What is Lepton: Definition and 60 Discussions

In particle physics, a lepton is an elementary particle of half-integer spin (spin 1⁄2) that does not undergo strong interactions. Two main classes of leptons exist: charged leptons (also known as the electron-like leptons or muons), and neutral leptons (better known as neutrinos). Charged leptons can combine with other particles to form various composite particles such as atoms and positronium, while neutrinos rarely interact with anything, and are consequently rarely observed. The best known of all leptons is the electron.
There are six types of leptons, known as flavours, grouped in three generations. The first-generation leptons, also called electronic leptons, comprise the electron (e−) and the electron neutrino (νe); the second are the muonic leptons, comprising the muon (μ−) and the muon neutrino (νμ); and the third are the tauonic leptons, comprising the tau (τ−) and the tau neutrino (ντ). Electrons have the least mass of all the charged leptons. The heavier muons and taus will rapidly change into electrons and neutrinos through a process of particle decay: the transformation from a higher mass state to a lower mass state. Thus electrons are stable and the most common charged lepton in the universe, whereas muons and taus can only be produced in high energy collisions (such as those involving cosmic rays and those carried out in particle accelerators).
Leptons have various intrinsic properties, including electric charge, spin, and mass. Unlike quarks, however, leptons are not subject to the strong interaction, but they are subject to the other three fundamental interactions: gravitation, the weak interaction, and to electromagnetism, of which the latter is proportional to charge, and is thus zero for the electrically neutral neutrinos.
For every lepton flavor, there is a corresponding type of antiparticle, known as an antilepton, that differs from the lepton only in that some of its properties have equal magnitude but opposite sign. According to certain theories, neutrinos may be their own antiparticle. It is not currently known whether this is the case.
The first charged lepton, the electron, was theorized in the mid-19th century by several scientists and was discovered in 1897 by J. J. Thomson. The next lepton to be observed was the muon, discovered by Carl D. Anderson in 1936, which was classified as a meson at the time. After investigation, it was realized that the muon did not have the expected properties of a meson, but rather behaved like an electron, only with higher mass. It took until 1947 for the concept of "leptons" as a family of particles to be proposed. The first neutrino, the electron neutrino, was proposed by Wolfgang Pauli in 1930 to explain certain characteristics of beta decay. It was first observed in the Cowan–Reines neutrino experiment conducted by Clyde Cowan and Frederick Reines in 1956. The muon neutrino was discovered in 1962 by Leon M. Lederman, Melvin Schwartz, and Jack Steinberger, and the tau discovered between 1974 and 1977 by Martin Lewis Perl and his colleagues from the Stanford Linear Accelerator Center and Lawrence Berkeley National Laboratory. The tau neutrino remained elusive until July 2000, when the DONUT collaboration from Fermilab announced its discovery.Leptons are an important part of the Standard Model. Electrons are one of the components of atoms, alongside protons and neutrons. Exotic atoms with muons and taus instead of electrons can also be synthesized, as well as lepton–antilepton particles such as positronium.

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  1. ribella

    A Question about the decay of the W boson to tau lepton

    Why is generally the decay of the W boson to tau lepton not taken in the calculations?
  2. ohwilleke

    I LHC To Announce Its Lepton Universality Violation Results On Tuesday (20-Dec)

    Experimental results tending to show lepton universality violations (i.e. a different probability for decays to tau leptons, muons, and electron-positron pairs respectively, mass-energy conservation permitting) are the most notable experimental anomalies from Standard Model predictions...
  3. ohwilleke

    I New Lepton Universality Data To Be Announced Tuesday, 18 October 2021

    One of the Standard Model's rules is that charged leptons (i.e. the electron, muon and tau lepton) are identical to each other in their properties except for their masses (and that their anti-particles are identical to them except for a charge-parity flip). But, in two kinds of rare...
  4. ohwilleke

    I Latest Tau Lepton Measurement Consistent With Koide's Rule

    The latest tau lepton mass measurement, from Belle II is 1777.28 ± 0.75 (stat.) ± 0.33 (sys.) MeV/c^2. The combined error is ± 0.82 MeV/c^2 (which is 0.38 sigma greater the the Koide's rule prediction). This is consistent at a one sigma level with the current Particle Data Group world average...
  5. Helena Wells

    A If Neutrinos are majorana particles, does this mean lepton number is not conserved?

    If neutrinos are majorana particles does this mean that lepton number is not conserved in particle reactions? And I only noticed neutrinos are only produced when the decay of a particle to some other particles is carried by the W bosons ( weak interaction ). Is it possible the weak interaction...
  6. PeterDonis

    A Exploring Rotating Lepton Model for Hadron Masses

    I just came across a 2016 paper [1] that claims to have computed reasonably accurate masses for hadrons using what it calls a "rotating lepton model" and "the relativistic Newton equation". An earlier 2001 paper by two of the same authors [2] appears to be the first introduction of the general...
  7. D

    A Hints for lepton flavor violation in B-hadron decays

    Hi everyone, I have to write a paper (like an essay) where I have to discuss the hints for lepton flavor violation (LFV) in B-hadron decays from LHCb, BaBar, Belle experiments. I have found that there are (rare) B-decays, mostly B-mesons, forbidden in the SM that signal flavor lepton violation...
  8. G

    I Conservation of Lepton and Baryon numbers

    I am considerably confused about conservation laws like lepton number (L), baryon number (B) and comparable. Unlike the conservation laws for energy, momentum, angular momentum and electric charge, the conservations of L and B are not rigorously covered in textbooks. So my questions -...
  9. C

    Solving for X's Baryon & Lepton Number

    Homework Statement Homework EquationsThe Attempt at a Solution I worked out that the baryon number of X is 0 and the lepton number is +1 which means x is a lepton. However, when I work out the charge of X, do I add W+ to the left hand side or right hand side of the equation? [/B]
  10. C

    Can charged baryons flow induce magnetic field?

    We all think that electric current is the electrons flow without mass transfer in conductor, i.e. charged lepton flow. But charged baryons flow can also deemed as "electric" current, e.g. ionic current. My question is that charged baryons flow can induce magnetic field? Same amperes, then same B...
  11. H

    I Why is quark charge exactly 1/3 or 2/3 the lepton charge?

    Even with the charge screening effect it is exact. Is there an explanation in the standard model? Can the fact there are 3 families of particles be involved?
  12. K

    I Violation of lepton universality theories

    in the event violation of lepton universality is measured and established at 5-sigma level what are some theories that extend the standard model that would explain violation of lepton universality, while consistent in other SM measurements? how would violation of lepton universality impact...
  13. S

    I Uncovering the Hidden Lepton Number of the Universe

    What are the observational constraints on the net lepton number of universe?
  14. kiwaho

    A Photon number needless conservation, consolidation possible?

    We know lepton conservation law, that means multiple neutrinos can not be consolidate to big single neutrino. But photon is boson not lepton, no need of conservation, does that mean it is possible to combine or fuse a bunch of photons into ONE big photon, or say, more energetic photon, i.e...
  15. V

    A Transforming lepton basis to diagonlise charged lepton mass

    To determine the mass of charged leptons, we rotate such that the matrix of yukawa couplings (which gives the mass matrix after EWSB) is diagonal. We also call this flavour basis for neutrinos, because the flavoured neutrinos couple directly to the correspondong flavoured lepton in weak charged...
  16. W

    I Lepton species and anthropoics

    There seems to be many exotic species of lepton, like the muon neutrino and the tao neutrino. Is there any evidence that these species and other exotic particles are relevant to the existence of life. If they didn't exist would that have the terrible consequences that is implied by some for the...
  17. Behrouz

    What are the charges of quarks and leptons?

    Hello everyone, I'm sorry if it's a dumb question, I'm very new in self studying particle physics. May I ask when we associate a charge (i.e. positive or negative) to quark and lepton, is it only because of their attraction or repulsion toward each other? in other words, is it just to name one...
  18. C

    How do I determine if a certain nuclear decay is allowed?

    Hi, I am struggling with a question where they want me to determine whether or not three different decay are allowed. From what I have understood all decays must follow a set of conservation law. These laws are: 1 Conservation of Baryon number 2 Conservation of Lepton number 3 Conservation of...
  19. F

    What is the justification for the branching ratios of tau lepton decay?

    Homework Statement Branching ratios of tau lepton decay: hadrons 66% muon 17% electron 17% Use your knowledge of the decay of W bosons to justify these rates. Homework EquationsThe Attempt at a Solution [/B] The W+ boson can decay to any combination of quarks with +1 charge, but the CKM...
  20. terra

    SU(2) lepton doublet conjugation rules

    I have a left-handed ##SU(2)## lepton doublet: ## \ell_L = \begin{pmatrix} \psi_{\nu,L} \\ \psi_{e,L} \end{pmatrix}. ## I want to know its transformation properties under conjugation and similar 'basic' transformations: ##\ell^{\dagger}_L, \bar{\ell}_L, \ell^c_L, \bar{\ell}^c_L## and the general...
  21. S

    How to simulate protons on a computer program to observe them?

    Well I'm in grade 11 and for an investigation I decided I'd observe the behavior of protons once the neutrons and electrons are removed from the atom. Specifically I want to observe how the acceleration at which the protons move apart from each other varies with the mass of the atom (i.e. #...
  22. J

    Electrons, Muons and Pauli Exclusion

    Lepton Universality and Pauli Exclusion Put in a possibly oversimplified way, lepton universality says that electrons, muons, and taus all behave in the same way except for mass effects. The question is “Does this apply to Pauli exclusion?” Due to the Pauli exclusion principle, only two...
  23. BiGyElLoWhAt

    On a scale of 1 to String, how speculative is this?

    http://arxiv.org/abs/1503.09158 Is this a legitimite thing? I've never heard of 4th color charge or quark-lepton unification.
  24. M

    Is there a difference between an electron anti-neutrino and a positron neutrino?

    Is an electron anti neutrino and a positron neutrino the same thing?
  25. M

    Naming of the Leptons - Find the Right Word

    Hi, I am currently an undergraduate student and I am writing my undergraduate research paper right now. The thesis is about searching for heavy Majorana type neutrinos. While writing the paper, I really need some 'right word' but I can't find it so let me explain it here, and it would be very...
  26. U

    Psi Meson Decay Modes: Spin, Parity, Quark Content & More

    Homework Statement (a) Explain spin and parity of mesons (b) State their quark content (c) Draw a feynman diagram of J/psi decay (d) Why doesn't ##\chi## undergo leptonic decay? (e) What is the minimum centre of mass? [/B] Homework EquationsThe Attempt at a Solution Part(a)[/B] Spin is...
  27. U

    Tau leptonic decay - Lifetimes and modes

    Homework Statement [/B] (a) Explain lepton universality. (b) Explain why decay mode is forbidden and find hadronic branching ratios. (c) Find the lifetime of tau lepton. (d) What tau decay mode would be suitable? (e) Find the precision. (f) How do you improve the results? (g) Why is it much...
  28. M

    Quark-Lepton Symmetry: What Happens When Particles Annihilate?

    If I understand correctly, conservation of baryon and lepton numbers imply that quarks and leptons are "basic" i.e. non-interchangeable particles? What happens when one such particle is annihilated, can the energy produced be used to "generate" the other type, or do some additional particles...
  29. B

    Question about pre-CMB & Lepton Number

    I have a question about the interaction between photons and leptons prior to recombination I understand that once the threshold temperature for electrons was reached early in the universe' history, the vast majority of the leptons and antileptons annihiliated into photons. Except for...
  30. J

    Lepton Universality: Branching Ratios & Cross Sections

    I'd just like to clarify something I read on Wikipedia: "The branching ratio of the common purely leptonic tau decays are: 17.82% for decay into a tau neutrino, electron and electron antineutrino; 17.39% for decay into a tau neutrino, muon and muon antineutrino. The similarity...
  31. H

    Why is hydrogen stable but lepton pairs aren't?

    I probably knew the textbook answer to this at one time, but I don't recall. Feynman states in the beginning of Vol II of the FLP that electrons don't collide into the nucleus due to the uncertainty principle. But an electron positron pair will mutually annihilate. I suspect the answer has...
  32. lpetrich

    Quark and Lepton Mass Matrices, Textures, Horizontal Symmetries

    Does anyone have any good introduction to theories of the quark and lepton mass matrices? Theories like textures and horizontal symmetry. My understanding of research into textures is that it often involves trying to make zero as many entries as possible in the mass matrices. Is that a fair...
  33. A

    Kaon decay to a lepton and associated neutrino

    Homework Statement Draw the lowest-order Feynman diagram for the decay: K^+ \rightarrow l^+\nu_l Homework Equations Kaon structure is given as u\bar{s} The Attempt at a Solution So I know that you can get a lepton and an associated neutrino from a W decay, where the strange...
  34. R

    Exploring the Higgs Theory & Koide Formula for Lepton Masses

    I was wondering if Higgs theory offers an explanation for why the Koide formula gives such precise masses for the leptons? I would assume that any theory of mass would be able to predict the masses of particles, especially for ones where precise relations have been found between them like the...
  35. P

    Understanding the Yukawa Term in Srednicki's Lepton Sector

    Hi all, I am just reading Srednicki, chapter 88: The Standard Model: Lepton Sector and I'm not sure if I really understand it. There are left-handed Weyl fields l, \overline{e}, \varphi in the (SU(2), U(1)) representations (2, -1/2), (1,1), (2, -1/2) Now there is also a...
  36. S

    Weak interactions that change lepton flavor

    Are there any W^\pm or Z^0 interactions that change lepton flavor? For example, turn an electron into a muon or vice versa? Thanks!
  37. L

    Probabilities for lepton processes

    Homework Statement Compare the probabilities for the following processes to happen: 1. e^+ + e^- \to \nu_\mu + \bar{\nu}_\mu 2. e^+ + e^- \to \nu_\mu + \bar{\nu}_e 3. e^+ + e^- \to \nu_e + \bar{\nu}_e Homework Equations N/A The Attempt at a Solution I really have no idea...
  38. DuckAmuck

    Z Bosons to Lepton Pairs: Why Equal for All?

    Hi guys, Quick question: Why is the branching ratio of Z bosons into a lepton-antilepton pair equal for all leptons? Is it not dependent on mass? Thanks DA
  39. S

    Calculating Decay Widths & Estimating Partial Width of τ− Lepton

    i was'nt sure whether to put this here in the homework questions because this is'nt really homework as its for my understanding... However my particle physics exam is tommoro and I'm having some problems understanding how to calculate decay width's of decays. i have the following from a past...
  40. bcrowell

    Electroweak stars, nonconservation of baryon and lepton number

    "Electroweak stars: how nature may capitalize on the standard model's ultimate fuel," De-Chang Dai, Arthur Lue, Glenn Starkman, Dejan Stojkovic, http://arxiv.org/abs/0912.0520 Pretty far out, and I'm not sure I believe it. One thing that surprises me is that they claim the standard model has...
  41. R

    Understanding Lepton Number Conservation in Particle Interactions

    Hello there! May I ask: I am learning about the conservation of lepton number. I understand that the lepton number is composed of different sorts of lepton number, L_{\mu} and L_{\tau} and L_{e}. And these are conserved in all interactions. I am confused however about the...
  42. kelvin490

    Question about conservation of lepton and baryon

    I would like to ask what is mean by conservation of lepton and baryon? Does this imply conservation of mass? In addition, does this theory contradicts mass-energy conversion E=mc2?
  43. J

    Tau lepton decay problem - weird answer?

    Hi, I've got a tauon weak-decaying into a charged pion and a neutrino. The tauon has total energy \ E_{\tau} = 2.5GeV, rest mass \ m_{\tau} = 1.777GeV/c^2, and the pion has rest mass \ m_{\pi} = 0.1396GeV/c^2. This is everything I am told. I have to find the maximum energy the pion can have...
  44. J

    Is a Unified Lepton Field Possible?

    Is it reasonable to suggest the possibility, that there could be a one field, whose first excitations would be the electrons, second the myons, and third the taus?
  45. J

    Number of Lepton Generations in string theory

    Is the number of lepton generations in string theory restricted to 3? Or is this "landscape dependent"? I don't really see what could limit the number of eigenmodes on a continuous object like the string.
  46. S

    Can String Theory Predict the Mass of Leptons?

    Do we have prediction of Lepton mass (Electron, muon, tau) from any “Theory of everything” ? PS The string theory is here for decades. If string theory is truly a “theory of everything”, the Lepton mass should be predicted all ready. what is the status to this issue ?
  47. U

    Exploring Conservation of Lepton Number in Electron & Muon Neutrino Mixing

    Homework Statement In some theory the electron neutrinos and the muon neutrinos mix like this: \mathcal{L}_{m} &= -\frac{1}{2}m\left(\overline{ \nu^{C}_{\mu\text{R}} }\nu_{\text{eL}} + \overline{ \nu^{C}_{\text{eR}} }\nu_{\mu\text{L}}\right) + \text{h.c.} Show that there exists a...
  48. W

    What is the Lepton Current for Tau Lepton Decay in the Rest Frame?

    I have a question that asks me to calculate the lepton current for the decay of a tau lepton into the tau neutrino and a pion. It asks me to work in the rest frame of the tau lepton, taking the spin to be fully polarized in the +z direction. The lepton current is given by: j^{u} =...
  49. T

    CDF ZZ Event Display: Lepton 1 Ambiguity

    I was looking at this event display from CDF on their discovery of ZZ production. Why is lepton 1 having an ambiguity of either an electron or muon ? http://fcdfwww.fnal.gov/physics/ewk/2007/ZZ/evd/r211311_e233113.html
  50. A

    Lepton beta decay when m near m_W?

    lepton beta decay when m near m_W? Has anybody seen somewhere, article or textbook or website, the calculation of beta decay of a charged lepton in the generic case, when mass is neither a lot smaller nor a lot greater than the mass of the W? It should be some result interpolating between the...