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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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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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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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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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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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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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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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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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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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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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 ...
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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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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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Amount CP violation needed
It is widely known that the CP violation provided by the Standard Model ( SM )is insufficient to explain the baryon asymmetry that we observe. However, I never came across any quantitative ...
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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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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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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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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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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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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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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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Another "real vs virtual particle" one: calculate the full diagram or $\sigma \times \mathrm{BR}$?
I have recently been forced to think about something I always thought that I understood but that, in reality, I could not be more confused about.
Suppose I want to study the LHC prospects for ...
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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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$2\nu\beta\beta$ vs. $2\nu$ECEC
Considering the recent observation of two-neutrino double electron capture ($2\nu$ECEC) by the XENON Experiment:
https://arxiv.org/abs/1904.11002
I was wondering why the half-life of this process is ...
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Why is the Higgs mass renormalization considered a fine-tuning problem, while the electron mass (in QED) isn't?
Don't both masses require infinite corrections in their renormalization procedure?
It is my understanding that the electron self-energy in QED increases to infinity with increasing cutoff value on ...
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How can strings be more fundamental than quantum fields?
I had the feeling that physics has moved on from the idea that spacially bounded objects located in spacetime (such as particles) can be fundamental. Instead, QFT describes everything by quantum ...
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Why is the branching ratio $H\to WW$ much larger than $H \to ZZ$?
The Higgs decay to a pair of W bosons is the second-largest ($\mathrm{BR}(H\to WW)=21.8\%$), while the one to $Z$ bosons is so much smaller ($\mathrm{BR}(H\to ZZ)=2.7\%$). Is there an obvious reason ...
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Fermilab $g-2$ muon experiment: How to obtain $a_{\mu}$?
In the $g−2$ muon experiment, we measure the anomalous
precession frequency $$\omega_a = \omega_s − \omega_c,$$ where $$\omega_s = \frac{g_{\mu}\left\vert q\right\vert B}{2m} + \frac{\left( 1-\gamma\...
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Why is there not enough CP violation to account for baryogenesis in the SM?
One of Sakharov conditions states that both C and CP symmetries must be violated by a theory which includes any baryogenesis mechanism. The Standard Model happens to be such a theory, and happens to ...
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Why are spin 1/2 baryons less massive/energetic than spin 3/2 ones?
I am just wondering why the lower spin state multiplets for both baryons (when looking at $L=0$ and considering only the u d and s quarks) are lower in energy i.e. the 1/2+ multiplet is lower than the ...
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Where do the intrinsic parities of particles come from?
It is known that some particles have negative intrinsic parity - for example pion $\pi$. I was wondering if this parity can be understood. I read somewhere that parity of quarks is defined to be ...
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Few questions regarding String-Net theory and the Standard Model
A friend today showed me this post and after reading Prof. Wen's answer, few questions came to my mind. Prof. Wen says:
all fermions (elementary or composite) must carry gauge charges (see cond-...
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What are hadronic uncertainties?
I am reading articles about recent findings at CERN and the possibility of New Physics contributions, and they keep saying that the discrepancies may be due to hadronic uncertainties...
What are they?...
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Why is Quark Mixing forbidden in the Lagrangian (pre CKM)
The corresponding term in Lagrangian for the coupling of quarks to gauge fields reads: $$ \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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Physical meaning of minimal coupling assumption
In SM, both for the gauge field $A_{\mu}$ associated with photon and for the gauge fields $W^{j}_{\mu}$, to restore the invariance of the lagrangian after the changing of the global transformation to ...
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When do pairs of quarks form jets vs mesons?
For certain processes with low momentum transfer, such as the Kaon decay shown below, quarks will form bound states of mesons.
Whereas for higher momentum-transfer processes, such as the decay of an ...
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Is the Higgs VEV unnatural?
Is there some way to argue that the Higgs VEV is unnatural, without even bringing up the Higgs boson mass? For example, can one show that the Higgs vev receives large corrections from new physics? The ...
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Why are these loop diagrams involving Goldstone bosons cancelling one another in the zero-momentum limit?
I have been working with one-loop corrections to certain processes. Let's say, corrections to the $Z\bar{f}f$ vertex. Of, course I need to consider the following diagrams:
Here $\phi^{\pm}$ is the ...
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Why doesn't the gluon fusion channel for Higgs production diverge?
The heavy quark triangle integral has a superficial degree of divergence of $D=1$, so one would naively expect it to diverge (more than logarithmically, in fact) in 4 dimensions.
It happens, though, ...
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Why does weak interaction violate time reversal symmetry?
Is there a theoretical explanation (or model) which explains the violation of time reversal symmetry by weak interaction, or it is simply an empirical fact?
user283378
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Parity of Particles
Like charge, spin,etc.. Parity is an intrinsic property of the particle. As I read in Grifith's introduction to elementary particles book , he states parity mathematically but I am not able to see it ...
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Fundamental invariants of the electroweak sector?
In a previous question, I asked what the matrix representation of the electroweak fields is, and I was told they are identical to the Faraday tensors, but come in a set of three ($W_i, i\in \{1,2,3\}$)...
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Equivalence Between the Algebras of the Standard Model and Connes Non-Commutative Geometry Model
I've been watching some lectures on Connes non commutative geometry model and one of the things I don't understand is why the algebra he considers, $M_2(\mathbb{H}) \oplus M_4(\mathbb{C})$, is ...
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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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What's the meaning of QCD phase transition temperature?
I read about the QCD energy level in various sources, eg: Wikipedia, and this gives a phase transition temperature of about 200 MeV, above which quark confinement breaks down and we may observe ...
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Virgin value of the Higgs field
https://profmattstrassler.com/articles-and-posts/particle-physics-basics/the-hierarchy-problem/naturalness/
According to Matt, the 246 GeV vev of the Higgs field came from:
"We get these two ...
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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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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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Can electron, muon, and tau be the particles of the same field, just different fluctuations?
The electron, muon, and tau somehow look the same, the only difference between them is their masses.
Can it be that they are the particles of the same field (not separate fields as an electron field, ...
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Why isn't weak hypercharge quantized to integer values?
Page 527 of Srednicki's QFT book lists the matter content of the Standard Model as
left-handed Weyl fields in three copies of the representation $(1, 2, −1/2)⊕(1, 1, +1)⊕(3, 2, +1/6)⊕(\bar{3̄}, 1, −...
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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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