Questions tagged [quantum-chromodynamics]
Quantum-ChromoDynamics (QCD) is the quantum field theory believed to describe the strong nuclear force.
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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 theory....
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Interpretation of vector mesons in QCD
It is well-known that scalar mesons are interpreted as pseudogoldstone bosons which is connected with spontaneous broken $SU(3) \times SU(3)$ symmetry to $SU(3) \times SU(3) / SU(3)_{chiral}$.
Is ...
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What are the equations for the weak and strong forces? [duplicate]
What are the equations for the strong and weak force, like how for the Electrostatics the equation is $F_c= k*Q*q/(r*r)$. I understand to an extend what the strong and weak forces are. However, I ...
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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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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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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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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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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. There's a ...
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Instantons and confinement
Do instantons tell us something about behavior of QCD when lowering an energies? I.e., is it possible that instantons provide confinement? Are there some calculations which tell us that instantons ...
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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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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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The counting of all possible baryons quark combinations
I would like to ask how do you count the number of possible quark combination that could possibly exist in a baryon. I know certain spin symmetry or orbital momentum symmetry have to be conserved. But ...
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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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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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Colour decomposition in QCD
I am looking to compute the matrix element for the process gg -> u ubar at leading order. It is straightforward to calculate the non-colour part of the usual s, t and u channels. I will call these ...
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What are non-Abelian plasmas?
I know what a non-Abelian group is: a group whose operation does not obey commutativity. Does this have anything to do with a non-Abelian plasma? I'm not really looking at a specific paper, but I'll ...
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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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Why cross section of $e^+ e^- \to \bar{q} q$ is 3 times larger than $e^+ e^- \to \mu^+ \mu^-$?
I know the usual answer: quarks carry color charge (let us denote them r,g,b), antiquarks anti color charge, and since the initial state ($e^+ e^-$) does not carry any color charge (and color is ...
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Is it possible to calculate neutron half-life theoretically?
Is it possible to calculate neutron half-life theoretically? For example, from lattice QCD or something?
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QCD and QED with unlimited computational power - how precise are they going to be?
My question is about quantum algorithms for QED (quantum electrodynamics) computations related to the fine structure constants. Such computations (as explained to me) amounts to computing Taylor-like ...
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How to calculate D0->K+K- decay with QCD factorization?
I am studying the decay of D0->K+K- and I calculated the result of Branching Ratio by Naive QCD and I wrote all the equations for amplitude and branching Ration and Hamiltonian and... for this decay ...
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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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Different ways of derivation of Gell-Mann-Okubo mass formula
Recently my teacher told me that there are many ways of deriving the Gell-Mann-Okubo mass formula by using group representation theory (by using dynamical group etc). Where can I read about these ways?...
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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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Has confinement been experimentally observed? [closed]
So, confinement has obviously been shown by lattice gauge theory to be a predicted aspect of QCD. However, to what extent has it been observed in experimental physics?
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Simplify QCD Lagrangian
How we can derive the electric charge of the field theory for each field?
For example lets say that we have the u-quarks (3-colors) and electron that we know that has charge -e) free Lagrangian
$$\...
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Why are the dineutron and diproton unbound?
It is known that there are no diproton or dineutron nuclei.
Does this mean that two protons or neutrons are not actually attracted to each other? Even if the attraction was weak, wouldn't it cause ...
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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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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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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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Uncharged Gluon Representation
I'm currently learning for an exam and in an old exam the question was posed:
If Gluons were uncharged, under which representation would they transform?
The ...
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Baryon in terms of quark fields – spinor index structure
What is the most general way to write down a current describing a baryon made from quarks $\psi_i^\alpha$? Let's say we suppress flavour indices but want to write colour $(i,j)$ as well as spinor $(\...
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Group theoretical reason that Gluons carry color-charge and anti-colorcharge
I was wondering how it is possible to see from the $SU(3)$ Gauge Theory alone that Gluons carry two charges colors: $g\overline{b}$ etc.
Some background:
The W-Bosons (pre-symmetry breaking) form an ...
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How do we know that nuclear physics corresponds to low energy QCD?
One often hears the phrase "most of nuclear physics is in the low energy regime of QCD, where strong coupling constant is large ...", with the following diagram being invoked:
How does one know where ...
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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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Dilaton field and Scale symmetry breaking
I have read at some places that a dilaton field is associated with the spontaneous breaking of scale symmetry in a theory. (While others would be difficult to trace right now, the most easily ...
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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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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 independent ...
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Intuitive explanation of how hadron mass emerges from the strong force
I'm not familiar with QCD, but I'm looking for intuitive explanation of this phenomenon (it could be that easy explanation does not exist).
What I've read is that large part of hadron masses arises ...
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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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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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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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Construction of Bound states in Lattice QCD
How the proton state is found and renormalized in lattice QCD? Is there any literature shows the method step by step? What about other bound states in QCD if we somehow know their quantum numbers?
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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 we'...
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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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Deriving Feynman rules from a Lagrangian for vertex factors for "more complicated" interactions
I am trying to derive Feynman rules from a given Lagrangian and I got stuck on some vertex factors. What for example is the vertex factor that corresponds to the four-scalar interaction that is ...
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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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What is anti-screening?
In his book "The Lightness of Being" Frank Wilczek (page 48) writes about screening and anti-screening. In screening, a bare charge attracts virtual particles of opposite charge which lessen the ...
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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 ...