# Tagged Questions

Quantum-ChromoDynamics (QCD) is the quantum field theory believed to describe the strong nuclear force.

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### Non-singlet and singlet flavor combination [duplicate]

In the perturbative QCD, specific partonic distribution functions (pdf) linear combination is called "flavor singlet" or "flavor nonsinglet" combination. What is the combination, and why are they ...
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### Non-singlet and singlet flavor combination [duplicate]

In the perturbative QCD, specific partonic distribution functions (pdf) linear combination is called "flavor singlet" or "flavor nonsinglet" combination. What is the combination, and why are they ...
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### 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 ...
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### Proton is made of 3 quarks of each spin 1/2, but the total spin of proton also 1/2, how it is possible?

Proton is made of 3 quarks (UUD) each has a spin of 1/2, so if you follow the constitution the total spin of the proton must be (3/2), but the spin of the proton is still 1/2. How it is possible? But ...
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### What is the mathematical motivation for complexifying momenta in BCFW?

One of the first steps in obtaining the on-shell BCFW recursion relations is complexifying the momenta of the external particles. Now complexifying things is not unprecedented (the dispersion program ...
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### Is CP problem the problem?

I've heard an argument that the question of smallness of QCD $\theta$ parameter is called the problem (namely, strong CP problem), since the other dimensionless couplings (like $\alpha_{s}$), are of ...
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### How many glueballs are there?

As I understand there are eight types of gluons (linear combinations of color/anticolor pairs with varying amplitudes) which can combine (for very short periods) to form glueballs. If there were no ...
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### Can glueballs, consisting of massles gluons, get a mass without reference to a Higgs field? [duplicate]

Glueballs, the by the standard model predicted combinations of short-lived combinations of massless gluons, have a mass between 1.4 and 5 (MeV). But where is this mass coming from? From interaction ...
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### Equality of renormalized coupling constant

I want to show, that the renormalized coupling constants of a SU(N) Yang-Mills field with fermions included, are all equal. In the most textbooks it is written, that this could been shown by the Ward-...
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### Semileptonic decay amplitude

I'm trying to do past exam papers and I am absolutely failing understand some things. I would very much appreciate any insights/intuition you have. The part I'm struggling is here: In the first ...
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### Pion decay from chiral anomaly

I have to compute the decay rate of the process: $$\pi^{0} \rightarrow \gamma \gamma$$ with 4-momenta $P \rightarrow K_{1}+K_{2},$ by the means of the following effective Lagrangian, coming from ...
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### How would one go about drawing a Feynman diagram of color screening and anti-screening in QCD?

Title really explains this one. I'm not sure how to put together a diagram of the lowest order for colour screening and anti-screening in QCD. Seems to be a lack of diagrams for this in the literature!...