Questions tagged [quantum-chromodynamics]

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

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1answer
121 views

What exactly is color superconductivity?

I'm having a hard time visualizing exactly what would happen. What similarities does it share with our usual superconductivity and what would be different?
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Understanding the prefactor $\frac{\theta g^2}{32\pi^2}$ of the $F\tilde{F}$ term in Yang-Mills theories

The most general Yang-Mills (YM) action consistent with Lorentz invariance, gauge invariance and renormalizability should contain a term $$\kappa F_{\mu\nu a}\tilde{F}^{\mu\nu a}\tag{1}$$ where $\...
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Is there some truth to the “vacuum as bubbling soup” story?

There are many discussions on this site about the nature and reality of vacuum fluctuations. The general consensus seems to be that this it falls into the Lie-to-Laymen category. On the other hand, ...
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How can we tell a theory is confining?

Physically, I understand what it means for a theory to be confining. The elementary particles are not observable, but only composite particles are. The classic example is QCD, where quarks are ...
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What was above and below the QCD chiral symmetry breaking temperature?

Above a critical temperature in the Universe, there was probably a phase of unbroken approximate QCD chiral symmetry. Mathematically, the symmetry breaking is triggered when the operators $\bar{\psi}{\...
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Soft interactions at high energy hadron-hadron collision?

I'm reading Peskin&Schoeder and in the section 17.4 (p.563) it says: "If one collides hadrons with other hadrons at very high energy, most of the collisions will involve only soft interactions of ...
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Closed quark loops written as traces

I'm working out the quark loop diagram. I draw it as follows: where the greek letters are the lorentz and dirac indices for the gluon and quark respectively and the other letters are color indices. ...
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How do you subtract colors and divide them by irrational numbers? (Gluons) [closed]

There is a gluon that is $$\frac{1}{\sqrt{3}} (red \cdot\overline{red} + blue\cdot\overline{blue} - 2\cdot green \cdot\overline{green})$$ This confuses me because I do not understand how adding and ...
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How experiments differentiate between valence quarks and sea quarks

The picture that baryon consist of three quarks and mesons consist of a quark,anti-quark pair is inaccurate.However this simple picture is enough to explain many properties of the hadrons as mentioned ...
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Understanding CI and DI in latice QCD

What I understand about the CI and DI, is that the one quark could be connected to another quark likes. This insertion is kind of wick contractions, between the operators. Since I do not have a very ...
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Intuition for Asymptotic Freedom

In QED, the $\beta$-function has a positive sign. This means that the coupling increases at higher energies, or equivalently, smaller length scales. This picture is made intuitively clear by the ...
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Why is color change between quarks without consequence?

As always I will preface this question with the fact that I only have a high school education, so I may be overlooking something, or unaware of something that is inherent to the question. That being ...
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How are quarks assigned color charge?

As always, I'll preface that I am wildly undereducated, so i may be overlooking something or be completely unaware of another relevant property. Color Confinement dictates that to "assemble" a baryon ...
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What does color represent in quarks?

Im sure I'm pretty undereducated to be asking a question like this, but what do the colors represent in quark chromodynamics? A I understand it, each color represents a sort of charge, but what kind ...
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Does it mean time reverses if all particles were antiparticles and vice versa?

First of all, I'm not major in physics and the question might seems stupid, as I'm layman studying for my self-interest and I really don't know much about it. I only think it might open some ...
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Peskin's treatment of Pions as Goldstone Bosons

After restoring the mass terms in the Lagrangian \begin{align} \mathcal{L}=\bar{u} i \not D u+\bar{d i} \not D d-m_{u} \bar{u} u-m_{d} \bar{d} d, \end{align} one obtains equations of motion for the ...
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Is there a first-order formulation of CP-violating QCD?

In QCD without a CP violating $\theta$ term, (I believe) we can express the gauge kinetic Lagrangian in a first-order form with the field strength $F_{\mu\nu}$ taken to be Lagrange multipliers. Up to ...
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Is it a coincidence that the maximum number of protons in a stable nucleus is on the order of $\alpha_s / \alpha_{EM}$?

The equivalent to the fine-structure constant for the strong interaction is about $\alpha_s \approx 0.5$ at the proton rest mass energy (and it runs pretty slowly with energy, so it's on this order ...
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Nielsen-Ninomiya Theorem on a 2+1 lattice

(a) In the paper on the proof of the Nielsen-Ninomiya theorem, it is shown for 1+1 and 3+1 lattices. Does the proof hold on lattices with even number of spatial dimensions (e.g 2 spatial + 1 time)? If ...
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Why is color screening in Quark Gluon Plasma not effective for light quarks mesons like pions?

The color screening of quarkonia states like $J/\psi$ (bound states of charm and anti-charm) and $\Upsilon$ (bound states of beauty and anti-beauty) is considered one signal of QGP (quark-gluon plasma)...
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Symmetry factor of gluon self-energy

In Peskin & Schroeder, p.523, they give the diagram contributing to the gluon self-energy that arises from the 3-gluon vertex, and they claim that the $1/2$ factor is a symmetry factor: How can ...
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Confusions with gluons. How many of them are there?

Gluons are bicolored objects. They are made out of one color and one anticolor. Therefore, there seems to be nine possible states $r\bar{r},r\bar{b},r\bar{g},b\bar{r},b\bar{b},b\bar{g},g\bar{r},g\bar{...
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A question about quarks and quantum chromodynamics

Penrose writes the following on pg 648 of his book "Road to Reality" How can we treat quarks as real particles, if they have the wrong spin-statistics relation? The way that this problem is dealt ...
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Measuring strong coupling constant

How can the strong coupling constant be measured?
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Asymptotic freedom and momentum exchange

Why is the momentum exchange very high for low distances? For sufficiently short distances or large exchanges of momentum https://en.wikipedia.org/wiki/Asymptotic_freedom While I think about ...
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How does the underlying symmetry of QCD imply the allowance of a 4-gluon vertex?

Quantum chromodynamics allows for a four-gluon vertex such as this, in a diagram Such a vertex would never be allowed in quantum electrodynamics, which has an underlying U(1) gauge symmetry. I know ...
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Is is cheating to use alpha strong (instead of V_QCD) to argue that there is asymptotic freedom of QCD?

For asymptotic freedom of QCD, people give the argument that alpha_strong decreases at high energy (thus there is freedom at low distance). But if we make the reasoning on the QCD potential, for ...
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Why do mesons decay?

Why do mesons decay shortly while baryons are more stable? And why do mesons decay, meaning that there are unstable, while they formed because meson is more stable than the two corresponding quarks? ...
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Is the asymptotic value (at high energy) of alpha strong null or non null?

If it is non null then why do we say that there is asymptotic freedom. Freedom would mean alpha strong equal zero
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Is the QCD potential really monotonic ? How does it prevent two quarks from meson to annihilate?

The QCD potential is made of two terms -(4/3) * alpha_s / r that describes the short distance and the term +k*r that describes the long distance Of course, alpha is a function of energy, so it is ...
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How can we reconcile the many different models in QCD that make inconsistent assumptions/claims?

For example, we have quark models, shell models, cluster models, etc. which seem to admit to different interpretations regarding nuclear structure and dynamics. Is there a way to resolve the fact that ...
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Theta-dependence of QCD quark condensate for $m=0$

I'm trying to understand the $\theta$-dependence of the following expression for the quark condensate in QCD, $$ \langle \bar{\psi}\psi\rangle = - \Sigma \cos(\theta)$$ taken from Eqs. (5) and (7) ...
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Could the Odderon intercept be equal to $\alpha_\mathbb {O}(0)=0.813$? [closed]

The directed percolation dynamical universality class is characterized by just three independent critical exponents. These exponents are (in a 3d space): $$\beta=\beta'=0.813(9)$$ $$\nu_\perp=0.584(5)...
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If quarks don't exist individually how can we say baryons are made up of three quarks?

We know the composite subatomic particles are made up of odd number of quarks (at least 3 for baryons ) or combination of quarks and antiquarks (mesons). My question is if they don't exist ...
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156 views

What does QCD look like in higher dimensions?

It was pointed out as a comment on my question on atomic physics in higher dimensions that that question implicitly rests on an assumption that QCD, and thus the structure of atomic nuclei, is pretty ...
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A Question about Wave-Function Renormalization Factor in SQCD

Here, I have a question about the one-loop computation of the wave-function renormalization factor in SQCD. According to Seiberg duality, the following electric $\mathrm{SQCD}_{e}$ \begin{gather} ...
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What is the real problem in the free body problem?

Ions are produced when an EM neutral atom gains EM charge by gaining or losing electrons, by collision with other charged particles or photons. The study of such collisions is of fundamental ...
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Why use $SU(3)$ and not $SL(3, \mathbb{R})$ for color charge? [duplicate]

Why do we use the group $SU(3)$ and not $SL(3, \mathbb{R})$ for color charge? As far as I can tell, the $SL(3, \mathbb{R})$ is volume and orientation preserving, by the fact that it has unit ...
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Understanding some lines from 't Hooft's paper on large-N QCD in 1+1d

In 't Hooft's paper "A two-dimensional model for mesons", the author shows that two-dimensional (1+1) QCD in the large-N limit interestingly gives a theory of mesons. 't Hooft calculates the "mesonic ...
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Regge theory and $SU(3)$ mass formula

In a video interview, Murray Gell-Mann states (at 40 seconds) "I have wondered all this time, and I still wonder, whether the straightness of those [Regge] trajectories, and the accuracy of the ...
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Pion mass in theta vacuum

Does the mass of the charged pion depend on the QCD vacuum angle? I've seen it said---e.g., in these TASI lectures---that when the quark mass matrix is real and there is a nonzero QCD vacuum angle $\...
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Why don't green and anti-green gluons immediately annihilate each other?

I can't believe I haven't found an answer elsewhere..... I have read repeatedly about blue/anti-blue gluons, etc., but no reason as to why they don't destroy each other immediately..... Or maybe they ...
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Color symmetries in variant QCD

Suppose we only have two colors, for example, red (R) and blue (B) to construct the wave functions of baryons and mesons and that the color symmetry is SU (2) and not SU (3). In this situation, ...
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How do soft Pomerons become hard?

The exchange of soft Pomerons (and Reggeons) ($\alpha_R(0)=0.55$ and $\alpha_P(0)=1.08$) seem to describe total hadron-hadron cross sections pretty well in the Regge limit. See, for example: https://...
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How does the strong interaction “create” the nuclear force?

I’ve recently read that the strong interaction, that holds the quarks together to form protons and neutrons, also creates a residual force called the nuclear force, which holds the neutrons and ...
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What happens to large gauge transformations in gauges different from the temporal gauge?

There are already several questions regarding the meaning and definition of large gauge transformations. Discussions of large gauge transformations typically only happen in the context of the ...
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Why does the axion solve the CP problem?

I don't understand why the CP violating term in QCD vanishes when we introduce the axion. The term is $$\left(\frac{a}{f_a}+\theta\right)G\tilde{G}$$ It is claimed that $\left\langle a\right\rangle=-\...
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Sufficient and Necessary Conditions for Chiral Symmetry Breaking

In their 2005 paper, the authors write (just below eq. 3.19) we see that a non-zero value of $F_0$ is a necessary and sufficient criterion for spontaneous chiral symmetry breaking. On the other ...
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Longitudinal polarization of gluons in loop

I have a short question about the possible gluon polarization in loop diagrams. For external gluons, we only want the 2 transverse polarizations. In Peskin-Schroeder it is explained that in Feynman-...
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Why do parton distribution functions not depend on the total momentum of the composite state?

When defining such quantity as the elastic form factor, I kinda understand why it turns out to be independent of the momentum of the charged particle, and only dependent on the momentum of the photon ...

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