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A somewhat meaningful and useful concept for massive particles is the Compton wavelength: https://en.wikipedia.org/wiki/Compton_wavelength#Limitation_on_measurement The Compton wavelength of a particle is determined by its rest mass. The position of a particle cannot be measured with a precision less than half its reduced Compton wavelength. In this sense, ...


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A point particle is an idealization of a particle. It simplifies calculations by using a 0 dimensional object instead of a normal particle in calculations where size, shape, and structure are irrelevant. For example, in the theory of, say, electromagnetism, scientists will talk about a point charge - a particle represented by a point that has a non-zero ...


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If a particle changes flavor, it's a charged-current weak decay. Example: $n\to pe\bar\nu$. If there's a neutrino in the final state, it's a weak interaction. Decay example: $\pi^+\to\mu^+\nu$. See also neutrino scattering. If parity isn't conserved, it's a weak interaction. Examples: $K^0 \to 2\pi$ and $K^0 \to 3\pi$. Note that kaon decays and $K\...


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The particle is not larger nor more massive. We normally describe particles using quantum field theory, and in QFT particles do not have a size. This is discussed in the answers to Why do physicists believe that particles are pointlike? and it would be worth reading through them. To say that a particle is a point is a bit of an over simplification. We ...


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Because interactions conserve the third component of weak isospin, $T_3$. So the incoming $T_3$ in a vertex must be equal to the outgoing $T_3$. For example:


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Lets look at the weak isospin doublet of for the first generation of quraks \begin{equation} \begin{pmatrix} u\\d' \end{pmatrix}_{L} \end{equation} Where $d'$ is represents the quantum mixing of $d$ and $s$ quark. From gauge theory, we know that, the role of gauge bosons is to transform a particle to another one, which living in the same mutiplet. So, if a ...


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String theory postulates that of the elementary particles we currently know about, each relates directly to low-energy string vibrations, the presence of multiple holes causes the string patterns to fall into families. Each hole in the Calabi-Yau space is a group of low-energy string vibrational patterns. If the C-Y has three holes, then three families of ...


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The heterotic group decomposes as $E_8~\rightarrow~SU(3)\times E_6$, The $\bf 248$ of the $E_8$ decomposes as $$ {\bf 248}~\rightarrow~(\bf 8,~\bf 1) + (\bf 1,~\bf 78) + (\bf 3,~\bf 27) + (\bf\bar 3,~\bf\bar{27}) $$ We have here the $(\bf 8,~\bf 1)$ of $SU(3)$ which is identical in form to the irreducible representation used for gluons, or the old nonet ...



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