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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 ...

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How do know that Goldstone Boson actually become the longitude degrees of freedom in W+,W- and Z boson?

In many Quantum Field Theory text books they says these about Spontaneous Breaking and Higgs mechanism like this In unitary gauge, the Goldstone Bosons are eaten by $W^\pm$ and Z and become their ...
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2answers
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How do we know that gluons travel at the speed of light?

Since gluons are located within nucleons and immediately outside of them, how do experiments determine parameters like their speed? Is it possible we could be assuming they travel at the speed of ...
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1answer
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Why does the upper component of a $SU(2)$ doublet has $T^3=1/2$ and lower component $T^3=-1/2$ and not the opposite?

For a $SU(2)$ doublet, why does the upper component have $T^3=1/2$ and lower component $T^3=-1/2$? I know that this can be answered in the Standard Model by using $Q=T^3+Y/2$. But that is because we ...
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1answer
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Do gauge bosons really exist? [on hold]

Do gauge bosons really exist or are they only a mathematical model? Have we ever detected them?
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1answer
85 views

Light neutrino number from the invisible decay width of $Z$ boson, and precluding heavy neutrinos as dark matter candidate

The $Z$ boson decays into pairs of quarks and leptons. While the decays to quarks pairs and charged lepton pairs can be observed, the decays to $\nu\bar\nu$ are cannot be. By subtracting the visible ...
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0answers
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Symmetry breaking with Kinetic term dominating the Potential on a Lagrangian

Suppose a Lagrangian $L$ for a scalar field $\phi$, consists of a kinetic term and a Mexican-hat type potential. I am aware that if the vacuum has symmetry $H \subset G$ while the Lagrangian has ...
5
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1answer
88 views

Is the electroweak $SU(2)$ gauge symmetry an exact symmetry in Standard Model before spontaneous symmetry breaking?

In Standard model, components of a $SU(2)$ doublet (for example $u$ and $d$) have different masses. This means there is no $SU(2)$ symmetry, but I think it is okay because the $SU(2)$ symmetry is ...
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Third and fourth order vertex for gluon self coupling

Are there only three and four order vertices for gluon self coupling?
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1answer
61 views

Pion decay constant: How to know which convention to follow?

As summarized by Wikipedia, different sources use different choices for the (pion) decay constant. This means that the numerical value can vary between $$ \sqrt 2\ f_\pi \quad\leftrightarrow\quad f_\...
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5answers
2k views

Why do we say there are four fundamental forces instead of some other number? [duplicate]

In my physics textbook (and in popular science culture) it is stated that there are four fundamental forces: electromagnetism, strong, weak, and gravity. But Wikipedia tells me that there is a ...
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3answers
313 views

Is spinor the sum of scalar, vector, bi-vector, pseudo-vector, and pseudo-scalar?

Is spinor $\psi$ actually the sum of scalar, vector, bi-vector, ..., pseudo-scalar? Before talking about spinors, we have to differentiate two kinds of spacetime, demonstrated with the example of ...
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1answer
770 views

Is it a coincidence that quarks have exactly -1/3 or 2/3 the electron's charge? [duplicate]

I have read these questions: Why do quarks have a fractional charge? Is there an explanation for the 3:2:1 ratio between the electron, up and down quark electric charges? Hypercharge for $U(1)$ ...
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0answers
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What Standard Model extensions are getting constrained by today's new measurement of the electron's EDM?

As published today in Nature: Improved limit on the electric dipole moment of the electron (free to view) and summarized on Live Science. They improve on the previous measurements by an order of ...
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1answer
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Coefficients of Color Potential in Quarks [duplicate]

I was wondering if there was a relation between the fractional coefficients $$2/3, 1/3$$ obtained when calculating color potential for quarks and the value of electric charge for the up and down quark....
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1answer
71 views

Displacement current quantum mechanical interpretation?

while there are quite many classical explanations of displacement current to make Maxwell's equations work, see e.g. here: Displacement current - how to think of it , it sounds just a little bit like ...
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1answer
47 views

Electroweak to Electro/Weak Bosons?

I apologise in advance, this is something I just can't seem to get my head around. So it's my understanding that before the electroweak force split, there were four bosons - $W1$, $W2$, $W3$ and $B$ - ...
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1answer
92 views

Is this Feynman diagram possible?

I have been introduced to Feynman diagrams in a particle physics University module and I just created this diagram to practice checking conservation laws: I have been told that charge, baryon and ...
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2answers
75 views

Why is boson spin number related to attraction and repulsion?

The accepted answer to this question says Since the electroweak interaction is mediated by spin 1 bosons, it is the case that "like (charge) repels like and opposites attract". Another answer ...
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1answer
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Question about the spin of the antiparticle of $Δ^-$

Ι came across a statement in a physics book that the spin of the antiparticle of baryon $Δ^-$ is 1/2. I know that the spin for regular $Δ^-$ is 3/2, and - as far as I know - the spin of particles and ...
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1answer
69 views

Rho and Omega meson mixing

I am new to particle physics. So this might seem to be a trivial question. I read about $\omega$ and $\phi$ meson states being written in terms of octet and singlet state. In the same way, can $\...
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0answers
35 views

Is there a published list of particles that can be produced from the fundamental particles in the Standard Model?

I read an article that has made me curious to ask if there is a list of all the possible particles that can be generated by the fundamental particles in the Standard Model. Here is the article http:...
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1answer
39 views

Does the weak force obey the CPT symmetry? How?

I have read that the weak force does not obey the charge symmetry, the parity symmetry, and the time reversal symmetry. Then how does it obey the CPT symmetry?
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1answer
72 views

What are significant applications of the Higgs Boson itself in the standard model?

I understand that the Higgs Field grants mass to all the matter in the universe, and is equally distributed in all parts of it, but it was the Higgs Boson that was discovered at the LHC, and ...
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1answer
213 views

Why should proton decay into positron rather than antimuon?

Grand unification theories, such as $SU(5)$/$SO(10)$/SUSY variants, suggest proton decay. The lack of observational evidence for proton decay rules out simple GUTs. But wait a minute! The GUT's ...
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0answers
53 views

Wave function of two identical quarks

I'm asked to discuss what the ground state wave functions of $qq'Q$ would look like (a combination of two quarks and some spin 0 particle $Q$ which transforms like a singlet under flavor $SU(3)_F$ and ...
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1answer
104 views

Can we visualize the standard model fermions as a 5-dimensional matrix with only the first 3 dimensions gauged?

Standard model fermions are usually represented by columns. However the column can carry different connotation depending on the matrix operator acting on it. For example, the Dirac spinor 'column' and ...
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1answer
145 views

A question about the notation of the Standard model

The ${\rm SU(2)_L}$ doublets, for example, the left-handed quark doublets $Q_{iL}\equiv(u_{iL}, d_{iL})^T$ are assigned quantum numbers $(\textbf{3},\textbf{2})_{+1/6}$, which means $Q_{iL}$ are ...
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1answer
51 views

What is meant by “the symmetry of an interaction”?

My understanding of a symmetry is this: apply an operation (e. g. parity inversion) to a system. If it behaves the same afterwards, it is symmetric under that operation. Now, quite often I see ...
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1answer
47 views

Why is it so important to measure CP violation in the lepton sector?

CP violation has been measured in the hadronic sector (K-mesons, B-mesons) and these baryons can decay into leptons, so why is it so important to measure direct CP violation into the lepton sector? I ...
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2answers
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Interacting Quantum Fields in Curved Spacetime

I was thinking this night that how would the fields interaction or the vacuum expectation value of Higgs field changes when spacetime is not flat? e.g., the higgs field interact with the electron ...
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0answers
47 views

Pion triplet $SU(2)$ representation [duplicate]

Writing pion as the triplet representation of $SU(2)$ my guess is that I can write them as combination of $u$ and $d$ as follows (as done for the triplet representation of spin) $$ \pi^-=\overline{u}d ...
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1answer
88 views

What happens during a weak interaction?

For e.g. during $\beta^-$ decay a $W^- $ boson is emitted changing an up quark to a down quark. This seems very weird to me as it looks like that up quark is not interacting with some other particle ...
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1answer
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What are the other components of the weak isospin?

Sorry for a lazy question, I was just looking this up on the internet for a basic grasp of Standard Model. However, I couldn’t find anything on Wikipedia or just somehow managed to miss it on my ...
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1answer
42 views

Pulling quarks from one another creates a new pair of quarks? [duplicate]

It is said that the force between two quarks increases as they are pulled apart which is ultimately resisted by the strong nuclear force. Correct me if I'm wrong, but my physics teacher once said ...
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0answers
47 views

What are the known ways to model baryons in string theory?

As I understand, string theory models baryons by including a so called "baryon vertex". Is this right? It seems that baryon vertex's are usually D/M branes. Is that always the case?
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1answer
43 views

Rule of thumb for identifying dominant quark contribution in loop calculations

I am trying to understand some calculations of $B$ meson decays and just stumbled upon the low energy effective weak Hamiltonian describing $\Delta S = 1$ $B$ decays: $$\mathcal{H}_\text{eff} = \...
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1answer
99 views

How to “build” a proton

Sabine Hossenfelder in her column The End of Theoretical Physics As We Know It made the following statement: For example, we have a perfectly fine theory that describes the elementary particles ...
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1answer
59 views

Higgs Doublet Transformations

If we consider two complex Higgs doublets with $SU(3)_{c} ⊗ SU(2)_L ⊗ U(1)_Y$ quantum numbers $\phi_{A,B}$ ∼ (1,2,1), such that $\phi_A = \begin{pmatrix} \phi_{A}^{\dagger}\\ \dfrac{1}{\sqrt2} (Re \...
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51 views

Particle Masses in $SU(3)$ gauge field theory

I have an $SU(3)$ gauge field theory with a scalar field $\phi$, and the Lagrangian: $ \mathcal{L} = - \frac{1}{2} $Tr $F_{\mu\nu}F^{\mu\nu} + \frac{1}{2}(D_\mu\phi)^T(D^\mu\phi)-V(\phi) $ Here, $ ...
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2answers
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Do color changing interactions between up and down quark exist?

In our Standard Model course we wrote down the term for the quark-gluon interaction of the left handed $SU(2)_L$ quark doublet $q_L$ as: $\mathscr{L} = … + \bar{q}_L \gamma^\mu iG^a_\mu(x) \lambda^a ...
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1answer
48 views

Parity conservation in electromagnetic decays

When a $\pi^0$ decays into photons, the only possible number of photons to which it can decay is 2$\times n$, with $n$ a natural number. This is because in electromagnetic decays charge conjugation is ...
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1answer
46 views

What dictates how the Higgs boson will decay?

As far as I know the higgs boson has been observed to dacay into gamma ray photons, the Tau, b quark and maybe top quarks. But what are the factors that govern what particles it will decay to? And are ...
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1answer
42 views

Can physical processes be deemed possible or impossible with Standard Model vertices alone?

My question is, can a physical process be deemed valid/invalid, such as muon decay, purely from the allowed standard model vertices? Or do we need to take into account things like lepton numbers and ...
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0answers
46 views

How do right handed quark doublets transform under $SU(2)_R$ transforms?

In the context of proving that the SM custodial symmetry is broken by Yukawa coupling to fermions using a 4D Higgs vector representation, $$\underline{\Phi}=\begin{pmatrix}\Phi\\i\sigma_2\Phi\end{...
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1answer
53 views

Do we / Can we construct field theories based on modelling fundamental properties rather than particles/forces?

This will probably sound ridiculous and stupid, so I apologize in advance. I'm wondering whether instead of modelling matter and force fields representing the various fundamental particles and forces,...
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1answer
41 views

Determining conservation of isospin

I am trying to understand a worked example. The problem is to determine if all conservation laws are conserved in $$ \pi^-+n \rightarrow K^++\Lambda^0 $$ The answer says that isospin is conserved, ...
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1answer
64 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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3answers
2k views

Do standard model particles actually exist or merely usefully describe behaviors of a medium?

I've read about how sound propagation can be modeled as phonon particles moving and interacting. I understand that this is a useful mathematical construct to describe the behavior of longitudinal ...
3
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2answers
125 views

What makes strange quarks stranger than any other quark?

Back in the 1950s, physicists noticed that certain heavy particles (sigma baryons, lambda baryons, & K mesons) have significantly longer lifetimes than physicists originally predicted just based ...
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1answer
54 views

Charged Current $\nu_\mu+e^-$ interaction?

Basically I want to know if there is something that forbids an interaction like $$\nu_\mu+e^-\rightarrow\mu^-+\nu_e$$ Having a look at the effective fermi interaction charged current lagrangian $$\...