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In the Wikipedia article of Muon, it says

...with unitary negative electric charge of roughly -1 and a spin of 1/2,

What are they trying to convey with the "roughly"? Aren't the allowed values of charge discrete?

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  • $\begingroup$ Thanks for the editing. I wrote the question on a phone so that was the best I could do. $\endgroup$ – Your Majesty Nov 28 '13 at 17:18
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    $\begingroup$ It is interesting to note that the Particle Data Group Summary Tables do not list the experimental uncertainty on the muon charge. $\endgroup$ – Emilio Pisanty Nov 28 '13 at 18:32
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I believe that the "roughly" term is applied because of the associated experimental error when measuring its charge.

The same cannot be said to the electron because "we" decided to make the electron the reference charge.

So, the reference charge is definitely -1. However the muon charge must be measured.

According to this paper,

Muon Mass and Charge by Critical Absorption of Mesonic X Rays. S. Devons, G. Gidal, L. M. Lederman and G. Shapiro. Phys. Rev. Lett. 5 no. 7, 330–332 (1960).

the measured relative charge of a muon is $e_\mu / e_e = 1 \pm 10^{-5}$

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    $\begingroup$ I would have thought you could do at least as well with mu-onium spectroscopy. But maybe the lifetime-driven line-widths get in the way. I'm off to search the literature. $\endgroup$ – dmckee Nov 28 '13 at 16:57

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