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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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24 views

Experimental evidence of interaction out of thermal euilibrium?

Sakharov gave three conditions for the matter-antimatter asymmetry observed in the universe. (1) Baryon number violation: Don’t have any experimental confirmation yet. (2) C-symmetry and CP-symmetry ...
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What do we mean with spontaneous symmetry breaking $SU(2) \times U(1) \to U(1)$ in particle physics?

I think my problem is with group theory. I understand that within the Standard Model the Lagrangian is invariant under $SU(2)_{L}\times U(1)_{Y}$ symmetry, but I do not understand what we mean (...
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F-type and D-Type Coupling between meson baryons

What is the basic definition and detailed meaning of the D-type (symmetric) and F-Type (symmetric) coupling between mesons and baryons. Why the name D and F. What is their origin?
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Isospin and $\Delta$ Production

Edited to add new information I have seen the relation between cross sections postulated: \begin{align} \sigma(\nu+p\to\mu^- + \Delta^{++})=9\sigma(\nu+n\to\mu^- + \Delta^{+}) \end{align} Motivated ...
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Mixing matrix in particle physics

It is familiar for us that the diagonalization of a mixing matrix which is written in the neutral basis gives the masses of the neutral particles. If I construct a mixing matrix which is written in ...
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1answer
109 views

What if there were only 1 generation of fermions?

The number of fermion generations has many effects beyond the mere existence of "3 copies of fermions," including determining scatterings, constraining CP violation, influencing electroweak symmetry ...
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3answers
83 views

Experimental proof for isospin conservation of strong interaction

I am having a surprisingly hard time to find direct experimental results for the (approx.) conservation of isospin in strong interactions. The canonical examples seem to be $\omega \! \to 3\pi^0$ and $...
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91 views

Question about the Lie group $SU(3) \times SU(2) \times U(1)$ and the concept of manifold

I don't know if this question is a duplicate, so I'll delete if is. Well, I'm in the very beginning of the study of contemporary topics such as gauge theories, I would say that I'm in a "science ...
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According to the Standard Model, were there 4 Higgs bosons at the beginning of the universe, before the electroweak symmetry breaking?

My question is in the context of the Standard Model, not about Beyond Standard Model. At the beginning of the Universe, before electroweak symmetry breaking, we have a complex Higgs isospin doublet. ...
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Standard Model Lagrangian and Euler-Lagrange Equations

First off, note that I only know physics through quantum mechanics, so forgive me if this is a foolish question. I've seen the Standard Model's Lagrangian density written out in full. My question is, ...
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How can multiple fermions combine to form a boson? [duplicate]

I understand that composite particles with integer spin form a boson. For example a helium nucleus is a boson because it has 2 protons and 2 neutrons. If all of the components on their own are ...
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A bilinear form or a dot product $X \cdot Y$ of emission X and absorpion Y of a vector boson W in the context of transformation of quark flavors

The reason for this question is related to the fact that I think something is missing when an absorpion and emission of a (intermediate) vector boson $W$ in the context of transformation of quark ...
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Deriving hadrons by their quantum numbers

While preparing for an exam I found a problem where one has to find the composition of hadrons given their quantum numbers. $$\text{ (Charge, Baryon number, Strangeness, Charm, Bottomness)=(Q,B,s,c,b)...
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Meson decay constants from PDG

I've been looking for data for meson decay constants (pseudoscalar and vector), but in the latest release of PDG review I only find information about these parameters for charged pseudoscalar mesons. ...
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Derivation of Jacobian peak in particle physics

I am reading some papers on particle phynomenology. The mathematical derivation of Jacobian peak in some paper is wrong. However, it gives the right prediction. If you have a W boson decaying in to ...
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Vev's in Beyond the Standard Model

Recently, I read a paper, the authors have mentioned that if we have $n$ stand for VEVs which are singlets of the SM gauge group, $k$ are associated with SM doublets, and $S$ is the VEV of the SM ...
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Formation of an electron

I recently asked the question (with title "electron charge") about the negative charge of an electron and I received several answers to tell me that what I had heard about the formation of the ...
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155 views

Particle notation: What does the asterisk stand for?

For instance, what's the difference between $\Sigma^0$ particles and $\Sigma^{0*}$ particles? I know they have the same quark configuration $sdu$, so how exactly are they different? Some context: I'm ...
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Physical meaning of whole state space for spin-one

The spin-one quantum states belong to $\mathbb{C}^3$, with a set of orthonormal bases taken as the spin component along a direction, which would correspond to the direction of the magnetic fild ...
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When are W-bosons emitted?

According to this Wikipedia page W-bosons are involved in beta decay. According to Wikipedia, this occurs when a down quark turns into an up quark and also emits a W− boson. When this happens in a ...
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1answer
164 views

Why is the smallness of Higgs mass not technically natural?

Technical naturalness The smallness of a parameter $\theta$ in the Lagrangian of a quantum field theory is said to be technically natural, if in the limit of vanishing $\theta$, the theory has some ...
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170 views

Measuring strong coupling constant

How can the strong coupling constant be measured?
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1answer
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What exactly is a gluon doing when acting as an gauge boson?

The question is in the title. I know that, e.g., W bosons change the flavour in electroweak processes. What do gluons do?
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1answer
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Why is $\Delta^0$ not decaying weakly?

Given the lower decay I wonder why it happens this way. Wouldn't it be possible to decay via a weak process as well?
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1answer
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Main decay of $ J / \psi $

The main decay of $ J / \psi $ is $ c \bar{c} \to ggg $. How can this reaction channel be observed? 3 gluons will hadronize immediately, or not?
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Deeply Virtual Compton Scattering

what are Transition Distribution Amplitudes respectively Deeply Virtual Compton Scatterings ? I'm so unsure about the terms I can't even distinguish (in case they are not really related to each other)....
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How does the introduction of the charm quark suppress FCNC?

I did some reading on the GIM mechanism today, and simply fail to understand how it works. I understand how the CKM-matrix can be used to do the basic calculation of the probability of, say, observing ...
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Why is $J/\psi$ not being able to decay into $\pi^+\pi^-\pi^0$ (Ozi-rule)?

There are already (very similar) questions like What is the precise statement of the OZI Rule? and Why does charmonium (and phi mesons) not decay via quark and antiquark annihilation? but, to be ...
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Regularization methods' equivalence and the hierarchy problem

There are several questions related to this one but whose answers just raise the doubt I'm going to describe here. Some facts that are everywhere are (i) The hierarchy problem results from the fact ...
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1answer
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How to find the most likely decays in 2 hadrons for $\Omega$?

I took a look at the Particle Data Group, and the most likely modes are $\Omega ^{-}\rightarrow \Lambda K^{-}$ (68%) and $\Omega ^{-}\rightarrow\Xi ^{0}\pi ^{-}$ (24%). I have 2 questions: 1) The ...
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Do we care about CFTs in particle physics?

This question is related to these others: mostly this one, but also this one and this one. Do we care about CFTs in particle physics? Let me explain. Suppose we don’t know anything about string ...
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Why do mesons decay?

Why do mesons decay shortly while baryons are more stable? And why do mesons decay, meaning that there are unstable, while they formed because meson is more stable than the two corresponding quarks? ...
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Where does decay widths in mass mixing matrix come from?

For some time ago I've seen people use complex mass mixing matrix including decay width of the particles. It kind of makes sense, but I could never fully justify it. I would be grateful if you could ...
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96 views

What are the basic ingredients in the Standard Model (SM) of particle physics? [closed]

What are the basic ingredients of the standard model (SM) and their applications?
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How does a quark $u$ or $d$ turn into one another?

The pseudo-explanation that the proton is stable because it has a lower mass than the rest is incomplete, because a particle formed by three u: uuu would have still a lower mass. I know that some ...
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66 views

Logic for Eq.(9.68) in Griffiths' “Intro. to Elementary Particles” (2nd edition)

Treating charged pion decay $\pi^{-} \rightarrow \ell^{-} + \bar{\nu}_{\ell}$ (where $\ell$ designates a lepton of first- or second-generation) by the representation of the figure below, Griffiths ...
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1answer
85 views

Where does the extra mass of a $W$ boson come from in particle decay?

I’ve seen everywhere explaining that it can exist because of time/energy uncertainty. I get this. I understand that’s WHY it exists but I’ve still never gotten WHERE the extra mass comes from. Is the ...
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3answers
72 views

Decay process of pions

$\pi^+$ and $\pi^-$ decay into muon(or electron) and neutrino and $\pi^0$ decays into photons. So there is a weak interaction in the decay process of $\pi^+$ and $\pi^-$. But the mean lifetime of $\...
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1answer
57 views

What determines the Lifetime of $Z$-Boson?

What determines the lifetime of the Z Boson? Is it it's mass? What else is involved? Is the coupling to Higgs involved?
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1answer
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Higgs critical temperature?

Higgs mechanism is defined to operate (give spontaneous symmetry breaking) below some critical temperature. As mentioned, for ex. in Wikipedia: Below some extremely high temperature, the field ...
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Heirarchy problem

Can anyone explain the hierarchy problem in context to Higgs mass corrections by scalar loop and fermion loop (the problem arising when we try to treat SM as an EFT)? and how do these corrections ...
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1answer
81 views

Do Quarks decay into Photons at high enough temperatures?

At high enough energy, particles lose coherence and are ripped apart into a Quark-gluon plasma where the individual quarks can break free of the atomic nucleus and fly around unencumbered. A black ...
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How does the Higgs field prevent the electron from going the speed of light?

Both the Photon and the Electron are point-like particles. Most massive particles can be thought of "bound collections of massless particles", but the electron assumed to be a "point particle with a ...
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Possible Feynman diagram for $\tau^+ \rightarrow p \mu^+ \mu^-$ and $\tau^+ \rightarrow \bar{p} \mu^+ \mu^+$?

I want to know the possible Feynman diagram for these two lepton family, lepton and baryon number violating tau decays. These decays are forbidden in the Standard Model. But the further extension of ...
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Are there compelling reasons not to organize the elementary particles by charge and spin? [closed]

I don't like the way the standard model is currently displayed (see second picture for reference). It does a poor job of abstracting the information into something understandable. As far as I ...
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What does matter look like in the absence of photons?

Another question inspired by this one on alternative standard models. Greg Egan's story Wang's Carpets (or the similar segment of the larger novel Diaspora) sketchily describes a universe with no ...
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Chemistry without electrons?

This question on the behavior of a universe with massless electrons got me wondering: what would atoms look like if electrons were not merely massless, but rather the lepton fields just didn't exist ...
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1answer
121 views

Do elementary particles have a density?

The SM supposes elementary particles are structureless unless composite objects like hadrons. For bosons, that can occupy the same state, we can define energy or mass density. The same happens but ...
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Are decay Feynman diagrams really Feynman diagrams too? Or just vertices?

So I was wondering, the vertex diagrams for the standard Model, can they also be feynman diagrams with on shell particles? For instance here is the W - boson vertex which decays into electron and ...
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How are quarks elementary when they can become leptons? [duplicate]

From a recently reignited [casual] curiosity into particle physics thanks to the Fermilab YouTube channel, I read about the g-2 experiment, followed by muons, naturally. Muons, it turns out have short ...