Questions tagged [standard-model]

A model of the basic particles and forces featuring six quarks, three charged leptons, three massless neutral leptons and four fundamental force carrying bosons. The twelve fermions are arranged into three generations, while the bosons serve to explain the electromagnetic interaction plus the strong and weak nuclear forces (and the Higgs mechanism). Do NOT use this tag for the standard model of cosmology, etc..

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54 views

Proton structure: are there photons, electrons and neutrinos in there too?

I've read that the proton is not simply made up of three (valence) quarks; rather, there is a continuous exchange of gluons between the three quarks, and these gluons can produce quark-antiquark pairs ...
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Unpolarized muon decay rate and Feynman diagram trick

In class, while evaluating the unpolarized muon decay rate, in the Fermi theory (low energy limit) we got the result for the matrix element $$\frac{G^2_F}{2}\text{tr}\left[\gamma^\rho(1-\gamma_5)(\...
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Standard model lagrangian derivation from symmetry principles [closed]

Is possible to derive Standard Model field lagrangian from symmetry principles? What symmetry principles are involved and required?
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Why do we need complex representations in Grand Unified Theories?

EDIT4: I think I was now able to track down where this dogma originally came from. Howard Georgi wrote in TOWARDS A GRAND UNIFIED THEORY OF FLAVOR There is a deeper reason to require ...
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Why protons are stable but Neutrons not?

just read something about standard model and find it very interesting, proton is made of there subatomic particle U,U,D and neutron have U,D,D . electromagnetic force between U,U,D wants to rip apart ...
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Apparent coincidence of spin-connection term and Higgs field?

In curved space time, there is a spin-connection term $\overline{\psi}\gamma^\mu\sigma^{ab}\omega^{ab}_\mu\psi$. Here's my apparent problem. If there were no Higgs field and no gravity, all particles ...
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In quantum field theory, is there a coupling constant for each field? (I.e. an equivalent to the fine structure constant for the EM fields.)

Given that the fine structure constant is a dimensionless ratio quantifying the strength of the coupling between a charged particle and the electromagnetic field, is there a symmetric set of such ...
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Help counting the number of real parameters in simple seesaw Lagrangian

The seesaw Lagrangian is $$-\mathcal{L}_{\rm mass}=\frac{Y^\ell_{ab}\langle H^0\rangle}{\sqrt{2}}\bar{\ell}_{aL}\ell_{bR}+\frac{Y^\nu_{ab}\langle H^0\rangle}{\sqrt{2}}\bar{\nu}_{aL}\nu_{bR}+\frac{1}{2}...
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Standard model : Why linear representation?

I'm trying to understand better the idea of the standard model, where particle states are described within vector spaces corresponding to irreducible representations of the group of symmetry of ...
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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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Why is $SU(3)$ chosen as the gauge group in QCD?

Why is $SU(3)$ chosen as the gauge group. Why not $U(3)$? Why does it even have to be unitary?
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Why is the standard model gauge group $SU(3) \times SU(2) \times U(1)$ and not $U(3) \times U(2) \times U(1)$?

The bilinears in the Lagrangian are invariant under the full group. Where does the restriction to unit determinant come from?
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2-body resonances study resources [duplicate]

As a part of my Particle Physics Course, I will need to give a talk on 2-body resonances - What are they? Some examples like $J(\Psi)$, Z boson and significance of those discoveries. I am looking for ...
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How to calculate pion decay amplitude?

Now I will calculate the amplitude for the decay $\pi^{+}\rightarrow l^{+}\nu$ at leading order. The effective Lagrangian for semileptonic weak interaction is \begin{eqnarray*} \Delta\mathcal{L} & ...
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Is neutron-neutron fusion viable?

Do I get unusual baryon maybe like pentaquark or just a pair of conjoined twin that is very unstable?
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1answer
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Charge conjugation of $|b\bar b\rangle$ states?

I know that for a state of a boson and its anti boson $|b\bar b\rangle$ the charge conjugation is $(-1)^{L+S}$ but I don't understand how this value is arrived at. Wikipedia says that is to do with ...
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1answer
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Feynman diagram and bosons

A decay is said OZI-suppressed if: if you can arrange it [the Feynman diagram of the decay] so that there's some time at which the quarks annihilate and all of the energy/momentum is carried by ...
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Does theory allow that one neutrino is massless and the other two not?

The question says it all. Is there the theoretical possibility that one of the neutrino mass eigenstates have zero mass, whereas the other two have not? Is there a theory that allows this option? I ...
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1answer
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Gauge singlet under SM

What does it mean when they say something is a gauge singlet under the Standard Model group? I would like to understand this concept of Singlets and Doublets. Thanks in advance.
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1answer
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Where can I find the mesonic decay constants in the particle data group listings?

I can't seem to find the mesonic decay constants in any of the relevant particle data group (PDG) listings. I'm specifically looking for the neutral Kaon decay constants. Link to the PDG interactive ...
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What is a/the $K^*$ meson, and why is it important?

I know that the standard pseudoscalar $K$ mesons are: $K^+$: valence quark content $u\bar s$; electric charge $+1$; spin $0$; parity $-1$; $K^-$: valence quark content $\bar u s$; electric charge $-1$...
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Why can we only measure the electromagnetism and gravitation in real life, and not the nuclear forces?

Why can we only measure the electromagnetism and gravitation in real life, and not the nuclear forces? All our senses are explained with EM forces and pressure values directly and heat transfer.
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Identical Particle Wavefunctions

Consider the process $$\rho^0\rightarrow\pi^0+\pi^0$$ The $\rho^0$ has $J=1$ whilst the two pions have $S_{tot}=0$ and thus require $L_{tot}=1$ by conservation of angular momentum. Consequently this ...
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What are the minimum/realistic energies needed for positron/electron annihilation to form $D$ mesons?

High energy positrons and electrons can form $D$ mesons when they interact with each other and annihilate, but I was wondering what those energy levels would have to be. Specifically, would a fixed ...
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What is Peccei-Quinn transformation?

In Srednicki's textbook Quantum Field Theory, Problem 94.2 considers a massless quark represented by a pair of Weyl fermions $\chi$ and $\xi$. Part a) asks us to show that the lagrangian is invariant ...
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Where is the mass of a proton coming from? From empty space or gluonfields?

Protons are made quarks and bosons. I think most physicists would agree that the mass of a proton is not coming from the quarks itself. About 90% is coming from something else, but what is that. I ...
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Why do physicists want to find a model that incorporates gravity into the standard model? [duplicate]

Gravity is just an effect of space itself curving. Space the metric itself expands faster than light, a property not of any particle, particles in the standard model do not expand, they are not ...
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1answer
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Particle physics jargons - generations, family and flavor

The question is pretty much in the title. What is the difference between the words generations, family and flavour?
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1answer
86 views

Quantum Field Question

I am watching a video on QFT (https://www.youtube.com/watch?v=jlEovwE1oHI). And have kind of a fundamental question(s)... Why are the fields that require hundreds of MeV (or more than 174 GeV like ...
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What is the evidence for 'billions of neutrinos pass through your body every second'?

This statement is repeated so often that it has become somewhat of a cliche: 'billions of neutrinos pass through your body every second'. For example see 1, 2, 3, 4, 5, 6. What is the evidence for it,...
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Hypothetical particle disintegrates into two pions $\pi^+ \pi^-$ or $2 \pi^0$, what are the possible isospins of this particle?

I had a question that I couldn't seem to find a clear answer so there it is. Let us assume an hypothetical particle $P^0$ which decays into 2 $\pi^0$ or in a pair $\pi^+$,$\pi^-$. What are the ...
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High intensity laser and neutrino pair production

Given the very small rest mass of the neutrino, visible photons have plenty energy for neutrino pair creation. What would be the probability of $\gamma \to \nu + \bar\nu$? Would the neutrinos be ...
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What’s lacking in our understanding of the strong interaction?

As far as I’m aware, our understanding of the strong interaction and strong nuclear force is pretty good. We can explain how particles move and stick together with good models. But from what I can ...
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What is the defect in the standard model with quark interactions

I am watching the Master Class Axions: Ghost Riders in the Sky as well as another lecture, this one given by Michael Turner. In his lecture, he states the Axion idea came about because of a defect in ...
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Is there a theoretical upper limit for the Yukawa-like coupling constant in a theory with a cut-off scale?

In the fermion mass terms in the Standard Model, $y_x\bar{L}H d_x$ or $y_x\bar{L}\tilde{H} u_x$ where $y_x$ are the Yukawa couplings, we have $y_x<<1$ except for the case of the top quark. ...
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1answer
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Positive Kaon decay into $\pi^+$ and $\pi^0$ suppressed

I am looking at the reactions of positive Kaons and am looking at $$K^+\to\pi^++\pi^0$$ Since the strangeness is not conserved, this has to be a Weak decay. When I try to make the Feynman diagrams ...
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What happens when a particle and anti-particle collides of different masses?

If Hydrogen and an 'anti-Helium' come together will we get a hydrogen and a photon? I was wondering if antiparticles can be used to fuel some nuclear reactions and used to develop particles which ...
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Do all the elementary particles in the standard model have their own wave functions?

Is it possible to derive the wave functions of all particles in the standard model? Would this be through the Schrödinger equation?
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$\Delta {b_{L}}$, $\Delta {b_{Y}}$ and $\Delta {b_{C}}$ and unification of couplings

In any standard book on QFT, I understand how to find out the beta function for Standard Model and for Minimal Supersymmetric Standard Model, but i could not understand how this beta function changes ...
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1answer
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Why do we know that the right-handed electrons exist?

I am wondering how come Weinberg and Salam put the right-handed electrons and did not put right-handed neutrinos* into the theory? In other words, are there any experimental/theoretical indications ...
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Physical content of $[D_\mu, D_\nu]=ieQF_{\mu\nu}$ and $[D_\mu, D_\nu]=igT^a G^a_{\mu\nu}$

For the abelian QED theory, $$[D_\mu, D_\nu]=ieQF_{\mu\nu}$$ where $D_\mu=\partial_\mu+ieQA_\mu$ is the gauge covariant derivative in QED and $F_{\mu\nu}=\partial_\mu A^a_\nu-\partial_\nu A^a_\mu$ is ...
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What is a sigma meson? [duplicate]

I have seen some articles talking about the role of sigma meson in nuclear force, but more articles omit it and only mention pion, rho and omega (e.g., Wikipedia). What is this sigma meson?
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Where can I find inclusive $p^+p^+ \to W^+W^-+X^{++}$ cross-section prediction at $14 TeV$ (NNLO)?

The title self explains.. I desperately need the theoretical NNLO (also NLO can work) prediction for inclusive diboson production (WW), a link to a paper would make my week!
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1answer
277 views

Trying to work out the CP-transformation property of the Higgs potential

The parity transformation property of a complex scalar field $\phi(x)$ is given by: $$P\phi(t,\textbf{x}) P^{-1}=\eta_P\phi(t,-\textbf{x})$$ where $\eta_P=\pm 1$. The charge conjugation property of a ...
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1answer
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Why is SM Higgs a doublet?

Based on my C grade of QFT (technically F in grad), I learned that a complex scalar field can give mass to a maseless gauge boson by SSB, with real component becoming a massive higgs, and orthogonal (...
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1answer
135 views

Why are fermion masses considered free parameters of the standard model rather than Yukawa coupling constants?

From what I understand, fermion masses in the standard model are due to Yukawa couplings to the Higgs field. Nevertheless, when listing free parameters of the standard model it is the fermion masses ...
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2answers
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Is the mass of the $Z$ due to mixing with the photon “precursor” $B$ or to interaction with the Higgs?

I want to get something clear that I do not seem to understand. I used to read that the photon $A$ and the $Z$ boson are (different) linear combinations between the $\mathrm{W}^0$ (neutral weak boson ...
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Why are all mesons unstable?

My question is: why are all mesons unstable? Shouldn't the lightest mesons like the pion be stable because they are made from the lightest quarks (and anti-quarks)? Or could the they annihilate (the ...
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1answer
286 views

Charge of $W$-bosons in Feynman diagrams

When looking at Feynman diagrams of particle decays, how would I be able to find out the charges of the $W$-bosons involved in the decay?
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Amplitudes with external gauge bosons in Feynman gauge

If I want to compute the tree amplitude for 2 particles annihilating into 2 gauge bosons, for example, $h h\to Z Z$ in the Standard Model (SM), and I use Feynman gauge, is it necessary to include ...

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