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Mar 15 at 17:56 comment added Andreas Blass To add a bit to @MichaelEngelhardt's comment, the two elements of $SU_2$ that correspond to a single element of $SO_3$ differ by just a $\pm$ sign. That's the $\pm$ that you noticed when comparing a $2\pi$-rotation with a $4\pi$ or 0 rotation.
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Mar 15 at 14:15 comment added Michael Engelhardt $SU_2$ is a double cover of $SO_3 $. The algebras are isomorphic, but there are two elements of $SU_2 $ for every one element of $SO_3 $.
Mar 15 at 14:06 comment added Andrea So, in a sense, $SO_3$ gives rise to the Lie-Algebra of $SU_2$? Thanks for the distinction with the quarks :-)
Mar 15 at 14:01 history edited Andrea CC BY-SA 4.0
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Mar 15 at 14:00 comment added Michael Engelhardt Yes, the mathematical structures here are Lie algebras and Lie groups. The exponential map is the usual one, it's just a different convention to write the "$i$" explicitly. By the way, every quark has only 3 colors (red, blue, green), and every antiquark has 3 colors (antired, antiblue, antigreen). These are distinct representations of $SU_3 $.
Mar 15 at 13:58 history edited Andrea CC BY-SA 4.0
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Mar 15 at 13:54 history edited YCor CC BY-SA 4.0
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S Mar 15 at 13:41 history asked Andrea CC BY-SA 4.0