Beta decay and the down quark.

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Summary:

If a down quark decays into an up quark in beta decay, why is it considered a fundamental particle?
I was learning about beta decay, and how a down quark decays into an up quark by emitting a W- boson which then becomes an electron and an electron antineutrino. I have two main questions - Firstly, how can the down quark be considered a fundamental particle, when it can break down to produce something else? Secondly, does it ever occur that the single down quark in the proton decays in the same way, and if it happens, what forms?
 

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A. Neumaier
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Essentially, the different quarks are different forms of the same particle, distinguished by their quantum numbers.

Just like electrons in different excitation levels. An excited electron decays into an electron in the ground state and a photon, although it is not considered as being composed of these.
 
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PeterDonis
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a down quark decays into an up quark by emitting a W- boson which then becomes an electron and an electron antineutrino
Note that a down quark only does this if it's part of a neutron or a nucleus that undergoes beta decay. If all down quarks did this regardless of their state, there would be no down quarks left in the universe, which would mean matter as we know it wouldn't exist either.

how can the down quark be considered a fundamental particle, when it can break down to produce something else?
Because that's not the standard definition of "a fundamental particle". The standard definition of a fundamental particle at present is a particle that has a fundamental field in the Standard Model of particle physics. That includes down quarks.

does it ever occur that the single down quark in the proton decays in the same way
No such decay has ever been observed. According to the Standard Model, such a decay cannot occur because there is no resulting bound hadron state that it could lead to. (Such a state would need to be a particle with charge +2 and a mass lighter than the proton, and no such particle exists.)

Various "beyond the standard model" theories do predict proton decay, but not by this mechanism.
 
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