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As far as I understand, electrons are infinitely stable since they are the least massive particle with non-zero electric charge. However, when accelerated to high-energies, the energy (or mass) of the electrons increases. Does this mean that at sufficient energies it is possible for these electrons to decay (e.g. into a muon, muon antineutrino, and electron neutrino) without scattering off any other particles?

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    $\begingroup$ Short answer: no. There's nothing for an electron to decay into without violating the conservation of charge, invariant mass or lepton number. $\endgroup$ – dukwon Dec 7 '15 at 21:35
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    $\begingroup$ The energy of the electrons increases. The mass of the electrons does not. $\endgroup$ – AccidentalFourierTransform Dec 7 '15 at 21:40
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    $\begingroup$ no matter how fast an electron is moving: in its reference frame it is still, and its energy is $mc^2\sim .511\ \mathrm{MeV}$. In that frame it cannot decay, so neither can it in any other frame. $\endgroup$ – AccidentalFourierTransform Dec 7 '15 at 21:49
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    $\begingroup$ The notion of "relativistic mass" has become controversial. Many authors (myself among them) discourage it; in part because it generates exactly the kind of question you just asked. The answer to your questions is "Obviously no" (for the reason that AccidentalFourierTransform gives), but the language of relativistic mass makes it seem to the beginner like a reasonable question. $\endgroup$ – dmckee Dec 7 '15 at 22:02
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    $\begingroup$ @dmckee: "has become*? Einstein warned about avoiding the concept of increasing mass back in 1948!. $\endgroup$ – Gert Dec 7 '15 at 22:20
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Particle interactions, including decays, must always satisfy energy and momentum conservation in all reference frames. This means that high-energy electrons in an accelerator don't count; to the other electrons traveling down the accelerator with them, everyone is relatively at rest.

An interesting counterexample involves a hard-to-escape reference frame.

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