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Apr
18
comment Why do we exclude the quark condensate in the OPE?
the chiral condensate violates chiral symmetry, and there are some correlators where only $m\langle\bar{q}q\rangle$ appears. The latter is also RG invariant.
Mar
31
comment Phase transition in mean field theory Ising model
Tc is not universal.
Mar
29
comment Itô or Stratonovich calculus: which one is more relevant from the point of view of physics?
By the way: In physical systems the noise and the drag are not independent. This is the content of fluctuation-dissipation (FD) relations. Physicists like Stra bettter, because FD relations take on a more transparent form, but its not obvious to me what is wring with Ito.
Mar
29
comment Itô or Stratonovich calculus: which one is more relevant from the point of view of physics?
I am not sure that I completely understand. In the limit that I turn of the multiplicative noise, $b\to 0$ in my notation, Ito and Stratonovich processes are the same. For additive noise, they are the same anyway.
Mar
21
comment Does an instanton couple equally to all flavors?
It couples equally to all massless flavors.
Mar
18
comment Why are sound waves adiabatic?
Actually, I just noticed that there is something wrong with your equation (it violates the second law). Should be $(\nabla T)^2$. What I meant is: You write ".. make the rhs negligible to the left ..", but this does not make sense, there is an = sign. What you mean is that the lhs (=rhs) is small compared to $\rho T s/T_P$, where $T_P$ is the period.
Mar
18
comment Why are sound waves adiabatic?
Veru good: Two minor comments. 1) Last sentence. Obviously the lhs is equal to the rhs, What you mean is that the time averaged value of $ds/dt$ is small compared to $\omega s$. 2) For standard sound waves large $\kappa$ does not make them isothermal, it makes them strongly damped. There are some cases where you can achieve large heat transport without dissipation, for example radiative transport in space plasmas.
Mar
10
comment Why don't neutrons cluster on their own?
Yes (but to first approximation they are). Also note that $10^{57}$ neutrons would form a bound state. This state is not the ground state, and about $10^{55}$ neutrons would decay into $pe\nu$.
Feb
28
comment Yang-Mills theories, confinement and chiral symmetry breaking
@Name YYY: I'm not an expert, but it would seem that the anomalous effective Lagrangian can always be constructed by heat kernel methods. Note that i) I must still require that the gauge anomalies cancel, ii) I need to know not only G but also G/H (where G is the global symmetry).
Feb
26
comment group velocity of acoustic waves
@Kirill I added a postscript.
Feb
26
comment group velocity of acoustic waves
1) The hydrodynamic limit is indeed subtle. 2) The hydro equations involve the conserved charges, so the corresponding moments of the collision term vanish. 3) To derive the hydro equations from the moment equations, we also need constituitive equations (we need to express the currents in terms of thermodynamic variables). The constituitive equations do depend on the collision term.
Feb
25
comment Yang-Mills theories, confinement and chiral symmetry breaking
@NameYYY : I added a postscript.
Feb
25
comment Nucleon-meson interaction
@NameYYY 1) You do what you can (this was at a time when Witten was still doing some phenomenological work). 2) Well, 30% error is not that great (no of way knowing that it's not a 100%). Also, using a specific lagrangian to find the Skyrmion is an additional approximation beyond large N.
Feb
23
comment Dependance of diffusion coefficient on size?
@becko I added a few more comments
Feb
23
comment Yang-Mills theories, confinement and chiral symmetry breaking
@NameYYY I could not explain it any better than 't Hooft himself inspirehep.net/record/144074
Feb
23
comment Nucleon-meson interaction
@NameYYY I added some comments about Skyrmions.
Feb
19
comment Total energy of the fluid
@nluigi I expanded my comment into an answer
Feb
18
comment What maximum energy can be stored in a gluon field flux tube
In a heavy quark meson, this is a well defined question. The string stores (string tension)x(mean $\bar{Q}Q$ separation). This is something like 200 MeV in a J/psi. In light mesons, the question does not make sense, because light quark strings break very easily.
Feb
18
comment Total energy of the fluid
You are missing the internal energy of the fluid (which depends on the equation of state).
Feb
18
comment What is mass constituted by quarks in a proton?
It is the difference in the mass of the proton in our world, where the Higgs field has an expectation value (vev) and quarks are massive, and an imaginary world in which the Higgs vev is zero and quarks are massless.