New answers tagged standard-model
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When is the Lagrangian a Lorentz scalar?
An obvious, kind of dumb, answer is that the Lagrangian corresponding to a given Hamiltonian will be a Lorentz scalar if the Hamiltonian has the form,
$$ \mathcal{H} = \pi^a \frac{\partial}{\partial t}...
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When is the Lagrangian a Lorentz scalar?
As far as I know, there are no good ways of stating what conditions on the Hamiltonian will cause the Lagrangian to be a Lorentz scalar other than to just say the Hamiltonian must be derived from a ...
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Accepted
$ \pi^0\to \gamma\gamma$ parity conservation
As @Cosmas Zachos said in his comment, the Levi-Civita pseudotensor $\epsilon$ has negative parity. But why?
This is the case because $\epsilon_{\mu\nu\rho\sigma} := \sqrt{-g}\ \varepsilon_{\mu\nu\rho\...
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Parity transformation of the $\pi^{0}\rightarrow\gamma\gamma$ process
... how do I apply P?
Spacelike components of a vector/tensor reverse their components. But the Levi-Civita symbol is a pseudotensor, so its three space like reversals amount to changing the ...
- 64.2k
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What force or field is associated with the weak hypercharge?
A clip from wikipedia:
Weak hypercharge is the generator of the U(1) component of the electroweak gauge group, SU(2)×U(1) and its associated quantum field B mixes with the W3 electroweak quantum field ...
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What is the associated charge for the weak neutral current?
In the standard model of particle physis, the weak neutral current Lagrangean has the general form $$-\mathcal{L}_{\rm nc}=\frac{g}{\cos \theta_{\rm W}}\sum\limits_\psi \bar{\psi} \gamma^\mu (T_3-\sin^...
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Charge conjugation and Transition form factors
Perhaps the conceptual confusion here regards the difference between gauge symmetries and global symmetries. Gauge 'symmetries' are really just redundancies---different descriptions of entirely the ...
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Have all the symmetries of the standard model of particle physics been found?
My answer is also ‘No’ but by direct construction: over the past few years, various research groups have understood new symmetries of the Standard Model.
Over the past decade, field theorists have ...
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Checks of anomaly cancellation
A lot of the information you gave me is beyond my knowledge, not being a science professional myself or even a student. But I like reading physics textbooks from time to time and the chapter on ...
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Why is the $\theta$ term of QCD violating charge and parity (CP) symmetries?
Old post, but I just heard of the following analogy recently so I thought I'd share. In the comments and elsewhere on this site, people have given a good answer about the mathematical answer to why ...
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Checks of anomaly cancellation
... if 𝐺 is a global symmetry of the classical Lagrangian, then one has to check 𝐺×𝐻² anomalies, where 𝐻 is one of the SM gauge groups.
On may check them for academic purposes, not consistency, ...
- 64.2k
2
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Accepted
Does electroweak theory have mass gap (not just Higgs mechanism)?
A Higgsed non-Abelian gauge theory has two would-be scales, a spontaneous symmetry breaking scale, and a confinement scale determined by the running of the coupling, analogous to $\Lambda_{QCD}$. ...
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How can a virtual $W$ boson turn into an electron and electron antineutrino?
Your question is a result of "just winging it" with quantum field theory. The idea that a $W$ boson with mass $M_W$ is created for a short period of time by borrowing energy via the HUP is ...
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How can a virtual $W$ boson turn into an electron and electron antineutrino?
Yes, the W boson in beta decay is virtual (off-shell). It is not necessary for the W boson to "last enough" for it to decay. It can decay into an electron and an antineutrino because the ...
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Accepted
Could there be states of matter that could avoid "matter decay"?
Baryon decay can be avoided if you build something out of stable non-baryonic matter. Given our currently known particles that leaves electrons/positrons, and neutrinos.
Stable electron clouds are ...
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What will happen if we keep bringing two protons closer and closer to each other, starting from a large distance?
Proton-proton collision induces parton (constituents of proton) scattering. From here1:
Protons consist of three valence quarks, two up-quarks and one down-quark, held together by gluons and a sea of ...
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What will happen if we keep bringing two protons closer and closer to each other, starting from a large distance?
The protons distract each other by the coulomb force. As you coerce them more and more close, you invest energy into the system.
At a point, beta decay of one of the protons becomes energetically ...
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Accepted
Why does the up and anti-up quark combine into a pion and not a photon?
Particle annihilation occurs when a particle meets its corresponding antiparticle, converting their mass-energy into two photons.
This is a useful oversimplification. Even electron-positron pairs, ...
rob♦
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What will happen if we keep bringing two protons closer and closer to each other, starting from a large distance?
If you did it slowly enough, you could create a very short-lived particle called a diproton, the 2He nucleus (Helium without any neutrons).
Wikipedia says its half time is 10-9 seconds because the ...
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Why does the up and anti-up quark combine into a pion and not a photon?
For a first-generation quark-antiquark pair (up-antiup or down-antidown) to have a high probability to form a neutral π meson ($π^0$) these conditions must be satisfied :
Color charges should be such ...
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What will happen if we keep bringing two protons closer and closer to each other, starting from a large distance?
This really depends on the energy scale at which you bring them together. At CERN they do proton-proton collisions, where the protons move at speeds close to the speed of light. At those energy scales,...
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