Questions tagged [electroweak]

Electroweak stands for the unification of the electromagnetism and the weak force. The unification energy scale is around 100~200 GeV, the mass scale of the Higgs boson. It is widely accepted to be formulated as a quantum field theory with gauge symmetries of U(1) symmetry in electromagnet forces and SU(2) flavor symmetry in weak forces. See also the [Wiki page](http://en.wikipedia.org/wiki/Electroweak_interaction).

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199 views

Elastic scattering of neutrinos on atomic electrons

In the elastic scattering of neutrinos on atomic electrons, why can tau and muon neutrinos only interact via weak neutral currents but electron neutrinos can interact with W or Z bosons?
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How do you know $W$ and $Z$ bosons are really bosons and not fermions?

It has been always said that $W$ an $Z$ bosons are bosons and not fermions but is there any experimental trial that prove that? Has anyone put two of them in the same quantum state or studies have ...
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$\tau \rightarrow \rho \nu $ relation with $ \rho \rightarrow e^+ e^-$

I'm reading "Okun, Leptons and quarks.", specifically subsection 13.3 "Semihadronic decays. General remarks." Okun says "The amplitude of the $\tau \rightarrow \rho \nu $ decay is related directly to ...
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ElectroWeak symmetry breaking and the early universe

I'm trying to piece together a coherent understanding of the EW symmetry breaking and its effect on early cosmology, from a number of somewhat incomplete sources. Any corrections to the following ...
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Monopoles from hypothetical Weinberg-Salam model with massless Z boson

We know that no monopoles exist in the Weinberg-Salam model (a good explanation is provided in the answer to this previous question). Now let us assume that the initial $SU(2)\times U(1)$ gauge ...
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Calculating $W$ - $Z$ bosons mass difference

Naively one might expect the W weak vector bosons to have slightly higher mass than the Z due to EM self-energy, while the opposite is true (80 vs. 91 Gev). Presumably due to the strong interactions ...
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Which $W$ boson is involved in electron capture?

I am learning about Electron Capture. $$p+e\to n+\nu_e$$ My question is whether the $W^+$ boson or the $W^-$ boson is involved in this transfer. I can consider this two ways: 1) Since the electron ...
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267 views

In electron-positron annihilation, why is photon exchange dominant at energies below the Z-resonance?

In a plot of the Z resonance from e+ e- collisions, why is photon exchange dominant below the center of mass energy below the Z peak?
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Weak hypercharge assignments in the Standard Model

While refreshing my memory on details of the Standard Model, I ran into a muddle. I might not exist to ask bizarre questions if the neutron, neutrino, and hydrogen atom were not electrically neutral. ...
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Weak isospin current

I cannot understand the product of a Dirac gamma matrix and a Pauli matrix in this formula of the weak isospin current: $$J_α^i(x)=\frac12\bar \psi_L(x)\gamma_\alpha\tau^i\psi_L(x),$$ where $γ_α$ is ...
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What Makes a $W$-Boson a Gauge Boson?

I understand that in beta decay, a quark changes its flavor and emits a $W$-Boson, and this $W$ (is the W Boson virtual, by the way?) quickly decays into an electron and an electron antineutrino or ...
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207 views

Are both Gell-Mann Nishijima formulas true for the same reason?

The Gell-Mann Nishijima formula states that $$Q = I_3 + \frac{Y}{2}$$ where $Q$ is the electric charge, $I_3$ is the third component of isospin, and $Y$ is the hypercharge. This was an empirical fact ...
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459 views

What is the isospin of the photon?

As already mentioned in this Phys.SE post, the PDG booklet says that the isospin of a photon is $0,1$. Is it referring to the strong or to the weak isospin of the photon? What would be the 3rd ...
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Form of Coulomb’s law before the electroweak phase transition

I’ve been thinking about the electroweak phase transition in the early universe, and I have some questions about the form of Coulomb’s law during that epoch. As I understand it, before the ...
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Total decay width of the W boson

I read in some particle physics book how they calculate the two body decays of the W boson, and then add all of them get the total decay width. I am wondering about other contributions, say, three ...
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Why is the Fermi coupling $G_F$ is measured from muon decay?

The decay rate of all weak processes, calculated from the $V$-$A$ theory will contain a factor of $G_F$, the Fermi coupling constant. However, $G_F$ is usually measured from $\mu^-$ decay which is in ...
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Calculation of the weak coupling constant

There are two ways to calculate the coupling constant of the weak interaction $g$. 1) From the electromagnetic coupling constant and the weak mixing angle, using the relation $${\sf e} = g\sin(\...
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592 views

$SU(2)$ to $U(1)$ symmetry breaking

Why does electroweak theory have $SU(2) \times U(1)$ to $U(1)$ symmetry breaking. Is it not possible to have simply $SU(2)$ to $U(1)$ symmetry breaking?
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How does a Higgs triplet transform under $SU(2)_L \times U(1)_Y$ when written as a $2\times 2$ matrix?

I recently learned that a Higgs triplet can be written as a $2 \times 2$ matrix: \begin{equation} \Delta=\begin{pmatrix} \frac{\Delta^{+}}{\sqrt{2}} & \Delta^{++} \\ \Delta^0 & - \frac{\Delta^...
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What crossed over at the electroweak crossover?

It's been known for quite some time now that the electroweak "transition" in the early universe is first-order for a Higgs mass of less than about 75 GeV, but for a larger Higgs mass (including the ...
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Renormalization scale dependence of S, T parameter of Electroweak precision test

In Peskin, Takeuchi(1992) paper(link), oblique parameters S and T, U are defined as (Eqn 3.12) $\alpha S\equiv 4e^2[\Pi'_{33}(0)-\Pi'_{3Q}(0)], \\ \alpha T\equiv \frac{e^2}{s^2c^2m_Z^2}[\Pi_{11}(0)-\...
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Are Photons Goldstone Bosons?

I'm not interested in more speculative ideas, like the one of Bjorken et. al. that photons are Goldstones of broken Lorentz symmetry. Instead, I want to understand if photons are simply the ...
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Why can increasing energy, the weak force and the EM-force become a similar force?

When the heat of the universe was much higher it seems that the weak force and the EM-force was combined into the electro weak force. For a layman the EM-force and the weak force look quite different,...
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Why the rate of Z boson decays produce 3 generations of matter?

I read a tons of article about Z boson, I know it is a carrier of weak force similar to photon except it is much much more massive than an electron and it is extremely short lived. Sadly almost all ...
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Is the expression for CP asymmetry wrong in Wikipedia?

In the wikipedia page about neutrino oscillation (or ate least in the current revision), the CP-asymmetry is written as $$A^{(\alpha\beta)}_{\text{CP}}=P(\nu_\alpha \rightarrow\nu_\beta)-P(\bar{\nu}_{\...
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Splitting of Forces at Low Temperatures?

Since the electroweak force split from the strong force during the electroweak epoch then the electroweak force split into the electromagnetic and weak force during the quark epoch, is this an ...
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Neutral kaon semileptonic decays, why is $\overline{K^0}\rightarrow l^+\nu\pi^-$ not allowed?

On several texts it is stated that the semileptonic decays are different for the $K^0$ and $\overline{K^0}$, and each only gives leptons of particular charges (see www.hep.ph.ic.ac.uk/~dauncey/will/...
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Could the electroweak spontaneous symmetry breaking explain dark matter and dark energy? [closed]

Electroweak spontaneous symmetry breaking is much harder for me to understand than the gluon-quark plasma phase transition. In the latter the physical laws remain the same and gluons and quarks just ...
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Why is the term $\bar{L}\phi R$ invariant under $SU(2)_L \times U(1)_Y$ where $\phi$ is the Higgs field?

In the explanation for the fermion masses, the term $$\bar{L}\phi R$$ is added to the Lagrangian where $L$ is a left-handed $SU(2)$ doublet, $\phi$ is the Higgs left-handed $SU(2)$ doublet and $R$ is ...
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Why are the masses $m_{W}/m_{Z}$ and the coupling constants $e/g$ related in electroweak unification?

Gottfried and Weisskopf write in their book, in the chapter on electroweak theory, that $\frac{m_W}{m_Z} + \left(\frac{e}{g}\right)^2 = 1$ In this expression, $m_W$ and $m_Z$ are the masses of the $...
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QCD pion and electroweak symmetry breaking

Equation (28.33) in Matthew Schwartz's QFT text book $$ \mathcal{M}(\pi^+ \rightarrow \mu^+ \nu_\mu) = \frac{G_F}{\sqrt{2}}F_\pi p^\mu \bar{\psi}_{\nu_\mu}\gamma^\mu(1-\gamma^5)\psi_\mu \tag{28.33} $$ ...
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Why do weak interactions exclude gluons?

Weak interactions seem the most universal after gravitation. A very few particles avoid them, only the photon and gluon, plus right leptons. The photon, however, is a part of the electroweak ...
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Is it good approximation to use the Fermi theory in terms of proton and neutron for the given processes?

Suppose the processes $$ p+\gamma \to n + \bar{l} + \nu_{l}, \quad p+\gamma \to p+\bar{\nu}_{l} + \nu_{l}, $$ where $p$ denotes a proton, $\gamma$ - a photon, $l$, $\nu_{l}$ - a lepton and ...
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Physical interpretation of weak hypercharge

When we have a doublet in electroweak gauge symmetry, we assign a weak hypercharge. The question is, if weak hypercharge is zero we could be talking about that there isn't electroweak interaction with ...
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The effective Lagrangian of composite Higgs model

I have a naive question about composite Higgs model. In https://arxiv.org/pdf/hep-ph/0412089.pdf (the original paper of $SO(5)/SO(4)$ minimal composite Higgs model), the authors say that, after ...
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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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What did electroweak symmetry breaking actually look like?

Approximately one picosecond after the Big Bang, the universe cooled down enough to pass through the electroweak phase transition. At this point the Higgs mechanism kicked in, the weak force became ...
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Calculation of Wilson Coefficients

In Flavour physics, amplitude of decay processes is generally expressed in terms of effective operators (reference). In the framework of effective field theory amplitude $\propto C_iO_i$, where $O_i$ ...
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Changing the hypercharge of the Higgs field

I'm trying to solve a question about the Higgs mechanism: If the hypercharge of the Higgs were to be $Y=1$ (Changing from $Y=1/2$ ), what would be the photon of this world? Meaning, how can I ...
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Since gluon fields do not appear apart from quarks, did gluon fields first appear during the quark epoch?

Big Bang cosmology and grand unification theories say that the strong force and the electroweak force separated during the electroweak epoch, i.e., $10^{-36}$ to $10^{-32}$ seconds after the Planck ...
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Why the four gauge bosons that correspond to the $SU(2)\times U(1)$ electroweak force before symmetry breaking are not listed in the Standard Model?

If I correctly understand this, the four gauge bosons that correspond to the electroweak force before symmetry breaking are the W1, W2, W3, and B. How come the W1, W2, W3, and B bosons are not listed ...
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Are electroweak particles stable?

Photons that [are associated with] the electromagnetic force are stable; while the W and Z bosons that [are associated with] the weak force are short lived. I guess that the high temperate electroweak ...
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Neutral vector boson from renormalizing IVB (intermediate vector boson) theory?

Consider the intermediate vector boson (IVB) theory, with the interaction Lagrangian density $$\mathcal L_I = \sum_l g_W (\overline{\psi}_{\nu_l} \gamma^\alpha(1- \gamma_5)\psi_l ) W_\alpha^\dagger + \...
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Cold neutron radiative capture by proton

What is the mechanism of cold neutron radiative capture by proton? I have seen some experimental papers on studies of parity violation via this reaction, but I haven't seen a fundamental description ...
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338 views

Why is Weak Hypercharge different for each fermion?

I'm having trouble with Electro-Weak Theory. I understand that one wants to develop a gauge theory with $SU(2)_L \otimes U(1)_Y$. What buggs me is that the coupling with different fermions has ...
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Why in the electroweak theory is symmetry 'hidden' at low energies? But energy is not a function of time $E=h\nu$?

Electroweak theory argues that the symmetry that accompanies them is obvious only to the high energies, while it is hidden with inferior energies. Scientist use top analogy to explain this concept but ...
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Can't the weak force be explained purely by electrostatics?

Again, super naive, but my basic question is: isn't the force of attraction between electrons and protons sufficient to explain their confinement to atomic orbitals. I know the W and Z particles have ...
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Why are monopoles resulting from electroweak symmetry breaking allowed?

I understand (i think) that for a magnetic monopole to exist as the result of a gauge group $G$ being spontaneously broken to a subgroup $H$ by the Higgs mechanism, that certain criteria must be ...
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337 views

What are singlet representations for the electroweak gauge group $SU(2)\times U(1)$?

This question comes from Srednicki's textbook "Quantum Field Theory". On page 532, the left-handed Weyl fields $\ell$ (a single lepton family, electron and its neutrino) and $\overline{e}$ are in the ...
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Why do we regard $W^\pm$ as physical fields but not $W^1$ and $W^2$?

When expanding the electroweak Lagrangian, we obtain terms like $$ \mathcal L = \frac{g}{2}(\bar e_L (W^1_\mu + iW^2_\mu)\gamma^\mu \nu_L + \bar \nu_L (W^1_\mu -iW^2_\mu )\gamma^\mu e_L)+ \dots $$ ...