Questions tagged [quantum-chromodynamics]

Quantum-ChromoDynamics (QCD) is the quantum field theory believed to describe the strong nuclear force.

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What is the modern status on the Skyrmion theory of baryons?

It was noticed in the 1960s that chiral perturbation theory, describing the goldstone-bosons (pions) of the breaking $SU(N_f)_L \times SU(N_f)_R \to SU(N_f)_V$ of the chiral symmetry, has solitonic ...
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Is the concept of bicolored gluons mathematically precise/meaningful? Please explain

Each flavour of quark carries a colour quantum number: red, green or blue. I know what it means mathematically. But elementary textbooks (e.g, particle physics by Griffiths) also say that gluons are ...
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What would the force arising from an $SU(4)$ gauge field operate like? (As in, how many charges, whether the boson would interact with the force, etc)

Heyo, i'm new to this all, and deadly curious what this would look like. If this isn't specific enough, lemme know.
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Chemical potential interpretation for QCD phase diagram

I'm studying the QDC phase diagram at finite densities and I'm having some trouble understanding the role of the chemical potential. It's clear how, if I increase the density of particles at $T=0$, I ...
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How are we sure that the measured proton spin puzzle will be also observed similarly in the neutron thus a neutron spin puzzle?

The EMC experiment in 1988 using muons' deep inelastic scattering, has reported that the contribution of the valence quarks triplet (i.e. up-up-down) in the proton was measured to contribute as little ...
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Quantum chromodynamics - why are there no $rrb$ or $ggr$ terms?

$$\Psi_{colour}^{qqq} = \frac{1}{\sqrt{6}}(rgb + gbr + brg -grb - rbg - bgr)$$ My lecturer stated that there cannot be any $rrb$ or $ggr$ terms in the expression above. I would like to understand what ...
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Why did the quark epoch occur earlier than the lepton epoch?

I don't understand the reason and hypothesis behind why the quarks appeared first—as per the big bang cosmology—shortly after the strong and electroweak force separated. We don't know what the quarks ...
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Can we say tension is the ability to generate interference by this paper?

https://arxiv.org/abs/1206.2021 Abstract : We propose a method for measuring the string tension in gauge theories, by considering an interference effect of mesons, which is governed by a space-time ...
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If a spin singlet cannot change, how can it interact with electromagnetism?

Let's say there's a Hydrogen atom in a spin triplet state. $$ | \downarrow\downarrow \rangle$$ Now let's say a photon with spin 1 came along abs was absorbed by the atom. We don't know if the proton ...
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Symmetry Factors in $n$-point one-loop function for QCD

I am calculating (the divergent part) of the gluon 3-point function and gluon 4-point function in the QCD Lagrangian. So I have found here what I believe to be all the 1PI Feynman diagrams at one-loop....
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What are all the 1PI gluon 3-point function Feynman diagrams at 1-loop?

As an exercise in renormalization, I want to calculate the divergent part of the gluon 3-point function and gluon 4-point function matrix element. What are all the 1PI one-loop Feynman diagrams?
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Boosting into a frame where two lightlike vectors are back-to-back

Consider a pair of lightlike vectors $a$ and $b$ which are reciprocal, that is $a\cdot b\overset{!}{=}1$. I would like to construct a boost $L(\beta)$ such that, for $L.a =a'$ and $L.b= b'$, the ...
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Does a free proton shrink when accelerated?

Reading about the charge radius of the proton and proton radius puzzle I started wondering if the cross section and therefore also the charge radius (i.e. size of proton) changes or more specifically ...
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Puzzle with the precession of the neutron spin?

More perplexing news about the neutron spin in this recent Nature Physics publication: arxiv.org/abs/2103.03333 The authors collaboration (more than 100 authors) report a newly experimentally found ...
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$SU(N)$ gauge theory in black hole background

Assume you have standard $SU(N)$ gauge theory in a (non-asymptotically flat) black hole background (i.e., near the center of the black hole). Given the extreme pressure and temperature, I assume that ...
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Do quarks have a non-zero electric dipole moment?

It is written here, in this PSE link that an electron has a measured perfect spherical charge density therefore a zero electric dipole moment (i.e. perfect monopole charge). My question is, are quarks ...
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In the history of the development of the standard model was there a version in which quark-antiquark pairs or flux tubes were the fundamental particle [closed]

Based on the observations of color confinement and no free quarks, I'm wondering if instead of a having quarks as fundamental particles, during development of the Standard Model was there ever a ...
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Reflection identity for color ordered gluon partial amplitudes

I'm currently studying various modern methods for calculating scattering amlpitudes, and I'm following Dixon's notes on the topic: https://arxiv.org/abs/hep-ph/9601359. When talking about the color ...
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Is there a simple explanation of why coupling constants run with $\log(E)$?

The inverse coupling constants run with $\log(E)$, where $E$ is the energy or four-momentum. Some coupling constants increase, some decrease with $\log(E)$. Is there a simple argument that explains ...
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How Do Pions Mediate The Residual Strong Force?

I know that the continuous exchange of gluons between quarks is what holds hadrons together, and that the exchange of pions between nucleons is what creates the strong residual force. However, how ...
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Lattice QCD Link Variables Meaning of $\mu$ and $\nu$

I'm currently coding a lattice QCD project and ran into an issue with my understanding. A link variable connecting two points could be in the $\mu$ or $\nu$ direction, for example, $U_\mu(x)$ goes ...
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Do Quarks Exist?

Do Quarks Really Exist? Is there any experiment which can confirm the existence of Quarks and what kind of force is that which binds them to form neutron and proton?
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Quark mass dependence of Glueball masses

In pure QCD, we have glueballs. Pure QCD can also be thought of as QCD where the fermion masses have been sent to infinity. If we vary the fermion masses (say, for simplicity, we deal with a single ...
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Lattice QCD $SU(3)$ Pseudo Heat Bath Algorithm in Practice

I'm doing a Lattice QCD project and would like to use the pseudo heat bath algorithm for updating links. I've been following Gattringer and Lang's "Quantum Chromodynamics on the Lattice". ...
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How does the Nambu-Goldstone mode explain the absence of parity doubling?

I've been doing some reading about chiral symmetry breaking since it was not touched in my particle physics course I found these slides As explained in the above link, if we take $|\psi \rangle$ as ...
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Writing fermionic correlation functions as a product of fermion propagators

I’m doing some work on Vafa-Witten and Weingarten type inequalities which realy on the positive definite path integral measure of vector-like gauge theories. In the original Weingarten paper , the ...
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Why does a gluon soft factor depend only on the adjacent legs?

The gluon soft factor can be obtained by taking the soft gluon limit in the $n$-point MHV amplitude $$A(1,2,\cdots, n)=\frac{\langle i,j\rangle^4}{\langle 1,2\rangle\cdots\langle n, 1\rangle},$$ to be ...
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Laplace transformation with mass

I am doing some QFT calculations with the fermion mass term, and it is making the calculation much more challenging. Say I have $$F(t,m) = \int_0^1 dx \frac{1}{1-x}\frac{1}{(t ~(1-x)+m(1-x)^2)^\...
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What are hadron parity partners?

I am studying Lattice QCD and there are many papers mentioning "Parity partners". What does this term mean?
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A gluon can have nine independent, bicolored states. How are some of the additions of these individual states, like $r\bar{r}+g\bar{g}+b\bar{b}$?

This came from the 25th page of the following pdf: http://www.gammaexplorer.com/wp-content/uploads/2014/03/Quarks-and-Leptons-An-Introductory-Course-in-Modern-Particle-Physics.pdf Sorry if I am asking ...
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Regge Theory references and predictive power

I am considering studying Regge theory and would like to listen/read some introductory lectures, reviews or textbooks. Does anyone has any good material for beginners in Regge theory. Furthermore I ...
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Is the pion valence PDF symmetric?

The positive-pion $\pi^+$ is comprised of 1 valence up-quark and 1 valence anti-down-quark. Assuming isospin symmetry, the parton distribution functions (PDFs) for these two valence quarks should be ...
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Why is the chiral condensate a negative quantity?

The chiral condensate serves as an order parameter for the chiral phase transition. Thus, it is a finite quantity in one phase and vanishes in the other phase. It is given as a vacuum expectation ...
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Is QCD parity conserving also non-perturbatively?

Since QCD is fundamentally non-perturbative at low energies one may ask if QCD is still Parity conserving. In the path-integral formalism using the Faddeev–Popov ghosts as gauge fixing terms the ...
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Renormalization group applied to a simple QCD problem

Consider page 551 of Peskin & Schroeder's book on QFT, where he is treating the process $e^{+}e^{-} \to \text{Hadrons}$. I have 3 misunderstandings regarding some lines of thought effected there: ...
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What is the structure of the nucleus?

I think my learning's gone awry: Somewhere along the way, I picked up the notion that there exists two separate valence structures within the nucleus (one structure composed of protons, the other of ...
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How do I calculate the Altarelli-Parisi splitting function for $g \rightarrow q\bar{q}$?

I'm trying to calculate the Altarelli-Parisi splitting function in the collinear limit for a gluon splitting into a quark-antiquark pair, but I keep getting stuck. Let $p$ and $k$ be the momenta for ...
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Has the quadruple gluon vertex of QCD been observed experimentally?

QCD has triple and quadruple gluon vertices in Feynman diagrams. Has the quadruple gluon vertex been observed? For example, did jet events confirm it? Or did some other experiments see evidence for ...
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What means Higgs branches in 5d?

I've been reading advanced string theory text but I came across with a paper that I definitely don't understand at all, what it means?, this text is Higgs branches of 5d rank-zero theories from ...
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How to derive Collinear amplitude proportional to Born amplitude

In the collinear limit, the squared matrix element factorises into (for partons 4 and 5 going collinear) \begin{eqnarray} \overline{\sum}|M_3(1+2 \to 3+4+5)|^2 \approx \overline{\sum}|M_2(1+2 \to 3+4')...
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Spinor index contractions for baryons

For an $SU(N)$ QCD with $N_f$ flavours one writes baryons as \begin{equation} B^{{a_1}…a_{{N}_f}}=\epsilon^{i_1…i_N}\psi_{i_1}^{a_1}… \psi_{i_N}^{a_{Ν_f}} \end{equation} by contracting the colour ...
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What does the Pontryagin index do in BPST instanton (solution to Yang-Mills theory)?

$$ \mathcal L = -\frac12\mathrm{Tr}\ F_{\mu\nu}F^{\mu\nu}+i\bar\psi\gamma^\mu D_\mu\psi $$ We take this Lagrangian for QCD, after this I need to calculate BPST instanton with topological Pontryagin ...
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Can a gluon split into three gluons: $g \rightarrow ggg$? (four gluon vertex)

I know $gg \rightarrow gg$ is possible, but is $g \rightarrow ggg$ possible? I don't see this anywhere although I think you could conserve colour charge (e.g. $r\bar{g} \rightarrow r\bar{g} + g\bar{b}...
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Restoration of symmetry explicitly broken by anomaly

What is the meaning of the restoration at finite temperature of a symmetry that is "broken" by the presence of an anomaly. If the symmetry is not there why is it restored at finite ...
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Symmetry group of QCD

In "Remarks on the chiral phase transition in chromodynamics" (1984) by Pisarski and Wilczek it is stated that the (classical) global flavor symmetry is $SU(2)\times SU(2) \times U_A(1)$ ...
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Is $SU(3)$ (and not $U(3)$) the symmetry group of color interactions because $U(1)$ is already used for EM?

I have already seen this question. It was answered that $U(3)$ can be decomposed into $SU(3) \times U(1)$, and $U(1)$ is already used for the EM interaction. Still, I wonder why the EM interaction ...
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Why is the QCD $\theta$-term aware of the topology of the space?

The QCD Lagrangian without the $\theta$-term $$\mathcal{L}_{QCD}=-\frac{1}{4}G_{\mu\nu}^aG^{\mu\nu a}\tag{1}$$ is not topological. However, the $\theta$-term $$\mathcal{L}_\theta=\frac{\theta}{32\pi^...
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How to calculate the branching fraction of hadrons? Or good resources on chiral perturbation theory

I am wondering how I could calculate branching ratios (or any effective diagram) for various hadrons. In particular I am interested in a rare pion decay via a virtual photon:$\pi^0\to \gamma \gamma^* \...
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Fundamental scales in QCD

How pion decay constant is linked to $\Lambda_{QCD}$? Are they the same thing unless multiplicative constants?
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Correlation of the strong force with fractional charges? [duplicate]

As far as we know the quarks are not free not because of their fractional charges but because of the independent strong force interaction that grows with distance within the radius of the proton or ...
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