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

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Similar masses and lifetimes of the $\Delta$ baryons

Why do the four spin 3/2 $\Delta$ baryons have nearly identical masses and lifetimes despite their very different $u$ and $d$ quark compositions?
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Experimental evidence for color-neutral gluons $( (r\bar{r}−b\bar{b})$ and $(r\bar{r}+b\bar{b}−2g\bar{g}) )$

Is there any experiment/measurement that would have a different outcome if one of the following scenarios is applied: The two color-neutral** gluons would not exist Those gluons would have very ...
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139 views

NRQCD: Why are quarks and anti-quarks treated independently?

I am studying these lectures on effective field theories and I am having some problems to understand how the Non-Relativistic QCD (NRQCD) Lagrangian is constructed. This theory is often used to ...
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138 views

What is the nucleon axial charge?

Can someone point me to a short definition of what the nucleon axial charge is?
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193 views

Glueball mass in non-abelian Yang Mills theory

How can the glueball mass be calculated in Yang Mills theory?
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75 views

Asymptotic freedom in QCD

From renormalization group equation $$ t \frac{d \bar{g}(t , g)}{dt} = \beta (\bar{g}(t , g)), \quad \bar{g}(1 , g) = g $$ (here $t$ is momentum scale factor, $g$ is initial coupling constant and ...
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54 views

How does the gluino change the strong force?

Given that the gluino is in the same representation of SU(3) that the gluon and so the same kind of interaction vertexes with quarks, I guess that it could have a role in the strong force. But there ...
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77 views

A Variation on Laplace's equation (context: Yang-Mills N-Instantons, Rajaraman's book)

Statement of the problem I need to solve the equation \begin{align} 0 = \frac{1}{\phi} \partial_{\sigma}\partial_{\sigma} \phi \hspace{20mm} (1) \end{align} where $\phi$ is a scalar field and ...
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62 views

Large wavelength limit of gluons

Does there exist a classical limit of QCD? I mean in the sense of wave particle duality of eg photons. Is there any similar thing for gluons?
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55 views

Baryon operators for $N_f<N_c$

Simple question. The answer is probably obvious, but I just don't see it. In section 3 of this paper http://arxiv.org/abs/hep-th/9509066 it is stated that in SUSY QCD if $N_f<N_c$ the classical ...
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80 views

Multiquark states and molecular QCD

Due to some recent "discoveries" (yet to be completely confirmed), we have hinted the existence of tetraquark particles. Why QCD exotics like multiquark states beyond n=2,3 (mesons, ...
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201 views

Vacuum polarization in QCD and gluon bubbles

In analogy to QED, the following Feynman diagram is a diagram contributing to the vacuum polarization effect, leading to anti-screening, asymptotic freedom and running of the strong coupling constant. ...
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213 views

Difference between Quark-Gluon Plasma and Color-Glass Condensate?

From the news. Apparently they are very similar, yet different.
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Parameterization of an arbitrary element of $U(2)_L \times U(2)_R$ (Chiral symmetry with two quarks)

When you write down the Lagrangian for two quarks : \begin{equation} \mathcal{L}_\text{QCD}^0 = -\frac{1}{4} G_{\mu\nu}^a G^{a\mu\nu}+ \bar\Psi i \gamma^\mu D_\mu \Psi \end{equation} you find an ...
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30 views

Hard processes in particle collisions

This is in reference to a MC generator called Pythia, but has wider applicability because the phrases are used widely in particle phyiscs, even though I have not managed to find a clear, ...
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222 views

How does flux tubes between quarks bind them together?

If you have, say, a proton it has gluon field fluctuations around it. Those flux tubes between the quarks suppresses the gluon field fluctuations and create a true vacuum between them(correct me if ...
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616 views

SU(3) gauge invariance in QCD

In QCD, the gauge-invariant lagrangian under the trasformation $ \psi \to \psi' = e^{ig T^a \theta^a(x)} \psi$ is written as: $\mathcal{L} = \bar{\psi}(i\gamma^\mu D_\mu - m)\psi - ...
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201 views

Parton Distribution Functions plot

I was looking at a plot of the parton distribution functions today and had a question. On the y axis, it seems like the value of x f(x) for gluons is greater than one at small x. I was under the ...
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264 views

Electromagnetic current-current correlators

Let the free electromagnetic current $J_\mu(x)$ be = $:\bar{\psi}(x)\gamma_\mu Q \psi(x):$ where $::$ is the normal ordering. In this expression why is $Q$ thought of as a "charge operator" instead ...
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284 views

Axion related questions

I have several question regarding axion. Could anyone give me some brief introduction to what is a axion string, axion field and how is this related to fermion zero mode and chiral zero mode?
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Difference between LO and NLO event generators?

I am performing undergraduate research in particle physics. In our research, we are using two event generators (PYTHIA8 and MC@NLO with HERWIG++) to study open bottom production. I am unsure of the ...
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42 views

Asymptotic behavior of Euclidean correlators in QCD

I am reading an old review paper (http://journals.aps.org/rmp/abstract/10.1103/RevModPhys.65.1). In the beginning it makes the following statement (I copy with slightly different notation). If we ...
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15 views

Decay of some particle involved quarks vs mesons as outgoing states

Let's have decay width of some mother particle into the state which involves hadrons. For simplicity, let's assume that creation of hadrons (on diagram) is possible only through electroweak vertices ...
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Different ways of derivation of Gell-Mann-Okubo mass formula

Recently my teacher have told me that there are many ways of derivation of Gell-Mann-Okubo mass formula by using group representation theory (by using dynamical group etc). Where can I read about ...
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70 views

Charge conjugation matrix in baryon current

In his paper Calculation of baryon masses in quantum chromodynamics (ScienceDirect), B.L. Ioffe considers currents describing baryons. In equation (13) he gives an interpolating current for the isobar ...
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26 views

Measure of interaction of two quarks and Casimir operators [closed]

Let's have two quarks, which refers to representations of $r_{1}$ and $r_{2}$ of color symmetry group. They create bounded state which refers to the representation $r$. There is a statement that ...
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109 views

Relationship between plasma physics and quark gluon plasma

To what extent do the ideas common in modern plasma physics, such as magnetohydrodynamics, cold plasma models, common types of plasma waves, Maxwell's Equations, etc, relate to the study of quark ...
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Is Gravity a part of the Strong Nuclear Force? [duplicate]

In episode 7 season 5 of Through the Wormhole, they say that gravity could just be a manifestation of the strong nuclear force and that the force carrier are a pair of Gluons. Is this valid in anyway? ...
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Similarities between laminar-turbulence transition and others like BCS-BEC crossover, quark-hadron transition etc

From my limited readings on fluid dynamics, my understanding is that as the system changes from near-laminar flows to full turbulence, the dimensionless Reynolds number changes from $ R << 1$ to ...
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63 views

Conversion of QCD cross section formula

I'm writing a program to calculate NLO cross sections for semi-inclusive high-$p_T$ pion production in proton-proton-collisions for my bachelor thesis. I've got a paper describing the production ...
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87 views

How to describe hadron-hadron strong interactions from QCD?

I'm trying to set up the problem of deriving the thermodynamics of hadronic matter. I know how to proceed in the case of an effective description such as mean field (Walecka/linear sigma model) but ...
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99 views

CP-violation in weak and strong sectors

There is a possible CP-violating term in the strong sector of the standard model proportional to $\theta_\text{QCD}$. In the absence of this term, the strong interactions are CP-invariant. In the ...
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Cancellation of the quadratic divergence in QCD

I am currently reading about QCD in QFT-Peskin&Schroeder. When calculating 1-loop diagrams for QCD and using dimensional regularization, the 3-vertex boson loop, 4-vertex boson loop and ghost loop ...
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In a composite Higgs, should the Z0 decay at the same rate that the neutral pion?

I am sorry to ask a question that obviously is a sum of separate questions about a process: Should the decay rate of Z0 be related to the decay of the "higgs" composite field it is eating? And, should ...
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Macroscopic chromodynamics

Lately I've been reading about gamma ray lasing phenomena, and I've been wondering about the applications of this. More concretely, the above fantastic question led me to wonder if we could somehow ...
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Does dark energy affect asymptotic freedom?

If the Hubble constant is extremely large, what will happen with quark confinement? I guess that quarks will remain confined because of asymptotic freedom. But can gravity or dark energy have any ...
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133 views

What is new with quark-Gluon Plasma and the bubble formed at RHIC?

RHIC experiment results of forming Quark-Gluon Plasma and a possible bubble was news few months ago. I cannot find any follow up news after that. Does anybody know anything new in this front? I am ...
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Gluon Fusion in Top Pair Production

In my textbook, there is a feynman diagram showing top + anti-top pair production with a highly energetic gluon resulting from the fusion of two gluons. I know that all vector bosons have spin 1, so ...
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172 views

The status of $SU(3)_C$ symmetry in the Standard Model

In the Standard Model of Particle physics the $SU(2)_{EW}$ symmetry and the $SU(2)$ isospin symmetry are broken. What about $SU(3)_C$? Is it broken too? if YES, what breaks the symmetry? If NO, what ...
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Why do the quarks constantly change colors?

In a hadron the quarks constantly change colors.They,then exchange gluons to remain color neutral.For example a red quark becomes blue by emitting a red and anti blue gluon and then the blue quark ...
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Do we know where newly formed quark-antiquark pairs come from in the process of hadronization?

The only explanations I have found are very vague, such as "spontaneously created from the vacuum" and because "it is more energetically favorable".
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Fermion field structure in non-abelian gauge theories

I am trying to understand the structure of the fermions in non-abelian gauge theories. Disclaimer: my question might be very trivial (I suspect the answer could simply be "a change of basis"), but I ...
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205 views

What are the problems in quark-gluon plasma? [closed]

Someone (T.D.Lee?) said that quark-gluon plasma would (after 1980?) be very important to understand high-energy physics experiments. I read some description of this new state in wikipedia ...
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186 views

In a gauge theory, are two states related by a global phase transformation identified?

In a gauge theory (non-abelian for this question), I am told that two states $|\psi\rangle$ and $|\phi\rangle$ are to be identified if they are related by a gauge transformation $U(x)$ ...
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41 views

Simultanously HOT and DENSE in QCD?

Take this form of the QCD Phase Diagram for example: This baryon density is a number density - i.e. number of baryons in some volume. Why are baryon density and temperature regarded as ...
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82 views

Resources for QCD colour factors

I'm looking for resources on the computation of (I think) very simple Feynman diagrams for QCD processes. The processes are q'qbar' -> qqbar and gg->qqbar (only at tree level, no loops involved) and ...
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47 views

Charge in terms of wavefunctions

For a charged quantum particle, say, an electron or a quark, how in the particle's wavefunction is the electric charge represented? Is it truly possible to represent electric charge using the wave ...
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105 views

Top quark production questions

I am just looking on the TopQuark production via proton antiproton collision and strong interaction. There seem to be three basic possibilities. $q + \bar q \rightarrow Gluon \rightarrow t +\bar t ...
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124 views

How to compute this loop integral? [closed]

I have a gauge boson that splits into two scalars and the loop is closed by a gauge boson as shown in the picture. The incoming boson has $\mu$ index while the boson that runs in the loop has momentum ...
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Quark anti-quark pair creation. Do we need a gluon?

What is the difference between these diagrams? Is the presence of the gluon taken for granted in the second one? and EDIT: I just zoomed into the picture, it is not a single quark state. ...