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This question is not about fission or fusion reactions, or changes in mass/energy as the speed of a particle changes. The number of protons (including protons within neutrons) and electrons pre- and post-fission and fusion are the same, and the number of protons and electrons are independent of velocity. Some think Einstein's equation means that mass actually can be converted to energy. But his equation is about equivalence, not conversion. So the question is: is there any reaction where electrons or protons (or the elementary particles (quarks) that constitute a proton) literally get converted to energy?

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yes. electrons are converted into pure energy when one collides with its antiparticle.

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  • $\begingroup$ Actually as proton is not an elementary particle, the annihilation is not 'perfect'. Other particles are created. $\endgroup$ – fanyul Aug 27 '20 at 21:26
  • $\begingroup$ When an electron collides with a positron (anti-electron), aren't two photons created? Do you call photons "pure energy"? If so, why? $\endgroup$ – md2perpe Aug 27 '20 at 21:50
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    $\begingroup$ photons qualify as pure energy. $\endgroup$ – niels nielsen Aug 27 '20 at 22:03
  • $\begingroup$ @fanyul, thank you , will edit. $\endgroup$ – niels nielsen Aug 27 '20 at 22:04
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    $\begingroup$ @md2perpe Photons might be considered as pure energy, because they are massless, and they can transfer all their energy to other particles. (For example to an electron through the photoelectric effect.) $\endgroup$ – fanyul Aug 27 '20 at 22:16
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It seems to me, after your comments to the correct answer of Niels to your question, the question is really:

" can the rest mass of an elementary particle be totally converted to energy .

The rest mass of protons and neutrons can be partially converted to energy, by the definition of "mass", the quarks feeding a quark gluon plasma. See what a proton is made of in terms of elementary particle four vectors added up to give its mass.

In the standard model of physics masses to elementary particles are provided by the symmetry breaking Higgs mechanism . When the energies are very high every elementary particle has zero mass , and in the cosmological models acquires a mass with symmetry breaking.

Of course this energy is not something that can be achieved in the present day laboratory, so the answer to the question is that elementary particles keep their rest mass.

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