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The condition for double beta decay to be the preferred mode are that the energy (mass) of the three states The initial nucleus (m) The would-be single-beta-decay daughter plus the electron and the anti-neutrino (s) The double-beta-decay daughter plus the two electrons and the two anti-neutrinos (d) have a relationship like  E_d < E_m < E_s ...

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They have 2 main differences. The first of them is very straightforward: They have different spins: As you pointed out, both are bound states of 2 spin 1/2 particles, therefore you can find the possible spins of such a bound state using the usual rules of angular momentum addition in Quantum Mechanics. 1/2 + 1/2 can give you either 3 spin 1 states (the ...

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A $\rho$ meson is the spin-1 (angular momentum, not isospin) excitation of the $\pi$ meson. We have, $$\rho = \begin{cases} \rho ^+ \quad u \bar{d} \\ \rho ^0 \quad \frac{ u \bar{u} - d \bar{d} }{ \sqrt{ 2}} \\ \rho ^- \quad d \bar{ u } \end{cases}$$ where each $\rho$ meson has a different isospin. However they are all spin \$ ...

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Mass so small that it cannot be observed, measured, tested, is unknown, but believed to exist, is not mass. it is pre-mass. It is alpha. The substance and evidence is confirmed only by our strong, unwavering belief that all things visible are made of things not visible.

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Electrons are leptons and are not composite particles and are therefore treated as point particles, aside from current experiments that are looking for electron dipole moments. When quantum mechanical spin was discovered various models tried to explain it via our standard notions of orbital angular momentum by having the electron rotate but it doesn't work.

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Particles in physics in current terminology is "elemenntary particles", which are the building blocks that form atoms molecules and radiation that we observe macroscopically in bulk. These can be created and destroyed during the processes of stellar evolution, and particularly photons, which are bosons, have no limit on their number at all. There is no limit ...

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That depends on whether the Universe is finite or not, which we don't know and probably will never find out.

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