Questions tagged [quantum-chromodynamics]

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

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Can I switch the convention of QCD by replacing coupling constant $g$ with $-g$?

There are two equivalent conventions in QCD that give two different definitions of the covariant derivative operator: ${D_\mu } = {\partial _\mu } - {\rm{i}}gA_\mu ^\alpha {T_\alpha }$ and ${D_\mu } = ...
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Real time lattice simulation

A typical simulation of lattice quantum chromodynamics uses imaginary time, so the weight function of the path integral is positive definite $e^{-S_E}$. In the case of metropolis-hastings, the ...
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Are gluon particles analogous to flux tubes?

I've always been told gluons are the force "particles" mediating the strong force. And I've learned that Fluxtubes are what hold quarks together. Are these fluxtubes the "particle" ...
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Suppressed indices - when and why?

When following textbooks in QFT I have many times seen the usage of 'suppressed indices'. This is a bit confusing to me, sometimes it is done without any explanation. For instance I usually see that ...
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Tadpole diagram in one-loop QCD gluon self-energy renormalization

I am trying to study QCD renormalization at 1-loop order. So, when I take into account the gluon self-energy corrections, the book I am studying from says that there are three diagrams who contribute ...
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Does each vector in $su(3)$ represent a different kind/type of gluon (infinite kinds/types of gluons); or, are they all considered the same kind/type?

According to Does gluons have names?, it seems that there is no way to give names to gluons because they are not "always the same". So, it seems to imply that the same gluon can change its ...
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Why are gluons color charged but not photon? Could there be a charged EM force carriers like gluons or neutral color charge carrier like photon?

Gluons have a color charge why don't photons have an electric charge like gluons?
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Global fit analysis of the Balitsky-Kovchegov equation

Ingredients: Suppose that I have the QCD BK equation $$\partial_y N(r,y) = \int d^2\vec{r}' \mathcal{K}(\vec{r},\vec{r}',|\vec{r}-\vec{r}'|;\{\lambda_i\}) \left[N(r',y) + N(|\vec{r}-\vec{r}'|,y) - N(...
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Transpose of a matrix element [closed]

A matrix element is just a number. Now, If I have the following matrix element: \begin{equation} \newcommand\bra[1]{\left<{#1}\right|} \newcommand\ket[1]{\left|{#1}\right>} A = \bra{B}\bar{b}\...
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$\alpha_s$ strength in quarkonium potential energy formula

The top answer to Is there an equation for the strong nuclear force? explains that there is a coulomb's law-like formula: $V(r)=−\frac{4}{3}\frac{α_s(r)ℏc}{r}+kr$ that describes how quarks and ...
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What temperature is required to affect stability of the nucleus

According to answers to "What is the temperature of an atomic nucleus?" the temperature inside a nucleus is in the thermal equilibrium with outside environment. So for an outside temperature ...
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How can a proton contain a charm quark that has a heavier rest mass?

Referring to this article: Protons contain Charm Quarks. What's going on here? Typical case of pop sci obfuscation of the actual physics? Are they only refering to the virtual particles that pop in ...
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Energy stored in the gluon fields -- in a proton, versus, in a pure Yang-Mills mass gap

I was reading this How much of the proton's mass is due to the Higgs field? and finds the discussion says: As Prof. Strassler explains in the link above, the proton mass is best throught of as ...
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Collinear Limit of QCD

According to the lecture, https://www.youtube.com/watch?v=u8uHI6WLz6c&t=1s ,specifically at 36:55 it says that $p^2 \rightarrow 0$ can be achieved if i assume that $p_1 || p_2$ where $p_1 + p_2 = ...
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What would a long-range three-charge force look like?

Suppose that gluons were colorless; without self-interaction, they would not be combined to flux tubes, and so could presumably spread out and, being massless, produce a long-range force like ...
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What is the physical meaning of the large $N$ expansion?

I know about the $1/N$ expansion for some time. Apart from the fact that as Witten suggests, it can be the correct expansion parameter of QCD Baryons in the $1/N$ Expansion (in a parallel that he ...
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Charge conjugation and Parity for QCD $\theta$ term

I am studying Charge Conjugation ($C$) symmetry and Parity symmetry ($P$) on QCD $\theta$ term: $\tilde{G}G= \frac{1}{2}\epsilon^{\mu \nu \lambda \rho}G^a_{\mu \nu}G^a_{\lambda \rho} \propto G^a_{0i}G^...
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The ratio of the Higgs and QCD condensate contribution to valence quark masses

According to https://en.wikipedia.org/wiki/Quark#Mass, the quarks have the following masses. I suppose these data in the table shows the valence quark masses, such that the uud valence three quark ...
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The concept of color dipole

Its a term that I recently came across when reading a reseach paper regarding perturbative QCD. It was stated that a meson was considered to carry a large color dipole. I am not sure what the term ...
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Hadrons as partons? e.g. hadron distribution function inside another hadron

Parton distribution functions (PDFs) are typically seen as describing the probability that a parton, e.g. a quark or gluon, can be found in a hadron with particular momentum fraction $x$. They can be ...
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Electromagnetic contribution to the mass of proton

It is a well-established fact that the main contribution to the mass of proton is the gluonic field inside a proton, as some answers explain (see here and here). But being the proton and the neutron ...
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On scaling cross sections

I don't understand how to recognize if a target is point-like or not just by looking at the cross-sections. Citing from Barr, Giles; Devenish, Robin; Walczak, Roman; Weidberg, Tony. Particle Physics ...
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Does QFT Imply Strings the Same Way that String Theory Implies Branes?

It is part of every introductory course on string theory (e.g. Tong) that the open bosonic string in $D = 26$ dimensions with Neumann-Neumann boundary conditions for directions $a \in \{0,\dots,p\}$ ...
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Color confinement vs. Weak charge confinement

Sometimes, color confinement is explained loosely by stating that the gauge group of QCD, namely SU(3), is non-abelian gauge group and, therefore, tends to form narrow "flux tubes" through ...
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Physical interpretation of the asymptotics of partition function in string theory

I would like to understand the physical interpretation of the asymptotic expansion of a partition function. The QCD partition function with gauge group $SU(N)$ as $N$ is large has been shown by Gross ...
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Would two helical magnetic fields attract stronger than two normal dipole magnets?

We know that we can artificially manipulate magnetic flux in space combining different discrete magnetic sources to create complex magnetostatic fields on air like chiral magnets or even helical ...
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Is it true that even-spin bosons give rise only to attractive forces? [duplicate]

In the standard model, electroweak and chromodynamic interactions are modelled as Yang-Mills fields. It also happens that the electroweak field is mediated by spin 1 vector bosons, and this leads to ...
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What is the temperature of an atomic nucleus?

I conjecture that atomic nucleus with $N\approx 250$ particles could be interpreted as a system in equilibrium (maybe the number is too low to consider the use of statistical mechanics as a reasonable ...
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What would be the consequences of asymptotic freedom and color confinement of QCD for neutron stars?

Neutron stars are conjectured to have densities between $10^{14}$ and $10^{17}\ \text{g/cm}^3$. In the latter limit, the neutrons could be so close that the interaction between them would not be the ...
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Transformation of $u_L$ under $SU(3)_c$ within Standard Model

Let $u_L$ denote the left-handed fermion fields of the up-type quarks. Within the context of the standard model as a $SU(3)_c \times SU(2)_L \times U(1)_Y $ gauge theory, when one gauges the field ...
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Do magnesium-32 nuclei look like soccer balls?

My understanding is that the strong interaction between nucleons, while very strong, is much weaker than the strong interaction between quarks within a nucleon. So even though an atomic nucleus in an ...
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Non-perturbative approach to high-energy physics

I know that main numerical approach to modeling high-energy physics events are Monte-Carlo event generators. But they are using perturbative description of collision and decay processes of particles. ...
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Why is there still disagreement over the mass of the bottom (or beauty) quark, but none of the others?

Wikipedia (among other places) lists two values for the alleged mass of the B quark, 4.18 and 4.65 GeV. Only one of the two possible masses listed has a link to another Wiki page explaining the ...
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With enough fermions, does QCD confinement disappear?

It's been a while since I thought about QFT, but the following thing was puzzling me. I've often read that theories with a beta function that is asymptotically free at lowest order will likely suffer ...
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Elastic quark-quark scattering

I try to write expression for qq->qq proccess with double gluon exchange I figured out the denominator, but i cant understand how to write numerator. My thoughts are: \begin{equation} numerator = g^...
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Semiclassical QCD

The classical Lagrangian density for Quantum ChromoDynamics (QCD) is given by: $$\mathcal{L}_\mathrm{QCD} = \bar{\psi}_i \left( i \gamma^\mu (D_\mu)_{ij} - m\, \delta_{ij}\right) \psi_j - \frac{1}{4}...
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How can I describe, in QFT, Weyl spinors coupled to a strong version of the gluon gauge field?

Let's assume a massless Weyl-spinor field. My aim is to let them interact by a gluon-like gauge-field, which has a bigger much bigger coupling, color charge, than the standard color interaction. This ...
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How to apply MHV rules to QED?

I've been working lately with scattering amplitudes using spinor helicity formalism. I understand how amplitudes with all positive (or negative) $A_n[\pm\pm\dots\pm]=0$ while working with gluons and ...
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Why is "a phase for the QCD vacuum" considered a fundamental constant and what does it even mean?

We can read about fundamental physical constants in Wikipedia: There is, however, no single "correct" way of enumerating them, as it is a matter of arbitrary choice which quantities are ...
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Phase Diagram of NJL model

I am trying to reproduce the phase diagram of NJL model given in this thesis, plot 3.1 in page 14, with this LINK. I have solved the gap equation and I found the phase boundary. The problem is I am ...
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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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