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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1answer
52 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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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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34 views

Photon vacuum polarization one loop diagram with W bosons [on hold]

I have to compute the one-loop integral for the photon vacuum polarization with a W boson loop in the unitary gauge SM with dimensional regularization. After performing the Lorentz algebra I end up ...
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42 views

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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69 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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3answers
160 views

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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72 views

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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41 views

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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96 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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Neutrino-lepton scattering in the full SM

I have to find the total cross section of the elastic scattering: $$\nu_\mu+e^-\rightarrow \nu_\mu+e^-$$ The exercise asks to do it keeping into account also the electron mass and the width of the $Z$ ...
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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 ...
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Right-handed neutrinos in Standard Model

Does SM allow for the right-handed (Majorana) neutrinos? Or right-handed neutrinos are beyond SM?
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68 views

Stability/decay, are they boolean or not, or does QM probabilities overrule this?

This is not a duplicate, I am not asking whether the proton is a stable particle, or why it is. I am asking about the definition of stability/decay whether it is boolean or not. I have read this ...
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Does the Standard Model have texture defects?

In the standard classification of topological defects, in a theory with vacuum manifold $\mathcal{M}$, $\pi_0(\mathcal{M})$ corresponds to domain walls, $\pi_1(\mathcal{M})$ corresponds to strings/...
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Why don't we add Wilson loops to the SM Lagrangian?

As the title says: why don't we add Wilson loops to common Lagrangians such as the Standard Model? They're gauge invariant and (correct me if I'm wrong, not sure on that) are renormalizable. Suppose ...
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Is the $U(1)$ in the standard model identified with quantum-mechanical phase?

I think there's a tension between two claims I've read: The standard model is Yang-Mills theory with gauge group $SU(3) \times SU(2) \times U(1)$. Here the $U(1)$ factor is data on the same level as ...
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References for Glashow-Weinberg-Salam model

I am looking for reference recommendations on the Glashow-Weinberg-Salam theory of electroweak symmetry breaking. In particular, I am looking for a discussion of the $SU(2)\times U(1)$ gauge symmetry "...
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Regge theory and $SU(3)$ mass formula

In a video interview, Murray Gell-Mann states (at 40 seconds) "I have wondered all this time, and I still wonder, whether the straightness of those [Regge] trajectories, and the accuracy of the ...
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Bare mass versus the mass form spontaneous symmetry breaking

Consider renormalization in $\phi^4$ theory $$\mathscr{L}=(\partial\phi)^2-\frac{1}{2}m^2\phi^2+\frac{\lambda}{4}\phi^4$$ where $m$ and $\lambda$ are respectively the unobservable bare mass and bare ...
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1answer
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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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1answer
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Theory behind the experimental signatures of Higgs-Boson

As I understand it, the Higgs-Boson itself is unstable and has a transient existence. So, the particle detectors cannot detect it directly. Instead, the detectors register all the decay products and ...
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Is the Standard Model UV complete? If not, why? [duplicate]

Below is my understanding of why QED is not UV complete. Please correct me if I am wrong. As a necessary condition, a UV complete quantum field theory must be renormalizable. But a renormalizable ...
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How $W^{\pm}$ bosons get their electric charge $\pm 1$ as opposed to $Z_0$ that have neutral electric charge?

In the 'Standard Model' book by Y. Grossman and Y. Nir, in chapter 7 (the leptonic standard model) on page 93 after defining the charge of the broken symmetry generator, i.e $Q=T_3+Y$ they say that ...
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Strangeness Conservation

I am stuck as to how to prove that strangeness is conserved in electromagnetic and strong interactions, while it is not conserved in weak interactions. I know that this can be shown by studying the ...
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Why can't two real photon, gluon, graviton, and $W$ and $Z$ fields interact by means of their virtual counterparts (the mediators of the process)?

It is a fact that two real (massless) photons, gluons, or gravitons can't react by means of their virtual counterparts (for example, two external photons that interact via one of these massless ...
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Could $\nu_e+p\rightarrow e^-+\pi^++p$ occur via the weak interaction? If so, how would be the Feynman diagram?

I checked if the reaction conservate baryon number, charge and leptonic number and it seems like it does. But I tried to draw the Feynman diagram and I don't understand how the proton interact, so ...
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38 views

In what sense $Z_\mu^0$ is orthogonal to $A_\mu$?

I am reading Standard model. Please explain in what sense the $Z$-boson $$Z_\mu^0=(g^2+g^{\prime 2})^{-1/2}(g A^3_\mu-g^\prime B_\mu)$$ is an orthogonal linear combination of the photon $$A_\mu=(g^2+...
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1answer
34 views

How lepton numbers and baryon numbers of a particle are decided?

Suppose a new particle is discovered how will it be assigned a lepton and baryon number? Moreover, what difference would it make if the electron had +1 lepton number and a positron -1? The same ...
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4answers
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How to understand the makeup of neutral pi and eta mesons?

I know that mesons are bosons made up of quark-antiquark pairs. But when I see the list of mesons, I can see that the makeup of neutral pions and eta mesons are noted in a strange way. $$\pi^0=(u\bar{...
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1answer
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Which sterile neutrino are we looking for?

I am trying to wrap my head around sterile neutrinos. As far as I understand it, sterile neutrino is a name for every particle that feels only gravitational force. The most notable example is a right-...
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How to determine which force mediates an interaction?

I have been given an assignement gives me a number of interactions, asks me to determine which, among these interactions are permitted under conservation laws (Charge, Baryon number, Leptonic numbers, ...
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58 views

An invariant of the gauge group $G$ that is totally symmetric with three indices in the adjoint representation

In Ch.19 of the textbook An Introduction to Quantum Field Theory by Peskin and Schroeder, on P.680 the property of a quantity $$\mathcal{A}^{abc}=\mathrm{tr}\left[t^a\{t^b,t^c\}\right]\tag{19.132}$$ ...
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1answer
56 views

Sizes of Elementary Particles

Present observation shows that elementary particles have no internal structure, and have no real size as they are described by wavefunction. Something that therefore confuses me is that on a lot of ...
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Dark matter composition [closed]

I assume that dark matter is made of heavy neutrinos.But wouldnt they be converted into neutrinos with less mass throygh weak interaction?So dark matter isnt made of neutrinos.I think there is truly ...
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1answer
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Why does charmonium (and phi mesons) not decay via quark and antiquark annihilation?

The decay of heavy quark/antiquark pairs (say $c\bar{c}$, $s\bar{s}$) is supposedly 'suppressed because of the Zweig/OZI rule', see for instance Phi meson. And they certainly have a longer lifetime ...
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Custodial Symmetry in SM Higgs

this excercise is about the custodial $SU(2)_R$ symmetry in the SM Higgs Mechanism. Step by step I have to develop the theory. I get the generel idea of the global custodial symmetry and why we need ...
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1answer
54 views

Why is the relation $M_W=M_Z\cos\theta_W$ true only at tree-level?

In Glashow-Weinberg-Salam electroweak theory, the relation $$M_W=M_Z\cos\theta_W\tag{1}$$ is said to be remain true only at the tree-level; it receives corrections from the loop diagrams. See here. ...
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1answer
47 views

why do quantum fields exist? [closed]

Why did the big bang give rise to the quantum fields it did, could the big bang have created the universe any other way? and why not more elementary particles?
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How can pure leptonic interactions be mediated by the weak force?

$\mu^+ \rightarrow e^+ + \nu_e + \bar \nu_\mu$ Consider the above decay, it is given in my lecture notes as a weak force interaction. I thought the weak force was only involved when quarks change ...
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What are direct experimental evidences that quarks exist? [duplicate]

Now we have very established model of quarks explaining fundamental strong interaction. What are experimental proofs for existence of quarks and what is the name of physicist which made them?
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When is a decay mediated by the weak force

If a $\pi ^+ \to \nu_\mu+\mu^+$, here quark flavour is lost entirely. I am aware the weak force mediates decays where quark flavour is not conserved. But If I consider a $p+\bar p$ annihilating, here ...
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1answer
82 views

Most of the momentum of a proton is carried by the quarks

I saw this in wiki https://en.wikipedia.org/wiki/Antiproton#Modern_experiments_and_applications "the valence quarks in the proton, and the valence antiquarks in the antiproton, tend to carry the ...
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2answers
51 views

How to figure out whether a force mediator is $W^+$ or $W^-$ in Feynman diagram?

How to figure out whether a force mediator is w+ or w- in feynman diagram? I always make sure each vertex is zero but sometimes I get my w+ or w- worng.
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Conservation of lepton

I was reading a bit about nuclear/particle physics when I came up to Cours et problèmes corrigés de physique nucléaire et de physique des particules by Philippe Miné in 2016 ISBN 2340011566, which is ...
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37 views

Difference between expected $p_0$-value and observed $p_0$-value?

In the ATLAS paper for the higgs discovery, they used 2 kinds of different $p_0$-values. One is the expected $p_0$-value, the other one is observed $p_0$-value. What's the difference between the ...
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1answer
44 views

Why do we have 3 quantum colours?

I understand the need to invoke colour as another quantum state to explain the observation of uuu ddd and sss baryons. I just wanted to know if there was some other property which explains why we only ...
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How does one calculate branching ratio of Higgs particle decaying into virtual particle?

Very often when people talk about decay they use branching ratio to describe a particular decay channel, for example: $$Z\rightarrow e^{+}+e^{-}$$ If both Z and the electron and positon are one-...