All Questions
Tagged with quantum-chromodynamics standard-model
155
questions
5
votes
1
answer
74
views
$ \pi^0\to \gamma\gamma$ parity conservation
Let's consider the decay process $\pi^0\to \gamma \gamma$. After we spontaneously broke the chiral symmetry of QCD coupled to an abelian gauge field $A^\mu$, we end up with the Goldstone boson ...
- 357
1
vote
0
answers
35
views
Evidence of more generations in the QCD beta function
We know that the beta function for QCD is
$$
\beta = -\left(11 - \dfrac{2N_f}{3}\right),
$$
where $N_f$ is the number of fermions in the theory. We have $\beta_{\text{SM}} = -7$.
Now, my question is, ...
- 1,611
0
votes
0
answers
31
views
Understading dimensions in quark bilinears
I have encountered myself with the following definition for $\pi$-fields as quark bilinears:
$$
\pi^a = i\bar{q}\tau^a \gamma_5 q \ ,\quad\text{with }\ q = \left(\begin{array}{c}u\\d\end{array}\right) ...
- 601
1
vote
1
answer
98
views
Quark condensate and VEV of $\pi^0$
In David Tong's lectures on the Standard Model I saw that there is a quark condensate, which is just a Vacuum Expectation Value (VEV) of the $\bar{q}_{Li}\, q_{Ri}$ operator,
$$
\left< \bar{q}_{Li}...
- 1,611
3
votes
1
answer
70
views
Isospin doublet and quark content from contraction of quarks
Let's introduce a quark $SU(2)$ doublet. We are in the $m_u \approx m_d$ limit. So we have
$$
q = \begin{pmatrix}
u\\
d
\end{pmatrix}.
$$
Then we can construct the Nucleonic field
$$
N := q q q = \...
- 1,611
1
vote
1
answer
118
views
Do all antiquarks carry and anti-color charge, or can they carry RGB color charges as well?
I know there are antiquarks with anticolor charges. Are there also antiquarks that instead carry color charges?
Basically, which of these lists describes the types of quarks that there are:
List one:
...
- 325
0
votes
2
answers
122
views
Is there any physical reason behind the choice of Lie group in a Yang-Mills theory?
A Yang-Mills theory can be constructed for any Lie group that is compact and semisimple. The motivation behind this is discussed in this question. Is there any physical reason we choose $SU(3)$ or $U(...
- 3,350
0
votes
2
answers
76
views
Difference between $ SU(3)$ and $ SU(3)_c $ group
I am reading quark model. I don't understand what's the meaning of a color $SU(3)$ or $SU(3)_c$ group and how it differs from a general $SU(3)$ group. Please elaborate.
- 377
3
votes
0
answers
67
views
When do pairs of quarks form jets vs mesons?
For certain processes with low momentum transfer, such as the Kaon decay shown below, quarks will form bound states of mesons.
Whereas for higher momentum-transfer processes, such as the decay of an ...
0
votes
0
answers
90
views
Beyond sexaquarks?
There are some candidates for tetraquarks, pentaquarks or even now sexaquarks/hexaquarks. But, I wonder, what are the highest molecular quark states taking into account QCD? Are there heptaquarks, ...
- 6,611
-1
votes
1
answer
156
views
Why is the electromagnetic force not an emergent property of the strong force? [closed]
Two up quarks in a proton lead to an imbalance, which results in the proton having the ability to attract electrons. Two down quarks in a neutron lead to balance in the electromagnetic force, leading ...
- 29
3
votes
2
answers
161
views
How do we known that $\langle \bar{\psi}_i \psi_j\rangle=(250 MeV)^3\delta_{ij}$?
I have started to read the phenomenology of QCD in low energy regime. I understand that, from the QCD renormalization group equation, the QCD becomes nonperturbative theory when energy scale is below $...
- 31
9
votes
3
answers
2k
views
Why are gluons massless as their range is finite?
The range of electromagnetic waves and gravitational force is infinity and the particles exchanged during these interactions are photons and gravitons respectively. Both are massless following the ...
1
vote
1
answer
233
views
Why Lambda Baryon and Sigma Baryon unequal sum isospin with "really" isospin?
For information isospin for $u$-quark = 1/2, isospin for $d$-quark = -1/2 and isospin for $s$-quark = 0.
Lambda Baryon consists $uds$ quarks,therefore sum isospin 1/2-1/2+0=0, It true.
But sum isospin ...
- 23
1
vote
0
answers
27
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 ...
- 171
-1
votes
1
answer
198
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 ...
- 115
4
votes
1
answer
302
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} $$
$SU(...
- 63
3
votes
1
answer
530
views
Why do gluon jets have more particles?
From papers on the concept of quark/gluon jet tagging, I've seen that gluon jets have more particles and a broader radiation pattern. Why does carrying more color charge allow them to produce more ...
- 979
9
votes
2
answers
286
views
Why are protons/neutrons spin-$\frac{1}{2}$ when they are made up of three valence quarks (also spin-$\frac{1}{2}$)?
If we take the composite angular momentum of the three valence quarks, we should have a proton/neutron spin angular momentum of,
$$S=\{ \frac{1}{2},\frac{3}{2} \}$$
using the general rule for adding ...
- 941
3
votes
1
answer
213
views
What gives rise to mass gap for gluons, even if they are massless?
It is known that QED does not have a mass gap.
On the other hand, at the heuristic level, QCD has a mass gap.
But photons and gluons are both "massless". Could anyone explain (at least at ...
- 1,669
0
votes
1
answer
100
views
Is there an energy release from gluon expansion and contraction?
When an electron drops to a lower orbit, energy is released as a photon. Gluons expand and contract in-between quarks, described like springs or rubber bands that have tremendous force when extended ...
- 4,783
1
vote
1
answer
81
views
How can I show the following contraction of the electromagnetic field strength and its dual? [closed]
Given the electromagnetic field strength $F^{\mu\nu}$, and its dual $$\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\...
6
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 ...
- 61
0
votes
1
answer
83
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
- 4,783
5
votes
4
answers
668
views
Why are gluons color charged but not photon? Could there be a charged EM force carriers like gluons or neutral color charge carrier like photon?
Gluons have a color charge why don't photons have an electric charge like gluons?
1
vote
1
answer
60
views
$\alpha_s$ strength in quarkonium potential energy formula
The top answer to Is there an equation for the strong nuclear force? explains that there is a coulomb's law-like formula:
$V(r)=−\frac{4}{3}\frac{α_s(r)ℏc}{r}+kr$
that describes how quarks and ...
- 451
1
vote
1
answer
105
views
Why is there still disagreement over the mass of the bottom (or beauty) quark, but none of the others?
Wikipedia (among other places) lists two values for the alleged mass of the B quark, 4.18 and 4.65 GeV.
Only one of the two possible masses listed has a link to another Wiki page explaining the ...
- 4,509
1
vote
1
answer
168
views
Why is "a phase for the QCD vacuum" considered a fundamental constant and what does it even mean?
We can read about fundamental physical constants in Wikipedia:
There is, however, no single "correct" way of enumerating them, as it is a matter of arbitrary choice which quantities are ...
1
vote
0
answers
106
views
Do quarks have a non-zero electric dipole moment?
It is written here, in this PSE link that an electron has a measured perfect spherical charge density therefore a zero electric dipole moment (i.e. perfect monopole charge).
My question is, are quarks ...
- 4,170
-3
votes
1
answer
209
views
Do Quarks Exist?
Do Quarks Really Exist?
Is there any experiment which can confirm the existence of Quarks and what kind of force is that which binds them to form neutron and proton?
- 31