# Tagged Questions

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

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### Is there an equation for the strong nuclear force?

The equation describing the force due to gravity is $$F = G \frac{m_1 m_2}{r^2}.$$ Similarly the force due to the electrostatic force is $$F = k \frac{q_1 q_2}{r^2}.$$ Is there a similar equation ...
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### Why have our eyes not evolved to see “gluons”? [closed]

Bit of a random question...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 (...
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### What does it mean that there is no mathematical proof for confinement?

I see this all the time* that there still doesn't exist a mathematical proof for confinement. What does this really mean and how would a sketch of a proof look like? What I mean by that second ...
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### What is the status of Witten's and Vafa's argument that the QCD vacuum energy is a minimum for zero $\theta$ angle?

The argument, which I reproduce here from Ramond's `Journies BSM', is originally by Witten and Vafa in ($\it{Phys}$. $\it{Rev}$. $\it{Lett}$. 53, 535(1984)). The argument is that for $\theta = 0$ (...
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### Is it pions or gluons that mediate the strong force between nucleons?

From my recent experience teaching high school students I've found that they are taught that the strong force between nucleons is mediated by virtual-pion exchange, whereas between quarks it's gluons. ...
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### How far can we separate two bound quarks?

If we try to separate two quarks bound into a meson or a hadron, the energy in the gluon field eventually will be large enough to spawn a quark-antiquark pair. How far can we stretch that gluon ...
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### What are the details around the origin of the string theory?

It is well-known even among the lay public (thanks to popular books) that string theory first arose in the field of strong interactions where certain scattering amplitudes had properties that could be ...
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### The divergence in QCD Series— How many are they, and what do they mean?

I am referring to this question, and especially this answer. In addition, QCD has - like all field theories - only an asymptotic perturbation series, which means that the series itself will ...
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### Mathematically, what is color charge?

A similar question was asked here, but the answer didn't address the following, at least not in a way that I could understand. Electric charge is simple - it's just a real scalar quantity. Ignoring ...
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### What is the significance of the QCD scale parameter $\Lambda$?

I see that it appears as a constant in the relation for the running of the strong coupling constant. What is its significance? Does it have to be established by experiment? Is it somehow a scale for ...
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### Why Cronin Effect Happens?

I'm looking for explanation on Cronin effect but unfortunately there's no Wikipedia entry or self-contained paper to start from. The statement of this effect is that: "At leading order, multiple ...
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### Bound states in QCD: Why only bound states of 2 or 3 quarks and not more?

Why when people/textbooks talk about strong interaction, they talk only about bound states of 2 or 3 quarks to form baryons and mesons? Does the strong interaction allow bound states of more than 3 ...
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### Why doesn't the color force between two quarks have an inverse square law?

From what I understand the color force between two quarks doesn't decrease with distance. Why is it that the color force doesn't decrease with the square of the distance if there are three dimensions ...
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### What is a mass gap?

I read that Quantum Chromodynamics is a theory with a mass gap. What is a mass gap in laymen terms? Why some theories have it? Which theories does not have it? Note: I searched for mass gap before ...
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### Is there a strong force analog to magnetic fields?

In special relativity, magnetism can be re-interpreted as an aspect of how electric charges interact when viewed from different inertial frames. Color charge is more complex than electric charge, but ...
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### Trace and adjoint representation of $SU(N)$

In the adjoint representation of $SU(N)$, the generators $t^a_G$ are chosen as $$(t^a_G)_{bc}=-if^{abc}$$ The following identity can be found in Taizo Muta's book "Foundations of Quantum ...
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### 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 ...
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### What's the deepest reason why QCD bound states have integer charge?

What's the deepest reason why QCD bound states have integer electric charge, i.e. equal to an integer times the electron charge? Given that the quarks have the fractional electric charges they do, ...