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

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Asymptotic Freedom - Qualitative Explanation

I am doing a (mostly qualitative) course on Particle Physics, and am confused about the concept of asymptotic freedom. The lecture notes basically say that a quark may experience no force/be "unbound" ...
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Potential of the pseudo Nambu-Goldstone boson

Pseudo Nambu-Goldstone bosons appear in breaking approximate symmetries, for instance, the QCD axions. Many literatures just write the action of those pseudoscalars without any explanation. In ...
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What are threshold corrections?

As the title goes, what are threshold corrections in quantum field theory? In particular, I would be glad if a good reference is provided. Standard QFT books such as Peskin, Weinberg, etc seem to ...
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buiding circuits from color superconductors

caveat: the sort of exotic matter engineering in here is currently beyond the reach of our technology, but, that having been said: Has their been any research on building models of these sorts of ...
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Where can I get the most accurate measurements of parton distribution functions?

Where would I look to get the most accurate experimental values of parton distribution functions for the proton? I know these functions aren't measured directly, but I'd basically like to find a fit ...
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882 views

How does Delta baryon decay conserve angular momentum?

I'm a chemist so bear with me: I understand the Delta baryons $\Delta^{+}$ and $\Delta^{0}$ to be in some sense spin (and isospin) quartet states of the proton and neutron. These can decay straight ...
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If virtual particles have negative mass why do they contribute positive mass to atoms?

According to Lawrence Krauss, atoms containing in our body consists of merely 10% (if I remember correctly) of our total mass. The rest come from virtual particles popping in and out of existence from ...
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What kinds of contributions can be neglected in the leading logarithmic approximation?

I'm looking for some good explanation on leading logarithmic approximation (LLA) in QCD; in particular, what types of contributions can be neglected while assuming LLA?
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Can massive fermions have zero modes?

I'm confused about fermion zero modes in relation to instantons. I understand that instantons can create fermion zero modes, but it's not clear to me when a fermion has a zero mode. For example, ...
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What constitutes a 'reliable' instanton calculation?

In Modern Supersymmetry, John Terning, on pgs 151, and 153 performs a so called 'reliable' instanton calculation when dealing with the ADS superpotential 'since the gauge group is completely broken'. ...
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Why does this proof show the gluon propagator comes from the first two terms?

I am reading the book "QCD: Renormalization for the Practitioner" and i am having trouble understanding something. In page 70 the gluon propagator is written as follows $$\begin{multline} ...
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131 views

The relation between classical and quantum vacua

First let me clarify what I mean by vacuum. Suppose we are concerned with a theory of fields $\phi ^i$ defined on a stationary globally hyperbolic spacetime $M$ (I want the spacetime to be stationary ...
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213 views

Colour decomposition of $n-$gluon tree amplitude

I have here a $SU(N_c)$ Yang-Mill's theory and let the index $i$, label the $n$-gluons, and $\{k_i, \lambda_i, a_i\}$ be its momenta, helicity and colour index and $\cal{A}_n^{tree/1-loop}(\{k_i, ...
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Is the color gauge group spontaneously broken in QCD by the Savvidy vacuum?

One model for confinement in quantum chromodynamics is the Savvidy vacuum. This is a spontaneous symmetry breaking of color gauge symmetry by the gauge fields. The vacuum is divided into Savvidy ...
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Origin of the quark condensate VEV

Consider the QCD lagrangian : $$L_{QCD}=-\frac{1}{4}G^a_{\mu\nu}G^{a\mu\nu}+\sum\bar{\psi}_q(i\not{D}-m_q)\psi_q$$ Textbooks explain that this lagrangian is spontaneously broken by the VEV of quark ...
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Origin of quark masses

Does all the mass of the quarks in the standard model come from the Higgs sector or is there also a contribution to quark masses due to QCD chiral symmetry breaking?
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For mesons, or baryons, do sea quarks contribute to the angular momentum of the bound state?

The total angular momentum of a bound state of quarks, such as a meson say, can be done by studying the spin and orbital angular momentum of the 2 valence quarks. What about the sea quarks why they ...
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Wilson loops and gauge invariant operators (Part 2)

These questions are sort of a continuation of this previous question. I would like to know of the proof/reference to the fact that in a pure gauge theory Wilson loops are all the possible gauge ...
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Why is the chiral symmetry only $SU(3) \times SU(3)$ and not $SU(6)$?

In the limit where the masses vanish, low energy QCD has a well known chiral symmetry (see http://arxiv.org/abs/hep-ph/0505265 for a very extensive review, and pg 19 for the section relevant for my ...
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Strong CP Problem

So, as far as I know, the Strong CP Problem in QCD results from the theta angle term in the action: $i\theta\int_X F_\nabla\wedge F_\nabla$ where $\nabla$ is the gauge connection and $X$ is a manifold ...
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Why does strong interaction increase with distance?

I read numerous times that strong interaction increases with distance. But how can one actually derive the force-distance relation from the lagrangian (quark field + gluon field + gauge coupling)? ...
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Does the number of left handed chiral quark superfields always equal half the number of quark flavours?

In Weinberg's "The Quantum Theory of Fields Vol III" page 267 we're told that $n_f = 2N_f$. Where $n_f$ are the number of flavours and $N_f$ is the number of left chiral quark superfields (or the ...
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Cusp anomalous dimension in QCD

What is the "cusp anomalous dimension", starting from the basics? I came across this term while reading some QCD papers.
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Is there something like Hawking radiation that makes protons emit component quarks?

If Hawking radiation can escape from black holes, could quarks perhaps become separated from protons despite it being "impossible" for that to happen?
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The Gluon - Does It Exhibit Wave Properties?

Do Gluons have frequencies and wavelengths? I assume that they do, but have been unable to find anything on point in SE or Wikipedia. Just beginning to study university-level physics here.
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Polarization of the gluon

I think that, by now, it's understood that the gluon propagator in QCD has a dynamically generated mass. Ok, so my question is the following: where does the extra polarization degree of freedom come ...
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Are “confinement” and “asymptotic freedom” two sides of the same coin?

On Wikipedia it says that the two peculiar properties of quantum chromodynamics (QCD) are: confinement and asymptotic freedom. Asymptotic freedom is the idea that at low energies we cannot use ...
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Is Higgs mechanism necessary in QCD?

As we known, we introduce Higgs mechanism into Standard model to explain mass of Gauge boson, lepton and neutrino(maybe?). Higgs mechanism is necessary for electroweak theory because gauge symmetry ...
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Permissible combinations of colour states for gluons

My lecturer has said that there are 8 types of gluons (I'm assuming that the repetition of $r\bar{b}$ is a typo that is meant to be $r\bar{g}$) $$r\bar{b}, b\bar{r}, r\bar{g}, g\bar{r}, g\bar{b}, ...
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Is QCD free from all divergences?

On page 8 in http://arxiv.org/pdf/hep-th/9704139v1.pdf David Gross makes the following comment: "This theory [QCD] has no ultraviolet divergences at all. The local (bare) coupling vanishes, and the ...
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Lattice QCD and string theory

I've heard the claim that some aspects of string theory are used to improve Monte-Carlo simulations of lattice QCD, for example by people working at the LHC. I know a bit about Monte-Carlo methods in ...
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Why are $SU(N)$ gauge theories easier to handle for $N\rightarrow \infty$?

I was wondering if there was a intuitive/heuristic argument to understand why generalizing the QCD gauge group $SU(3)$ to $SU(N)$ and taking $N\rightarrow \infty$ simplifies the analysis of the ...
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Color-charge conservation in proton decay

In some extensions of the Standard Model of particle physics (Supersymmetry with R-parity violation being a prominent example), the proton is allowed to decay, e.g. via $p\to e^+\pi^0$: While this ...
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Playground of Forces

Why is it that the gravitational force acts on large sized objects while the strong and weak nuclear forces act at subatomic levels only? What is that stops each other to enter each others domain?
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How to compute scattering amplitude $\gamma\pi^+\to\pi^+\pi^0$

I wish to find the amplitude for process $\gamma\pi^+\to\pi^+\pi^0$ at low energies. I am familiar with the basic concepts and techniques of QFT but have never dealt with the scattering processes ...
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Variation of the kinetic quark term of the QCD Lagrangian under gauge transformation

A simple kinetic quark term would look like $$\bar{\psi}(\gamma^{\mu}\partial_{\mu} - m){\psi}.$$ Imposing SU(3) symmetry the Dirac spinor transforms like $$\psi(x) \rightarrow \psi'(x) = e^{ig_s ...
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From which dimensionful constants does proton mass arise?

It is well known that the most of the proton (or any other hadron with light quarks) mass is not made up from quark masses, but it is dynamically generated by QCD mess inside. I've also heard that, ...
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What is the $t\bar{t}$ production supposed to bring up

The $t\bar{t}$ production, I've read, that will somehow confirm the QCD and might bring up new physics. Why are we studying $t\bar{t}$ production from $p-p$ collisions at the LHC? What are we trying ...
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Non-existence of gluon singlets: Any recent theoretical progress?

An unanswered question from last year (2012) on gluon singlets asked whether there is any theoretical explanation for the experimental absence of the ninth or colorless (singlet) gluon. This is the ...
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Simulating a proton

How much computing power would it take to simulate a single proton from the bottom up, without taking any shortcuts whatsoever? My current understanding is that: A proton is basically a seething ...
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all multipolar terms of nuclear fields must be confined?

When usually speaking about QCD confinement, one refers to the fact that color charges over lengths above the hadron size are fully neutral, which implies that monopolar charges over an extended ...
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Are there new physics scenarios that predict low lying hadrons?

There is a significant ongoing experimental effort to search for new hadrons with masses in the GeV range. This is used to find the spectra of QCD bound states, with a particular emphasis on finding ...
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How non-abelian gauge coupling runs below confinement or QCD scale?

I know the famous beta function of asymptotic free, but that seems describe the running coupling beyond confinement/QCD scale so that a perturbative analysis can apply. But how coupling runs below ...
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String breaking in QCD

I have trouble understanding string breaking in QCD. I have read an article on arxiv (http://arxiv.org/abs/hep-lat/0505012), and I still don't understand what truly happens. My understanding of ...
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Matrix elements of four-quark operators [closed]

In weak interaction phenomenology, especially in strangeness changing processes, effective four-quark operators are used. Such as $Q_1 = (\bar{s}_\alpha \gamma_\mu (1-\gamma_5) d_\alpha) ...
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Why is the strong CP term $ \theta \frac{g^2}{32 \pi^2} G_{\mu \nu}^a \tilde{G}^{a, \mu \nu}$ never considered for $SU(2)$ or $U(1)$ interactions?

The Lagrangian one would write down naivly for QCD is invariant under CP, which is in agreement with all experiments. Nevertheless, if we add the term \begin{equation} \theta \frac{g^2}{32 \pi^2} ...
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Why scattering of red and blue quark only involves $G_8^\mu$?

According to the author C. G. Tully (Particle physics in a nutshell), the scattering of a red and blue quark only involves $G_8^\mu$. How come this is so? I thought $G_3^\mu$ and $G_8$ only mediate ...
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Infrared divergences in QCD

As we know, we can remove infrared divergences by summing over all final states with arbitrary number of soft photons. But in QCD this does not work, since gluons are not "neutral" because they carry ...
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Cronin enhancement in p-A collisions

What is the physical picture behind Cronin enhancement in proton-nucleus collisions at intermediate transverse momentum?
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How can we see that there is superconductivity/superfluidity in the boundary theory in the holographic principle?

For example in the models for holographic superconductors we can calculate the conductivity. Also there is an energy gap. I can understand that it describes a superconductor. However I have also heard ...