regular matter.

    Electrons and protons have the same rotational quantum RPMs. Spectrums. Because they have the same structure.

    The classic theory of mass. And the classic explanation of the periodic table.
    Reply
  • rod
    I found this on *inverted matter* using MS BING. "Matter (not people) that is from the opposite planet is called “inverse matter,” and inverse matter burns after a certain amount of time in contact with regular matter."

    I found this report on leptons. lepton, https://www.britannica.com/science/lepton
    Reply
  • Classical Motion
    Matter....mass...and inertia are distinct concepts. There are related, but not the same thing. Mass and inertia are properties of matter. There is only one form of matter, our science calls it charge. But know little about it. -e and +e is the only matter, the only physical entity in this cosmos. You may think of it as a set number of E field lines, or a specific density at a certain size. One may also consider it to be a constant length. However you happen to think of an E field.

    The area of space that this constant occupies, can be varied. THAT area, gives us the property of mass. When we squeeze or stretch that area......we change the mass of the matter. The matter remains constant and when we vary the density of that matter it is called mass. Mass is adjustable, matter is not. Inertia is the force necessary to accelerate mass. If mass is stationary, it takes a certain force to accelerate it. If the mass is moving with momentum, it will take more acceleration to change it. So it will have more inertia. Inertia depends on the motion of mass.

    Matter is a constant amount, mass is the density of matter, and inertia is the acceleration needed to change that motion of mass.

    Where does the motion come from? Why does matter move? Why and How does matter move?

    All -e and all +e have an internal electrical pressure which is powered by the self-repulsiveness of the e itself. e is always trying to explode out into space. e hates itself. It's trying to super nova. At c.

    This outward c acceleration of e, generates a common M field, because of the common outward e direction. This M field rolls and turns the little bits of e. The outward V of e, has been turned into a rotation of e at c. This expanding super nova has turned and rotated to a certain size. This happens at c. At c, M = E. And the charge spins, a self powered spin. By the conversion of linear V to angular V.

    This rotation has TWO spins, not one. The E is rotating and the M is rotating. In the right handed e(+e), the M and the E are rotating in the same direction. In this configuration, the mass adds, because the E and the M momentums are moving in the same direction. You will measure and experience high mass.

    In the left handed e(-e), the E and M are in opposite directions. The mass subtracts. The mass you measure is the difference in the directions......and measures very light. Almost no mass. But that is only apparent. Because the momentum is in opposition.

    Giving the left e an empty personality. Light and no energy or mass.

    And giving the right e, a full personality, heavy and lots of energy and mass.

    Two personalities with the same amount of matter.

    Physicality is much simpler than the standard model.
    Reply
  • rod
    Classical Motion said:
    Matter....mass...and inertia are distinct concepts. There are related, but not the same thing. Mass and inertia are properties of matter. There is only one form of matter, our science calls it charge. But know little about it. -e and +e is the only matter, the only physical entity in this cosmos. You may think of it as a set number of E field lines, or a specific density at a certain size. One may also consider it to be a constant length. However you happen to think of an E field.

    The area of space that this constant occupies, can be varied. THAT area, gives us the property of mass. When we squeeze or stretch that area......we change the mass of the matter. The matter remains constant and when we vary the density of that matter it is called mass. Mass is adjustable, matter is not. Inertia is the force necessary to accelerate mass. If mass is stationary, it takes a certain force to accelerate it. If the mass is moving with momentum, it will take more acceleration to change it. So it will have more inertia. Inertia depends on the motion of mass.

    Matter is a constant amount, mass is the density of matter, and inertia is the acceleration needed to change that motion of mass.

    Where does the motion come from? Why does matter move? Why and How does matter move?

    All -e and all +e have an internal electrical pressure which is powered by the self-repulsiveness of the e itself. e is always trying to explode out into space. e hates itself. It's trying to super nova. At c.

    This outward c acceleration of e, generates a common M field, because of the common outward e direction. This M field rolls and turns the little bits of e. The outward V of e, has been turned into a rotation of e at c. This expanding super nova has turned and rotated to a certain size. This happens at c. At c, M = E. And the charge spins, a self powered spin. By the conversion of linear V to angular V.

    This rotation has TWO spins, not one. The E is rotating and the M is rotating. In the right handed e(+e), the M and the E are rotating in the same direction. In this configuration, the mass adds, because the E and the M momentums are moving in the same direction. You will measure and experience high mass.

    In the left handed e(-e), the E and M are in opposite directions. The mass subtracts. The mass you measure is the difference in the directions......and measures very light. Almost no mass. But that is only apparent. Because the momentum is in opposition.

    Giving the left e an empty personality. Light and no energy or mass.

    And giving the right e, a full personality, heavy and lots of energy and mass.

    Two personalities with the same amount of matter.

    Physicality is much simpler than the standard model.
    "Physicality is much simpler than the standard model."

    So, does this new model (at least new to me) replace the standard model in use in physics today?
    Reply
  • Classical Motion
    This model is an updated model of the Parson's Magneton. It was studied and updated by a group of scientists that tried to corral plasma in a bowl. And the Standard Model and QM failed them. And still does. We still can't stir plasma in a pot.

    This updated model explained why they can not stir that pot. I have read that it was simulated and resulted in more accurate results than the standard model. It even predicted new spectrums and nuclei. Which were confirmed in the lab. And explained the source known atomic spectrums.

    The first model came out before the neutron was known. And if you look at the structure, it's easy to see why a neutron is needed. AND how a neutron is made.

    I have used this model to explain all dynamics I can think of for the last 8 years. And it has never failed me.

    -e and + e is all that is needed for the periodic table and this universe.

    Will it replace the standard model? I doubt it. This has no magic. It's straight electro-mechanical concepts. A 16 yr. old can understand it.

    Edit: This model predicts and physically explains the periodic table much more accurately than the standard model.
    Reply
  • Classical Motion
    What's neat about this model is that it explains the recently witnessed quick contraction of matter in fusion ignitions. This predicted property and dynamic was un-reconized for many decades. But this is the physical dynamic that explains the so called "quantum effect". This model explains how all the varied ratios, of the particle's properties, change, with a physical cause.

    But not realized yet......is that this also is the mechanism for emission from monopole emitters.

    And will lead to the true understanding of the dynamics, of dipole emission.
    Reply