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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Upper bounding the Kolmogorov Complexity of the Standard Model

The Kolmogorov complexity of a hypothesis/theory/model is the shortest computer program that simulates it, regardless of how inefficient executing that program may be in terms of memory and time. I'm ...
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Is the QCD Lagrangian without a $\theta$-term invariant under large gauge transformations?

In his book "Quantum field theory", Kerson Huang states that we need to add the term $$\frac{i\theta}{32\pi^2}G_{\mu\nu}^a \tilde{G}_{\mu\nu}^a$$ to the Lagrangian, to make it invariant under large ...
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263 views

What is the cause of the minor mass difference between the proton and the neutron?

As known, the proton is from two up and a single down quark, while the neutron is from a single up and two down quarks. The down quark is a little bit more massive and the up, and so the neutron is ...
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Why is the $\theta$ term of QCD violating charge and parity (CP) symmetries?

From the non-trivial nature of the QCD vacuum, the Lagrangian is augmented with a term like \begin{equation} \theta \frac{g^2}{32 \pi^2} G_{\mu \nu}^a \tilde{G}^{a, \mu \nu} \end{equation} where $ \...
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226 views

Chiral fermions from torsion flux in M-theory?

Witten's 1981 paper "Search for a realistic Kaluza-Klein theory" is frequently cited for its observation that, in a compactification of d=11 supergravity on a manifold with SU(3) x SU(2) x U(1) ...
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224 views

Why might $B$ meson decays violate lepton universality?

Scientists from various institutions have recently discovered that there might be a break in the application of the Standard Model, particularly with a fundamental principle called the lepton ...
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504 views

Effective Field Theory (EFT) decoupling top

The decoupling theorem of Appelquist-Carazzone says that if you want to decouple a particle, the low energy resulting theory need to be renormalizable. You can't do that for the top, because you break ...
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129 views

Does the Standard Model plasma develop a spontaneous magnetisation at finite temperature?

Reference: arXiv:1204.3604v1 [hep-ph] Long-range magnetic fields in the ground state of the Standard Model plasma. Alexey Boyarsky, Oleg Ruchayskiy, Mikhail Shaposhnikov. The authors of this paper ...
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195 views

How many orders of magnitude in energy spans the Standard Model phenomenological spectrum?

I am wondering if it makes sense to state that the upper limit is roughly 1012 eV (up to know the physics probed by the LHC seems to be pretty consistent with the SM) and the lower one is ... the ...
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1k views

On the naturalness problem

I know that there are several questions about the naturalness (or hierarchy or fine-tunning) problem of scalars masses in physics.stackexcange.com, but I have not found answers to any of the following ...
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267 views

What is the rate of B violation expected in the standard model during high energy collisions?

In a recent question Can colliders detect B violation? I asked about detecting B violation in collisions. Here I am interested in the theory aspect. (I asked both questions originally in the same ...
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200 views

Up and down quark content of neutral pion

If isospin were a perfect symmetry, then the neutral pion would have equal $u\bar u$ and $d\bar d$ content, but since up quarks are slightly lighter than down quarks, the neutral pion, being the ...
7
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1answer
366 views

To what extent are lepton and quark generations tied in the Standard Model?

The Standard Model of particle physics splits both the leptons and the quarks into three generations, with mass and instability going up from the first to the third generation. These are normally ...
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425 views

750 GeV diphoton resonance: KK graviton?

As everybody of you may know at LHC they found this probable resonance (https://cds.cern.ch/record/2114808, https://cds.cern.ch/record/2114853?ln=en). It may be a scalar or a KK graviton mode. Now, ...
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Question about the vacua of the Standard Model

This question is probably based on a misunderstanding. Please correct me if I'm wrong, and if unclear, I'll try to put it in a clearer language. In Yang-Mills theory such as the theory of strong ...
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277 views

Where in fundamental physics are Lie groups actually important (and not just Lie algebras)?

I was wondering where in fundamental physics the global structure of a Lie group actually makes a difference. Most of the time physicists are sloppy and don't distinguish groups and algebras ...
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Are QFT solitons expected to represent standard model particles? Or strings?

Is work on solitons in QFT's focused on finding solutions that could represent the fundamental particles of the Standard Model, or is the work focused on finding particles Beyond The Standard Model? ...
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258 views

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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196 views

Why is Quark Mixing forbidden in the Lagrangian (pre CKM)

The corresponding term in Lagrangian for the coupling of quarks to gauge fields reads: $$ \sum_{i} \bar Q_i D_\mu \gamma^ \mu Q_i .$$ Considering the Yukawa terms, it is generally stated that no ...
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From $U(3)$ to $SU(3)\times U(1)$ Color symmetry. There is a “gluon” photon-like?

Suppose that $U(3)$ was the gauge group. We can decompose this as $U(3)=U(1)\times SU(3)$, which implies that in addition to the $SU(3)$ that has eight generators corresponding to eight gluons, there ...
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If the gauge symmetry is not broken by spontaneous symmetry breaking, what symmetry is broken?

In this post, the answer by buzhidao showed that the $U(1)$ gauge symmetry is not broken by spontanous symmetry broken and Higgs mechanism. What role does "spontaneously symmetry breaking" ...
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161 views

Why is there not enough CP violation to account for baryogenesis in the SM?

One of Sakharov conditions states that both C and CP symmetries must be violated by a theory which includes any baryogenesis mechanism. The Standard Model happens to be such a theory, and happens to ...
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136 views

Why are spin 1/2 baryons less massive/energetic than spin 3/2 ones?

I am just wondering why the lower spin state multiplets for both baryons (when looking at $L=0$ and considering only the u d and s quarks) are lower in energy i.e. the 1/2+ multiplet is lower than the ...
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124 views

Where do the intrinsic parities of particles come from?

It is known that some particles have negative intrinsic parity - for example pion $\pi$. I was wondering if this parity can be understood. I read somewhere that parity of quarks is defined to be ...
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117 views

Few questions regarding String-Net theory and the Standard Model

A friend today showed me this post and after reading Prof. Wen's answer, few questions came to my mind. Prof. Wen says: all fermions (elementary or composite) must carry gauge charges (see cond-...
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554 views

Intrinsic parity

When we apply parity on a field two times, we demand that we should get back the same field. This gives us, $P^{2} =1$, which implies, $ P \psi = e^{i \theta} \psi$ . This extra phase factor is ...
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What are hadronic uncertainties?

I am reading articles about recent findings at CERN and the possibility of New Physics contributions, and they keep saying that the discrepancies may be due to hadronic uncertainties... What are they?...
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1answer
73 views

Scattering matrix symmetries and standard model

I am not able to get around the following question (if it make sense): Suppose I can derive the scattering matrix S for any particle scattering process. Suppose that the standard model is actually ...
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388 views

Why does the neutron have spin 1/2 and not 3/2?

The neutron is thought to consist of three tightly bound quarks, each with spin 1/2. Simple addition of angular momentum would tell us that the resulting system (neutron) could have either spin 1/2 or ...
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$\mathbb{Z}_6$ symmetry in the Standard Model

It's a well-known result that the spontaneous symmetry breaking of $SU(5)$ would lead not to the usual $G_{SM}=SU(3)\times SU(2)\times U(1)$, but to $G_{SM}/\mathbb{Z}_6$. However, it's also often ...
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1answer
85 views

running couplings using 'broken' and 'unbroken' SM lagrangian

In this question let me call unbroken Lagrangian simply the form of the SM Lagrangian $\mathcal{L}$ before the redefinition of fields involved after performing the shift $h \rightarrow h+v$, where $v$ ...
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Gauge Theory Motivation

I have been studying the mathematics of gauge theories for the past several months and now that I am beginning to understand, I find myself struggling to put everything in context historically. I have ...
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141 views

Does the Flavor symmetry forbid $uu\rightarrow cc,ss$?

This question comes from the reading of this paper. They suppose a flavor symmetry group $G_F = U(3)_q\times U(3)_{d} \times U(2)_{d}$ which acts on the three LH quarks $q_L$, three RH quarks $u_R$ ...
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145 views

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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Are mass terms forbidden in the Lagrangian because of parity violation or because fermions live in a complex representation?

Normally one argues that we can't write down Lorentz AND gauge invariant mass terms, because of parity violation, i.e. l-chiral and r-chiral fields transform differently. This means that mass terms ...
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Is Witten's claim that gauge group representations get exchanged with its dual under parity correct?

I'm currently reading Physics and Geometry by Witten, which I really liked up to the point where he claimed that we exchange representations $R$ and $\tilde R$ under parity transformations, where $R$ ...
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How can we count 17 particles in the standard model

This may be a bit of numerology, but I'd like to be able to make a statement like "There are 17 particles in the standard model" with some logical definition of a particle. But this statement is ...
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Understanding the effective low-energy Lagrangian for hadrons

My course in Higgs Physics is discussing a two-nucleon low-energy effective theory of hadron interaction. With $\psi=(p,n)$, the pion is defined as $\vec{\pi}= i \bar{\psi}\vec{\tau} \gamma_5 \bar{\...
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How does the Higgs Particle Decay?

Background: It is my understanding that the Higgs Boson is a fluctuation of the Higgs field. It also is not very stable and decays into a plethora of other particles. If all other particles interact ...
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flavor splitting of quark masses

It seems pretty clear that flavor symmetry splits the quark masses. It seems extremely odd that a global gauge symmetry of the Standard Model fields could or would produce such extreme splitting as ...
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140 views

Were fermions ever massless?

In a discussion of the Standard model and Higgs mechanism it was claimed that accordingly: "During an early phase of the cosmos all fermions were massless." I wonder whether this claim can be ...
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263 views

How can one activate the decay of the quark b with PYTHIA event generator?

This is my problem and I hope finding a solution. _In the simplest alternative, MSTJ(22) = 2, the comparison is based on the average lifetime, or rather (c*tau "time life") , measured in mm. Thus ...
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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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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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56 views

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-...
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How Is Are Parton Distribution Functions (PDFs) Determined In Practice And In Theory?

I have put my actual questions in bold normal font, and the rest of this question clarifies what I mean to be asking, especially in terms of the depth and kinds of information I am looking for. ...
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Detemining whether a particle is stable with respect to the electromagnetic, EM or strong force

If we have some beautiful hadron , how can we decide based on the quantum numbers and masses of the other beautiful hadrons whether it is stable under the electromagnetic, weak or strong interaction . ...
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1answer
43 views

Interpretation of a CLs exclusion plot

I have some trouble with the interpretation of this plot. The plot on the right shows the CLs values for the SM Higgs boson hypothesis as a function of the Higgs boson mass in the range 110–145 GeV. I ...
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Higgs decay at higher energies

If I look at the expected Higgs decay BR's vs. $m_H$ the WW and ZZ dominate over the tt decay mode in the over tt threshold region. From the mere couplings: $g_{ffH} = \sqrt2 m_f /v$ $g_{VVH} = 2 ...
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A question on the existence of elementary particles with relation to time

Suppose that the universe started existing at time $t = 0$ and we chose another arbitrary point in time $t =\tau$. Is it true that for every positive real number $\tau$ every single elementary ...