Questions tagged [leptons]

Leptons are elementary fermions that do not experience the strong interaction: electrons, muons, tauons, and neutrinos.

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Is Lepton Flavour Universality an accidental symmetry of the Standatd Model?

Is Lepton Flavour Universality an accidental symmetry of the Standatd Model? If it is, why? How does it emerge from the Standard Model?
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Could the equal amounts of positive and electric charge point to equal amounts of matter and anti-matter?

The universe is electrically neutral because the electric charges of all quarks and leptons cancel. If particles are electrically charged, then there are equal amounts of positive and negative ...
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Anomalous magnetic moment form factor of $e^-,\mu^-$ and $\tau^-$ in QED

The anomalous magnetic moment form factor $F(q^2)$ of an elementary fermion at $q^2=0$, calculated at one-loop, is $$F_2(0)=\frac{\alpha}{2\pi}.$$ At least at this order (order-$\alpha$), $F_2(0)$ ...
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Is leptons annihilation one process of 4 particles simultaneously or two independent processes of 3 particles? [duplicate]

Why leptons annihilation is not 4-particles, and it is represented in the form of two 3-particles? If the pair of an electron and a positron has formed a positronium particle, then what is virtual ...
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Conservation of energy in nuclear physics

In terms of whether a nuclear process is "allowed" or not, is conservation of energy relevant? If lepton number, charge, colour and baryon number are conserved, and there exists a vertex ...
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Fundamental reason for the color and flavor group to be the same?

The answer to this question might just be a straight-up "no, it's just a coincidence", but since coincidences are rarely a thing in physics, I thought I'd ask. Is there a fundamental reason ...
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Does Lepton flavour universality arise natural in the SM

I always thought LFU is stated by the SM , but I recently read that it was just assumed because measurements suggested this universality. If thats true, proving LFU violation would not automatically ...
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Is the electromagnetic coupling constant of the muon exactly the same value with the electron $α \approx1/137$?

I am asking because I want to know If I can use the Schwinger equation: $$a_{e}=\frac{\alpha}{2 \pi} \approx 0.0011614$$ to calculate the one-loop QED contribution to the anomalous magnetic moment of ...
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Why do we say lepton flavour violation is against the standard model if no symmetries are associated with lepton flavour?

Or if a symmetry does exist what is it? If possible could you compare this to a symmetry that leads to a conservation?
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The effect of the fifth force of nature being detected?

The article https://www.sciencealert.com/physicists-may-have-discovered-new-force-of-nature-in-lhc-experiment speculates that possibly the effect of the fifth (so far unknown) force of nature has been ...
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Explanation for exact amounts ratio between up,down quarks and electrons in a universe

As I've understood in the quark era $\approx (10^{-12} - 10^{-6}) \,\text{s}$ unified interaction has decoupled into Leptons and Quarks. Given that now Hydrogen+Helium atoms accounts for $97.9\%$ of ...
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Why is the hadronic $\tau$ branching ratio into 2 pions prefered?

In the PDG the $\tau$ branching ratios are listed and Br$(\tau^- \rightarrow \pi^-\pi^0 \nu_\tau)\approx 25$ %, while Br$(\tau^- \rightarrow \pi^-\nu_\tau)\approx 11$ %. How can this be since the ...
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Higgs Mechanism Confusion

I find myself confused about the Higgs mechanism, but more specifically about how the leptons get their mass. The intuitive picture I have in my head is that since the leptons are interacting with the ...
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If neutrinos were Majorana fermions, then what would be the lepton number of the neutrino?

Majorana particles are their own antiparticles and the lepton number of antimatter is -1 whereas for matter is +1, so if neutrinos were Majorana fermions, then what would be the lepton number of the ...
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How could Leptoquarks explain Lepton Flavour universality (LFU) violation?

I recently read about the possibility of Leptoquarks and that this new particle could also explain a possible LFU violation. Why would introducing a new particle explain LFU violation?
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Lepton Universality

Does universality of lepton also mean that two lepton families have the same renormalized coupling if they interact with a particular particle?
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Conservation of lepton number in neutrino oscillation

I was wondering how neutrino oscillations are possible and lepton number is conserved, as for example in the decay of an anti-muon: $$\mu^+ = e^+ \nu_e \bar{\nu_{\mu}}$$ all lepton numbers are ...
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How is this Yukawa coupling term invariant?

I have the seen the following term used to describe Yukawa coupling for the lepton and Higgs field: $$ \epsilon_{ij}\phi^{i}\bar{e}_{R}f_{L}^{j} $$ Under $SU(2) \otimes U(1)$ and so expected the ...
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Feynman diagram for the decay $\tau \rightarrow \nu_{\tau}\pi^+\pi^-\pi^-$

$$\tau \rightarrow \nu_{\tau}\pi^+\pi^-\pi^-$$ This is a possible decay mode of the $\tau$ lepton (see Wikipedia). Given that a neutrino is present this must be the weak interaction and therefore we ...
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Is There A Reason lepton flavor violation (LFV) In $B^+$ Meson Decays Can't Be Explained By $K+πº$ Decays?

Physicists at Moriond 2021 reported New results on theoretically clean observables in rare $B$-meson decays from LHCb 3.1 sigma away from the SM prediction of lepton universality in the $B+→K+μ+μ- $vs....
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What is $a_\mu$ in particle physics? In terms of a muon; and its magnetic moment? What is, '$a_\mu$ × $10^9$ - 1165900 '?

What does $a_\mu × 10^9 - 1165900$ mean in the muon $g-2$ experiment at Fermilab?
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Is it possible that rescuing the Idea of a Baryo/lepto-dynamic Field had some Relevance in the FermiLab Result?

We know that Fermilab, Brookhaven, and other laboratories have found discrepancies with the SM. One option that almost immediately comes to mind is that the so-far exact conservation law of baryon and ...
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Review of experiment $g-2$ of the muon

This week a new development was announced to measure the pressure of the muon magnetic moment and the variation of its result on the theoretical value. The question is: what is the meaning of this ...
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Could the muon's shape be attributing to our measurements of the $g$ term for the magnetic moment?

I read that the $g$ term in the magnetic moment is being studied carefully these days in muons. I was wondering if the $L$ term was also being studied? I know that electrons have been found to be ...
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Does the Higgs field interact differently with muons compared to electrons?

Muons decay into other particles. Does the Higgs field know that muons will decay and does it interact with them differently compared to how it interacts with electrons?
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Measure cosmic ray pions and protons below surface of the Earth?

In my particle physics lecture, the prof asserted that apparently cosmic ray muons can still be measured 1km below the surface of the earth (e.g. in a mine). This led me to ask the following two ...
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Is it accidental or deeply meaningful that the Higgs and the left-handed lepton have the same quantum numbers?

Is it accidental or deeply meaningful that the Higgs and the left-handed lepton have the same quantum numbers? Precisely, under $SU(3) \times SU(2) \times U(1)$: the Higgs has the quantum numbers: ...
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8 votes
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Why is the conservation of lepton number a thing?

Since there is no way to tell neutrinos and antineutrinos apart, does conservation of lepton number make any sense? As far as we can tell, the neutrinos and antineutrinos are indistinguishable so from ...
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Why is this decay of the neutral pion suppressed?

I got given a homework problem to check various particle decays, whether they're allowed under the different conversation laws (Baryon, Lepton, charge etc.) Currently im stuck at the following one: $$ ...
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4 answers
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What is the experimental evidence for creation of a gravitational field by electrons or other leptons?

What experimental evidence do we have that leptons (electrons, muons, tau leptons, neutrinos) create—rather than merely respond to—a gravitational field? General relativity (GR) predicts that all ...
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What is the difference between an Electron, a Tau, and a Muon?

What are the basic differences between a Tau, Muon, and Electron? I can read the wikipedia article, but was wondering what specifically distinguished the three leptons and in what experimental ...
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What would the world look like if muons were stable?

Since they are a sort of 'heavy electrons' also 'muonic atoms' could form. These have ben also produced in the laboratory. However, they are unstable. But I was wondering what consequences it would ...
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Why is the $t$-channel diagram for Bhabha scattering allowed?

I thought that the individual lepton numbers $L_e$, $L_{\mu}$ and $L_{\tau}$ are individually conserved for each vertex. At least this is how I explained to myself that this Feynman diagram, ...
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4 votes
1 answer
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Can protons and neutrons be completely converted into Leptons?

In beta decay, a neutron releases an electron and turns into a proton. The inverse happens, though usually not naturally, in positron emission, where a proton emits a positron and becomes a neutron. ...
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Why are all quarks and leptons of this universe the same?

We know the composition of stars by spectroscopic analysis. The EM waves generated by them are blue- or redshifted. We could have said, "Look, the wavelength is slightly different so it may be ...
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Which term in QED Lagrangian represents the muon?

I often heard that electron-muon scattering is a QED process. So I suppose that muon is a field in QED. However, when looking at the QED Lagrangian, it is basically F^2 + psi D_A psi where D_A is the ...
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What happens if I replace an electron in a $\rm Li$ atom by a muon?

According to my knowledge the exclusion principle won't affect it, so it will jump to the muonic 1s orbit (strongly deformed by the electrons' repulsion). The electrons fill the electron 1s orbits (...
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2 votes
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What is the flavour problem? In particular the leptonic flavour problem

I'm studying from these lecture notes by S. Pascoli. She mentions several times the leptonic flavour problem but she never actually defines it. I've looked a bit online but I only found vague ...
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Does tau decay to rho meson + tau neutrino?

In the particle data group table: http://pdg.lbl.gov/2020/tables/rpp2020-sum-leptons.pdf ,only $\tau \rightarrow \pi + \nu_{\tau}$ is documented. But does $\tau \rightarrow \rho + \nu_{\tau}$ as well? ...
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Crossing Symmetry between tree-level diagrams of $e^+ e^- \rightarrow \mu^+ \mu^-$ and $e^- \mu^- \rightarrow e^- \mu^-$

According to Peskin and Schroeder (P&S)'s book, on pp. 156-157, the two processes $e^+ e^- \rightarrow \mu^+ \mu^-$ and $e^- \mu^- \rightarrow e^- \mu^-$ are connected via $s \leftrightarrow t$ ...
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The Collins-Soper frame

I am looking for some thorough notes on the Collins-Soper frame. This is a reference frame used in Drell-Yan process in hadron colliders.
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Is lepton number conserved by definition?

In ordinary beta decay, an electron and an anti-neutrino, together with a proton, are emitted. The proton has zero lepton number, the electron +1 lepton number, and, it is said, the neutrino-type ...
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What is the colour factor, when calculating width for Tau decay?

I am trying to answer a question that asks to find the branching fraction of $\tau^{+}$ decays. In the answers, it gives that the widths are as follows: $$\Gamma(\tau^- \rightarrow ν_{\tau} d \...
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Electrons are Leptons. Other Leptons are muons, tauons and neutrinos but where are they found in the Atom?

We all know about Electrons and we use them every day. WE SEE THEM in all the basic diagrams of ATOMS TOO - BUT where are the other LEPTONS? Are they so short-lived that they really don't play a ...
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Decay of electron? [closed]

Have we detected any decays of electrons to an electron neutrino and $W$-boson in Fermilab or in CERN? Are neutrinos the only possible stable leptons inside an electroweak field?
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3 answers
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What are the advantages of an electron-electron collider vs. An elec./positron one? Has one ever been built?

The current LHC is a proton-proton collider (instead of proton-antiproton) for reasons I have read about elsewhere. Why has there never been an electron-electron collider? Also, I am reading now ...
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Does lepton family number always have to be conserved?

So for example, is the following reaction possible?: $$ \mu^- \rightarrow e^- + \gamma $$ In the textbook it says this is possible, as it conserves the lepton number. However, the lepton flavour is ...
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What tells quarks, electrons, etc. to move the way they do? [closed]

I've recently had some interest piqued in physics, and this is one of the many questions that I have continuously found myself trying to figure out; what tells/told particles to go in the direction ...
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Is there a lepton anti-lepton asymmetry?

I dont know is this a stupid question or not but I wanted to ask We all familiar with the fact that there must be some more baryons w.r.t antibaryons. Since we do not observe any anti-baryons we ...
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Is there a reason, other than mass hierarchy, that we associate each quark generation with a particular lepton generation?

The Standard Model contains three generations of quarks, and three generations of leptons. We generally pair off these generations into the "light" generation ($e, \nu_e, u, d$), the "medium" ...
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