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Most quantum field theories cannot be solved exactly, so physicists have developed approximation schemes. The perturbative approach to quantum field theory relies on expansions in the interaction coupling constant. For example, in QED one expands quantities in powers of the fine-structure constant $\alpha=e^2/\hbar c$, which measures how strongly electrons ...


5

In physics, once one has a theory, a mathematical model, one starts using it to compute measurable quantities to test against data, or predict the result of experiments. QCD is a gauge theory, and it is part of the standard model which is the theory for particle physics, SU(3)xSU(2)xU(1). In physics, a gauge theory is a type of field theory in which the ...


4

This is a question that entails the ability to manage QCD at low energies and this is an active field of research yet as we are not able to do it, unless for lattice computations. It can be considered as part of the more general problem of the determination of the phase diagram of QCD (see my answer here). The idea of chiral symmetry breaking in strong ...


3

In my view, one of the best way to define what means "confinement" is to calculate Wilson loop. For instance, one can consider euclidean compact abeliean theory, 4D compact QED (it is defined on a lattice!) and calculate Wilson loop in two different limits (strong coupling & weak coupling). Having performed computation, you can see that in the limit of ...


3

For your first question, fixing $\kappa$ depends on what our convention is for $\theta$. Our convention is we want $\theta$ to be a parameter such that $\theta$ and $\theta+2\pi$ lead to the same physics. Given that is the case, since $\theta$ appears in the action like $\exp(i\theta \kappa\int d^4x F\bar{F})$ we clearly want to choose $\kappa$ to be such ...


2

Ordinary (electronic) superconductivity is characterized by 1) the Meissner efffect (screening of magnetic fields), 2) a gap in the excitation spectrum of single electrons, and 3) perfect conductivity (zero resistance), Color superconductivity in dense quark matter is characterized by 1) the Meisner effect (screening of color magnetic fields), 2) a gap in ...


1

The normalization is of course arbitrary, because we can always redefine what we mean by $\theta$. However, given that topological charge is quantized, we know that $\theta$ is periodic. This means it makes sense to define the topological part of the action so that $\theta$ has period $2\pi$, that is $$ S = \theta Q_{top} $$ where $Q_{top}\in Z$ is an ...


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