Questions tagged [quantum-chromodynamics]
Quantum-ChromoDynamics (QCD) is the quantum field theory believed to describe the strong nuclear force.
1,022
questions
1
vote
2
answers
20
views
Meson-Meson Scattering Data
Recently, I have been interested in the scattering amplitudes of mesons in the high-energy fixed-angle regime. I have come across a simple result, due to Brodsky et al., that predicts the scaling of ...
- 11
0
votes
0
answers
15
views
How to compute the susceptibility of the Polyakov loop in Monte Carlo lattice field theory?
I am having troubles understanding the definition of the susceptibility of the Polyakov loop give, for example, in the book by Gattringer, Lang "Quantum Chromodynamics on the Lattice", page ...
- 155
1
vote
0
answers
27
views
Gluons and their States
I've learned that gluons, which mediate strong interactions among quarks, always exist in a superposition of two or more color charges. To my knowledge, any superposition of eigenstates of an ...
- 894
0
votes
0
answers
26
views
Relation between (Anti-) Quark-Gluon-Quark Correlators
I'm having trouble in understanding the derivation for the realtion between Antiquark-Gluon-Quark and Quark-Gluon-Quark correlators given in a PhD thesis which I sadly can't find a link of to share ...
- 11
-1
votes
0
answers
18
views
Can you tell about Nucleus mass defect according to reference of proton having sea of quarks [closed]
Related with particalphysics and also chromo quantum dynamics please tell the suitable answer
1
vote
0
answers
56
views
Can quarks emit gluon waves?
Fundamentally, particles with a coupling constant to a gauge field interact with that force and can emit waves when accelerating. We observe this with particles with electric charge emitting EM waves ...
- 1,720
1
vote
0
answers
23
views
Background cuts [closed]
I am considering background events like, $p p \rightarrow t \bar{t}$ and QCD events where t is a top quark. I want also to reduced the QCD events such that QCD events will be less observed than that ...
3
votes
1
answer
81
views
Structure functions for mesons
I was researching about structure functions and understand how it's a probability density function that describes the distribution of quarks inside hadrons. However, since mesons also have quark gluon ...
- 595
3
votes
1
answer
93
views
Renormalization of quark bilinears
I'm looking at the one-loop corrections to the amputated quark two-point functions ($\Gamma_i$) with insertions of quark bilinears (indexed by $i\in\{S,P,V,A,T\}$) with off-shell legs in Euclidean QCD....
- 71
0
votes
0
answers
77
views
One-loop renormalization of the gauge coupling
Quoting Yuji Tachikawa, chapter 3 of "${\cal N}=2$ Supersymmetric Dynamics for Pedestrians":
Recall the one-loop renormalization of the gauge coupling in a general Lagrangian field theory $$...
- 616
0
votes
1
answer
64
views
How do scientists attempt to break quark confinement?
From what I understand, when energy is supplied to quarks, it elongates the gluon tube and when enough energy is put in, new quarks are formed, keeping quark confinement. However, I am inquisitive ...
- 595
1
vote
1
answer
130
views
How Does QCD Explain Nuclear Fission?
This question is brief and simple. As a layman, it is my understanding that QCD (Quantum Chromodynamics) explains the nuclear force as a "residual" force, mediated by pions (mesons), which ...
- 11
0
votes
1
answer
65
views
Contraction of Lorentz indices in gluon propagator of QCD
In QED, the photon propagator has a factor of $g^{\mu \nu}$, and both $\mu$ and $\nu$ contract with the $\gamma$ matrix indices, which come from the fermion antifermion photon vertices on either end ...
- 932
0
votes
1
answer
102
views
Are all mass eigenstates also spin eigenstates?
Is there a rigorous way to show that a mass eigenstate of a particle must also be an eigenstate of the total spin operator? If this wasn't true, you could imagine that a composite particle in a mass ...
- 719
2
votes
1
answer
51
views
Expansion at first order ${\cal O}(\alpha_s)$ in counterterms for the QCD vertex renormalization at 1-loop
What is the meaning of the expansion at first order ${\cal O}(\alpha_s)$ in $\delta_2$ and $\delta_3$ at the second step in the last line? These quantities are not "small" - on the contrary,...
- 153
0
votes
1
answer
52
views
How to write a naive Dirac matrix for Lattice QCD?
I'm trying to write down the naive Dirac matrix (with fermion doubling) for a LQCD simulation with one quark, for now. I initialized the $SU(3)$ gauge field and the quark field. The quak field has 4 ...
- 31
1
vote
0
answers
60
views
How do we measure the gluon scattering?
In QFT, we often calculate the 'Feynman amplitude' for gluon scattering. As we know, Gluon is gauge boson mediating the strong force between quarks. By color confinement, Quarks are never seen in our ...
- 76
0
votes
1
answer
77
views
Superposition of charge states in the Neutral Pion
The quarks of neutral pions don't exist in a pure flavour state, and instead are described as a superposition of up-antiup with down-antidown:
$\frac{u\bar{u}-d\bar{d}}{\sqrt{2}}$
However up and down ...
- 13
0
votes
0
answers
45
views
Some questions about Peccei-Quinn-Weinberg-Wilczek (PQWW) model
In PQWW model,we have two Higgs doublet($\phi_1$,$\phi_2$),and the Yukawa interaction of quarks is:
\begin{equation}
\mathcal{L}_{Y}^q=-f_{i j}^{(u)} \bar{q}_{L j} \phi_{2} u_{R i}-f_{i j}^{(u)} \phi_{...
- 1
0
votes
0
answers
36
views
Calculating Diagrams with Wilson Lines to One Loop
I have a question concerning the calculation of amplitudes containing Wilson lines.
I want to calculate the jet function defined by equation (3.3) from this paper.
$$\mathcal{J}(\text{arguments})u_s(p)...
- 2,865
2
votes
0
answers
34
views
How to update $SU(2)$ Higgs fields with Heat Bath algorithm?
I'm trying to update the Higgs field coupled with a pure gauge $SU(2)$ theory through Heat Bath algorithm. Pure gauge and Higgs configurations should be updated separately. For the pure gauge part the ...
- 31
2
votes
1
answer
105
views
Can black holes have colour charge? [duplicate]
Does the no hair's theorem take into account all the elementary particles and forces, and in particular can a black hole have color charge?
- 1,628
1
vote
0
answers
15
views
Why are there more opposite sign events than same-sign ones in the QCD background of three-prong tau decays?
So im dealing with three-prong tau decays. Specifically, two taus are produced via the Drell-Yan process and one decays leptonicly into a muon and the other decays hadronicly into three charged pions (...
- 11
3
votes
1
answer
131
views
Global symmetries QCD goldstone bosons
Beside the local $SU(3)$-Color-symmetrie The QCD Lagrangian also has global symmetries:
$$L_{QCD}=\sum_{f,c}\bar{q_{fc}}(i\gamma^\mu D_\mu - m ) q_{fc} - \frac{1}{4}F^a_{\mu \nu} F^{a \mu \nu} $$
$SU(...
- 53
3
votes
1
answer
501
views
Why do gluon jets have more particles?
From papers on the concept of quark/gluon jet tagging, I've seen that gluon jets have more particles and a broader radiation pattern. Why does carrying more color charge allow them to produce more ...
- 979
1
vote
0
answers
34
views
Neutron pair annihilation cross-section
I'm having a surprisingly difficult time finding this information. My expectation is that the total pair annihilation cross-section is something like $\langle\sigma v\rangle_{n\bar n}\sim \Lambda_{\rm ...
- 1,261
0
votes
0
answers
43
views
Physical interpretation of collinear divergences in QCD
Consider the following Feynmann graphs:
what is the physical interpretation of collinear divergences? Why I should get a divergence when the gluon of momentum $l$ and the quark of momentum $k$ (graph ...
- 1
3
votes
1
answer
233
views
Why $\pi^+$ meson possible combinations of quarks is only three?
I am a starter at nuclear and particles physics. I am reading particles and nuclei an introduction to the physical concepts. There was a paragraph about Colour-neutral particles and how only ...
- 73
1
vote
0
answers
80
views
Can I do anything instructive by simulating QED on a lattice?
For learning something about the degrees of freedom and underlying path integral math, is it possible to do some kind of scalar QED or normal QED simulation on a lattice in the same way Lattice QCD is ...
- 7,121
9
votes
2
answers
190
views
Why are protons/neutrons spin-$\frac{1}{2}$ when they are made up of three valence quarks (also spin-$\frac{1}{2}$)?
If we take the composite angular momentum of the three valence quarks, we should have a proton/neutron spin angular momentum of,
$$S=\{ \frac{1}{2},\frac{3}{2} \}$$
using the general rule for adding ...
- 739
2
votes
1
answer
100
views
Do Neutron stars have free quarks?
Do Neutron stars have free quarks? Also Can Quark Stars be formed also due to this reason?
Because of Asymptotic freedom high energy causes quarks to be free but according to the mass-energy ...
user348071
1
vote
1
answer
80
views
Can you rewrite the QCD lagrangian in terms of hadron?
Is it possible to (exactly) rewrite the QCD lagrangian in terms of hadrons?
I get that it's probably practically too difficult to do, but would it be possible in principle?
- 1,777
0
votes
0
answers
18
views
How many multiquark stable (or metastable) and observable states are there in QCD and the SM?
I was wondering about the last news about multiquark resonances. We get some tetraquark and pentaquark states...But, why to stop there?
Could we have long-lived enough hexaquarks, heptaquarks, ...
- 6,171
1
vote
3
answers
142
views
Which configurations are important in lattice QCD?
The failure of perturbation theory in describing strongly-coupled QCD is because it can't account for field configurations that are 'large'.
My questions is: from experience in lattice QCD, what kind ...
- 357
3
votes
0
answers
42
views
Topology active subgroup for the QCD vacuum
I've been reading about the nontrivial topological structure of the QCD vacuum and, when studying the different equivalence classes created by the pure gauge fields, all papers say that it is possible ...
- 96
3
votes
1
answer
126
views
What gives rise to mass gap for gluons, even if they are massless?
It is known that QED does not have a mass gap.
On the other hand, at the heuristic level, QCD has a mass gap.
But photons and gluons are both "massless". Could anyone explain (at least at ...
- 1,281
0
votes
1
answer
64
views
Could there be a hybrid particle made of one quark and one or several gluons?
The question's background is that such a quark-gluon bound state
might be colour-less.
But it seems that no such particle is ever mentioned.
Is there a reason speaking against its existence?
ADDED:
...
- 577
4
votes
1
answer
131
views
How does AdS/CFT help us understand non-perturbative aspects of QCD?
I've heard AdS/CFT has found applications in many areas of physics where nonperturbative aspects leave us crippled in making any simple calculations.
Among these applications, I also have heard that ...
- 1,141
0
votes
0
answers
48
views
Notation of trace in QCD
I have been reading about QCD, and I have found a notation about the trace at Schwinger Dyson equation like $Tr_D[exp]$ and I don not know whats does means. Thank you.
Update reference: equation 27
...
0
votes
0
answers
47
views
What is the angle between two $b$-quarks when a Higgs boson decays to a $b$-quark pair, in the rest frame of Higgs boson?
I am generating events of $ep -> Xjv$, ($j$ is a jet, and $v$ shows a neutrino) where $X$ particle decays to a $b$-quark pair. I am going to calculate the angle between two b-quarks in the rest ...
- 1
0
votes
1
answer
72
views
Is there an energy release from gluon expansion and contraction?
When an electron drops to a lower orbit, energy is released as a photon. Gluons expand and contract in-between quarks, described like springs or rubber bands that have tremendous force when extended ...
- 4,008
0
votes
0
answers
20
views
A validity of a perturbation towards a extreme high density/temperature QCD
It is often said that a QCD at a extreme high density/temperature region is well described by a pQCD. It seems right because a distance between quarks are very small at the extreme high density, and a ...
- 713
0
votes
0
answers
13
views
Energy conservation in proton-electron soft collisions
Recent quantum chromodynamics calculations seems to indicate that the proton can briefly spawn a charm/anti-charm quark pair. From the article Inside the Proton, the ‘Most Complicated Thing You Could ...
- 259
0
votes
0
answers
41
views
Does the scattering data, in any way, justify the simplistic picture of hadrons? [duplicate]
The structure of the neutron or the proton is extremely complicated. It is not a simple bound state of three valence quarks. However, the simplified picture allows us to explain various properties of ...
- 10.4k
1
vote
0
answers
49
views
How well does Lattice QCD handle relativity?
In Lattice QCD space-time is approximated by a grid.
To me this doesn't seem to handle relativity well. Due to
(1) A Lorentz transformation of the grid will distort the hyper-cube volumes into ...
- 6,670
1
vote
1
answer
74
views
How can I show the following contraction of the electromagnetic field strength and its dual? [closed]
Given the electromagnetic field strength $F^{\mu\nu}$, and its dual $$\tilde{F}^{\mu\nu} =\dfrac{1}{2}\varepsilon^{\mu\nu\alpha\beta}F_{\alpha\beta},$$
how can I show that
$$\tilde{F}^{\mu\nu}F_{\nu\...
5
votes
3
answers
2k
views
Can free quarks exist?
I know that currently free quarks do not exist but can they exist under any circumstance? The Physicsworld article Quarks break free at two trillion degrees states that they do but I want a ...
- 51
1
vote
0
answers
37
views
Electon-positron annihilation ratio and vacuum polarization in QCD
It is commonly defined the ratio
$$R(s)= \frac{\sigma(e^+e^-\longrightarrow \textrm{hadrons}; s)}{\sigma(e^+e^-\longrightarrow \mu^+\mu^-;s)}$$ where $s$ is the center-of-mass energy squared. I found ...
- 11
0
votes
0
answers
31
views
Glue vs spring, the approach
Why does quantum mechanics use the term "glue" instead of "spring"? Is the strong force wchich keeps quarks together so different from the behavior of springs that they must have a ...
- 119
0
votes
1
answer
68
views
Are glueballs confined without a connection to quarks?
Without being bound to quarks through the strong force, is there any reason why glue balls would be confined? Are they confined
- 4,008