Questions tagged [quantum-chromodynamics]
Quantum-ChromoDynamics (QCD) is the quantum field theory believed to describe the strong nuclear force.
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Why are gluons massless as their range is finite?
The range of electromagnetic waves and gravitational force is infinity and the particles exchanged during these interactions are photons and gravitons respectively. Both are massless following the ...
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Diquark propagators in color superconductivity
I’m studying color superconductivity referring to “The Phases of Quantum Chromodynamics From Confinement to Extreme Environments” by John B. Kogut and Mikhail A. Stephanov (link).
In chapter 9, the ...
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If isospin is only approximately conserved by strong interaction, why do we never see isospin violation?
Due to the mass difference between the $u$-quark and the $d$-quark, SU(2) isospin symmetry is only an approximate symmetry (even in a universe devoid of weak and EM interactions). This suggests to me ...
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Integral of the square of the spectral density in a quantum field theory
The quantity of interest is
$$
\int_0^\infty dE \, \rho(E)^2
$$
where $\rho(E)$ is the spectral density in a Quantum Field Theory. I am wondering whether
it has any physical meaning, and
it can be ...
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Why, in QCD, are quarks in the fundamental representation of $SU(3)$?
QCD is built from the notion that Dirac's Lagrangian should be invariant under gauge colour transformations.
Here, quarks are elements of $\psi_{\alpha,f,c}(x)$, where $\alpha$, $f$ and $c$ stand for ...
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Mass of the kaon (1460)
When looking at the particle listing in PDG most of the hadron masses and excitation include an experimental error. But this is not the case for the Kaon (1460), does anybody know why? This means the ...
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Quantum Chromodynamics and Group Theory
I know nothing of QCD, but I was watching this youtube video and pondered on whether the additive structure described is a group, and if so, which is it?
As of now, I know the group must contain the ...
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Justifying the identification of eight gluons with the ${\bf 8}$ from ${\bf 3}\otimes{\bf 3}^*$
When we combine the fundamental ${\bf 3}$ and antifundamental ${\bf 3}^*$ of color $SU(3)$ of QCD i.e. single quark of three colors and a single antiquark of 3 anticolors, nine states are obtained. ...
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Why does full QCD not invariant under the $Z(3)$ symmetry group?
Hello and thank for the time you will take reading my question.
The $Z(3)$ symmetry can be defined as a simple global phase transformation: $$Z(3)=\left\{1, e^{\frac{i2 \pi}{3}}, e^{\frac{-i2 \pi}{3}}\...
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Spin and rotation of gluon field components
The gluon field has 4 spacetime components: $A_\mu = (A_0, A_1, A_2, A_3)$, like the electromagnetic field.
Firstly, how to find each component of the gluon field?
And how to perform a rotation on ...
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Theta vacua eigenstates
I have been trying to prove the very simple result that the eigenstates of an operator with matrix elements
$$
\langle n^\prime | H | n \rangle \sim g(|n^\prime-n|),
$$
in a basis $\{|n\rangle\}^{+\...
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How is the pion related to spontaneous symmetry breaking in QCD?
In chapter 19 of An Introduction to Quantum Field Theory by Peskin & Schroeder, they discuss spontaneous symmetry breaking (SSB) at low energies in massless (or nearly massless) QCD, given by
$$\...
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Goldstone bosons in 2 and 3 quark flavor symmetries [closed]
In my (undergraduate) advanced elementary particles class last semester, we learnt that for a 2 quark (u/d) model the symmetry of the Lagrangian is (and breaks as)
$$
U(2)_L \otimes U(2)_R = SU(2)_L \...
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Can a sea quark swap places with a valence quark within a proton?
I understand that a proton is composed of three valence quarks and a very large number of sea quarks that briefly come into existence and then annihilate as quark-antiquark pairs.
Is it possible for ...
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$\mathrm{SU(3)}$ structure constant values
The $\mathrm{SU(3)}$ structure constants $f_{abc}$ are defined by
$$[\lambda^a,\lambda^b] = 2i f^{abc} \lambda^c,$$
with $\lambda^a$ being the Gell-Mann matrices. In three different books, I find its ...
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