# Tagged Questions

Isospin is a symmetry in nuclear physics under the interchange of neutrons and protons. The strong nuclear force conserves isospin, but the symmetry is broken by the electromagnetic force, which only operates between protons.

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### Why is quark flavor just a SU(N) group?

In the standard model one has U(1) for electromagnetism, SU(2) for the weak sector and SU(3) for the color sector. One could say that in the quark part of the fermions, there are  \underbrace{6}_\...
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### What is the cause of the minor mass difference between the proton and the neutron?

As known, the proton is from two up and a single down quark, while the neutron is from a single up and two down quarks. The down quark is a little bit more massive and the up, and so the neutron is ...
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### If weak isospin is not conserved in time, what does the Noether theorem tell us?

As far as I understand weak isospin is only conserved in interactions but not as time evolves. Nevertheless, we get from Noethers theorem, because of global $SU(2)$ invariance a conserved quantity ...
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### Isospin In Kaon Decay

The decay $K \to \pi \pi$ at zero-strong interaction level is calculated by considering the matrix element of the operator $Q_2 = (\bar{s}u)_{V-A} (\bar{u}d)_{V-A}$ for two kinds of processes: ...
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### Why does the alpha particle not excite the isovector transitions in nuclei?

The $\alpha$ particle is a very commonly used projectile for studying inelastic scattering. It has T=0 ans S=0. But it only excites the isoscalar vibration modes in target nuclei, and not isovector ...
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### Error in setting $m_{proton} = m_{neutron}$

Is the following reasoning correct, I'm doing mostly relativistic calculations so basically all masses come in squares. Suppose I have some expression that contains both the proton and the neutron ...
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### Up/Down Quark Color and Isospin

I'm reading some notes that say the reason that the strong interaction conserve isospin is because the up and down quarks have the same color... but I'm not very convinced. Is this a universal truth? ...
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### Does change in temperature affect the proton spin of molecular hydrogen?

How come the amount of parahydrogen present increases as the temperature lowers? Does change in temperature play a role in changing its proton spin? Also it lists that "At 0°K, hydrogen contains ...
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### Global and local symmetry for Isospin/Strangeness etc

Why some symmetries $\big[SU(3),SU(2)$ and $U(1)\big]$ of the Standard Model are local, and some others remain global, like Isospin and Strangeness. Is there a fundumental reason for that? Doesn't it ...
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### Isospin states of deuteron

I have a stupid question to ask being the newbie amateur that I am to physics. Deuteron in the ground state normally exists in the isospin singlet state at least according the wiki site: https://en....