All Questions
Tagged with quantum-chromodynamics quantum-electrodynamics
59
questions
5
votes
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
views
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
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(...
- 3,370
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:
$$...
- 163
-1
votes
2
answers
206
views
How exactly does a proton form from quarks? What is the exact sequence and mechanism?
What are the steps that lead to the bonding of two up quarks and one down quark into a proton? For instance, does an up quark "bind" with a down quark in quark-gluon plasma, which then binds ...
- 29
4
votes
0
answers
491
views
Can I do anything instructive by simulating QED on a lattice?
For learning something about the degrees of freedom and underlying path integral math, is it possible to do some kind of scalar QED or normal QED simulation on a lattice in the same way Lattice QCD is ...
- 7,219
0
votes
0
answers
65
views
What is the angle between two $b$-quarks when a Higgs boson decays to a $b$-quark pair, in the rest frame of Higgs boson?
I am generating events of $ep -> Xjv$, ($j$ is a jet, and $v$ shows a neutrino) where $X$ particle decays to a $b$-quark pair. I am going to calculate the angle between two b-quarks in the rest ...
- 1
1
vote
1
answer
81
views
How can I show the following contraction of the electromagnetic field strength and its dual? [closed]
Given the electromagnetic field strength $F^{\mu\nu}$, and its dual $$\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
...
- 183
4
votes
0
answers
399
views
Is the physical mass of the electron a gauge invariant quantity?
In Lattice, one cannot calculate gauge non-invariant quantities, such as the quark mass. This is because one averages over the gauge and gets 0.
One way to get around the issue is to fix the gauge. ...
- 41
2
votes
1
answer
378
views
Why is the typical lifetime so different depending on the type of interaction?
Why is $\tau$ so different when it comes to Quantum Electrodynamics ($10^{-16}$s), Quantum Flavordynamics ($>10^{-13}$s) and Quantum Chromodynamics ($10^{-23}$s)?
Does this have something to do ...
- 1,723
2
votes
0
answers
194
views
Understanding mathematical explanation for Chudakov Effect
I'm trying to better understand some properties of QCD and currently I'm looking into understanding color coherence. I've got the book "Elementary Particle Physics: Foundations of the Standard Model, ...
- 161
3
votes
1
answer
942
views
Is the non-perturbative approach to QFT a path integral approach? If so then how, given we don't have simple path integral formula for Dirac equation?
Here is my understanding of the scenario. Please correct me if i go wrong somewhere.
Initially, the perturbative approach to QED (Feynman diagrams) was very successful. But the same approach to QCD ...
- 2,326
1
vote
1
answer
171
views
How would the photon having a charge change the electromagnetic interaction?
This question is motivated by my recent foray into Quantum Field Theory. Just to make this clear straight off the bat; I am not suggesting in any way shape or form that the photon has a charge. I am ...
- 703
1
vote
0
answers
302
views
Radiative correction to the charge form factor $F_1$ in QED
In QED, one can calculate the correction to the form factor $F_2$. To the lowest order, $F_1=1$ and $F_2=0$. At one loop, it is found that $F_2(0)$ receives a non-zero finite correction which is ...
- 26.8k
2
votes
1
answer
365
views
Quantization prescription for an interacting field theory
To my understanding, unlike free fields, interacting fields cannot be expanded in terms of Fourier modes, with the Fourier coefficients representing creation and annihilation operators. Then is it ...
- 26.8k
2
votes
1
answer
506
views
Understanding type of force interaction in particle decays
Are there any fundamental rules of thumbs that can be used to identify the type of force interaction (weak, electromagnetic, strong) in a particle decay without drawing the Feynman diagrams at the ...
- 310
4
votes
0
answers
951
views
Sudakov double logarithm
I have calculated a few NLO corrections in QED and in the final result the Sudakov double logarithms have always canceled. So I thought that they have no physical meaning. On the other hand I have ...
- 1,273
36
votes
4
answers
6k
views
Why have our eyes not evolved to see "gluons"? [closed]
The photons are the propagators for QED, and we rely on photons to see the world around us.
The gluon is the propagator in QCD. Why have our eyes not evolved to see gluons (either on top of being ...
- 1,634
1
vote
1
answer
79
views
QCD-Process with superposition-particle
I am a total beginner with non-abelian gauges. To write down a process from a neutral pion ($\pi^0 = \frac{1}{\sqrt{2}}(u\overline{u}-d\overline{d})$) I expect to have to write it as this ...
- 11
5
votes
1
answer
863
views
Why Conserved Current Should Not Need Renormalization?
May be this is trivial but I need to understand why the renormalization of conserved current is not necessary ? As for example, in this paper, they demand (2$^{nd}$ paragraph of the 2$^{nd}$ column in ...
- 407
1
vote
1
answer
120
views
Looking for a reference for $\gamma_a e^{a}_{\mu} D^\mu \gamma_b e^{b}_{\nu} D^\nu =D^\mu D_\mu - \tfrac{1}{4}R$
I am having trouble finding references for the following identities:
Dirac Operator:
$$
\gamma_a e^{a}_{\mu} D^\mu \gamma_b e^{b}_{\nu} D^\nu =D^\mu D_\mu - \tfrac{1}{4}R
\tag{1}
$$
QED Operator:
$$
...
11
votes
1
answer
3k
views
Polarization Sums in QCD for the calculation of parton model splitting functions
Before i state the actual problem, here's a premise. In the case of a Spin 1 massive particle it's possible to demonstrate that $$\sum_{\lambda=0,\pm1}\epsilon_{\lambda}^{* \ \mu}\epsilon_{\lambda}^{\...
- 2,263
7
votes
2
answers
728
views
If proton spin emergence from quarks and gluons is mysterious, why is silver atom spin not?
A recent Scientific American article brought up an old issue, which is this: According to quantum chromodynamic models, the emergence of exactly 1/2 unit of spin in a proton (or a neutron, or any ...
- 22.1k
16
votes
2
answers
1k
views
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 ...
- 2,073
3
votes
1
answer
146
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?
- 5,569
1
vote
0
answers
63
views
Quantum fluctuations in the non-relativistic limit
Is there any way to describe quantum fluctuations in ordinary quantum mechanics? For instance, a proton fluctuating into a proton-$\pi^0$ state and then back to a proton?
What are the relevant ...
- 5,569
8
votes
2
answers
436
views
No non-trivial UV asymptotically free and IR free
How it could be proven that a non-trivial theory cannot be both asymptotically free and IR free (g=0 both in the UV and IR with some interpolating function in between)? This is of course contrary to ...
- 1,707
-1
votes
2
answers
175
views
Is quark electric charge also confined?
Since we will (maybe) never observe free quarks which has fraction charge, is it logically correct to say that the fraction (1/2,2/3 etc) electric charge is also confined. I.e. the charges that we do ...
- 5,569