All Questions
Tagged with quantum-chromodynamics quantum-electrodynamics
59
questions
5
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1
answer
76
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
60
views
Spinor-helicity formalism: relationship between 1 and 2 reference vector setups
The spinor-helicity formalism is usually set up so that for a massless vector boson (photon or gluon) with momentum $k$ an arbitrary reference momentum $p$ is introduced and the corresponding ...
- 312
3
votes
2
answers
130
views
Can an electron be produced inside a proton?
We know that inside a proton there is a sea of quarks, antiquarks and gluons. This happens as the valence quarks emit gluons which then split into a quark-antiquark pairs. These pairs become gluons ...
- 85
0
votes
0
answers
29
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Uniquness of the vacuum in a theory with/without mass gap
Context
I read the note Light Cone Quantization and Perturbationwritten by Guillance Beuf. He gives a argument in section 3.3.2, p17, 2nd paragraph :
In particular, in a theory with a mass gap, ...
- 108
1
vote
0
answers
77
views
What is the meaning of twist in OPE?
In Operator Product Expansion (such as explained in Peaking) there appear a quantity for an operator called twist, defined to be $d-s$ where $d$ is the scaling dimension of the operator and $s$ is it'...
- 41
0
votes
2
answers
122
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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(...
- 3,370
0
votes
0
answers
52
views
Using Compton scattering to derive the deep inelastic cross-section for the parton model
In the second volume of The Quantum theory of Fields, Weinberg provides the inelastic cross-section for the scattering of an electron from a nucleon with four momentum $p$ based on the parton model:
$$...
- 163
-1
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2
answers
206
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How exactly does a proton form from quarks? What is the exact sequence and mechanism?
What are the steps that lead to the bonding of two up quarks and one down quark into a proton? For instance, does an up quark "bind" with a down quark in quark-gluon plasma, which then binds ...
- 29
4
votes
0
answers
491
views
Can I do anything instructive by simulating QED on a lattice?
For learning something about the degrees of freedom and underlying path integral math, is it possible to do some kind of scalar QED or normal QED simulation on a lattice in the same way Lattice QCD is ...
- 7,219
0
votes
0
answers
65
views
What is the angle between two $b$-quarks when a Higgs boson decays to a $b$-quark pair, in the rest frame of Higgs boson?
I am generating events of $ep -> Xjv$, ($j$ is a jet, and $v$ shows a neutrino) where $X$ particle decays to a $b$-quark pair. I am going to calculate the angle between two b-quarks in the rest ...
- 1
1
vote
1
answer
81
views
How can I show the following contraction of the electromagnetic field strength and its dual? [closed]
Given the electromagnetic field strength $F^{\mu\nu}$, and its dual $$\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\...
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?
2
votes
0
answers
77
views
Transpose of a matrix element [closed]
A matrix element is just a number. Now, If I have the following matrix element:
\begin{equation}
\newcommand\bra[1]{\left<{#1}\right|}
\newcommand\ket[1]{\left|{#1}\right>}
A = \bra{B}\bar{b}\...
- 41
2
votes
1
answer
161
views
Electromagnetic contribution to the mass of proton
It is a well-established fact that the main contribution to the mass of proton is the gluonic field inside a proton, as some answers explain (see here and here). But being the proton and the neutron ...
- 1,640
1
vote
0
answers
55
views
Does a free proton shrink when accelerated?
Reading about the charge radius of the proton and proton radius puzzle I started wondering if the cross section and therefore also the charge radius (i.e. size of proton) changes or more specifically ...
- 4,170
2
votes
1
answer
280
views
Is there a simple explanation of why coupling constants run with $\log(E)$?
The inverse coupling constants run with $\log(E)$, where $E$ is the energy or four-momentum.
Some coupling constants increase, some decrease with $\log(E)$.
Is there a simple argument that explains ...
user85598
1
vote
0
answers
46
views
Extensions of the BCFW recursion relations
In their "original" form the BCFW recursion relations are used to calculate tree-level (color-ordered) $n$-gluon amplitudes. However is it possible to extend this formalism to more general ...
- 387
0
votes
0
answers
41
views
Structure function in DIS calculation step
There is one step in the calculation that I am not understanting, but there are many things to present so one can understand and help me:
i)$k^2$ and $k_T^2$ are small, so can be neglected;
ii)$k^\mu=\...
1
vote
1
answer
131
views
What happens to an electron when it radiates a photon?
I recently came across this Feynman diagram:
For a more simplistic diagram, I suppose even this would be adequate:
As you can see in these diagrams, they radiate these virtual photons. The virtual ...
- 126
2
votes
1
answer
80
views
Is Compton Scattering the "Abelian limit" of $qg \rightarrow qg$?
I have calculated the average over initial and sum over final states of the squared amplitude for both Compton scattering $e^-\gamma \rightarrow e^-\gamma$ (QED) and quark-gluon scattering $qg \...
- 321
0
votes
0
answers
49
views
$e^+e^- \to p^+p^-$ cross-section?
Does anyone know of a calculated or measured cross-section somewhere for $e^+e^-$ annihilation into a proton-antiproton pair, say at the LEP? Seems to me that it would be rather strongly energetically ...
- 417
0
votes
1
answer
271
views
How do I compute the amplitude for this QCD diagram?
Studying the following scattering process at a tree-level:
$$\bar{q}^i (p_a) + q^j (p_b) \to \gamma(k_1) + \gamma(k_2)$$
Considering the “reduced” amplitude that is obtained by stripping away the
...
- 753
2
votes
0
answers
155
views
Can one distinguish QED (or QCD) from QED (or QCD) plus higher field powers?
The Lagrangian of QED is based on minimal coupling; it contains a term proportional to $F_{\mu\nu}F^{\mu\nu}$ for the electromagnetic field.
The QED Lagrangian is the simplest Lagrangian with U(1) ...
- 684
3
votes
0
answers
305
views
Is it a coincidence that the maximum number of protons in a stable nucleus is on the order of $\alpha_s / \alpha_{EM}$?
The equivalent to the fine-structure constant for the strong interaction is about $\alpha_s \approx 0.5$ at the proton rest mass energy (and it runs pretty slowly with energy, so it's on this order ...
- 48.4k
1
vote
0
answers
96
views
What is the real problem in the free body problem?
Ions are produced when an EM neutral atom gains EM charge by gaining or losing electrons, by collision with other charged particles or photons.
The study of such collisions is of fundamental ...
- 29k
2
votes
1
answer
814
views
Topological theta-term as a background electric/magnetic field?
The topological $\theta$-term in the Schwinger model (1+1-dimensional QED) can be interpreted as a background electric field, as explained in Chapter 7.1.2 of Tong's lecture notes. The same holds true ...
- 1,783
5
votes
3
answers
5k
views
Most trivial neutral pion decay
Literature states neutral pion decay by QED cannot occur directly because the pion is uncharged.
However, I cannot see why Photons are not allowed to interact with the Quarks directly.
Please ...
- 59
2
votes
1
answer
461
views
Why is the fine structure constant the probability for photon emission by a charged particle?
I see in page 31 of Martin, Hanzel "Quarks and Leptons", that the fine structure constant is the probability for photon emission by a charged particle. Also I read from Lubos Motl's answer on this ...
- 700
2
votes
0
answers
106
views
How to get group U(1) from SU(N)?
I have read that the unitary group is somehow given by the direct product $U(N)=U(1)*SU(N)$ and it follows that for $N$ going to zero we get just $U(1)$.
How it can be possible? What does it mean $SU(...
- 479
3
votes
1
answer
692
views
Wilson Loops and Confinement in QED
In [1] Kenneth G. Wilson proposed a mechanism for confinement using lattice paths what leds him to the concept of Wilson loop. It seems to me that he is using mainly a single abelian field. He says
...
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