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I think it's fairly common knowledge that matter and antimatter annihilate each other on contact. But what happens if two dissimilar atoms collide?

For example, what happens if a Hydrogen atom (1 proton, 1 electron) contacts an antihelium atom (2 antiprotons, 1 antineutrons, 2 positrons)? Are we left with a new antideuterium atom? Do all the remaining particles fly apart? Does something even more exotic happen?

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Annihilation typically happens on the quark level. Nucleons and antineucleons are made up of quarks and antiquarks, and rearrangement can give leptons or mesons as output states, usually escaping with high energy. This can be accompanied by photons. The exact mix you get in different experiments will depend on the the statistical branching ratio for the reaction.

In your case, you will have a preponderance of antiquarks, so you will end up with some antibaryons in the output -- antiprotons, antineutrons or antideuterons. Again, the exact mix you get from experiment to experiment will depend on the the statistical branching ratio for the reaction.

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  • $\begingroup$ Of course two antibarions will be left. I suspect that the deuterium will rarely stay connected. $\endgroup$ – BartekChom Nov 28 '15 at 0:35
  • $\begingroup$ So, in other words, antideuterium is just one possible outcome, and it's possible (with some statistical probability) that there will be more released energy and less remaining antiparticles? And even if the outcome is right, those remaining antibaryonic particles may not form an anti-atom? $\endgroup$ – Bobson Nov 29 '15 at 14:42
  • $\begingroup$ @BartekChom Maybe... it's not obvious. Remember that the deuteron will have a binding energy, making it energetically favourable. But that's not the only consideration $\endgroup$ – Gremlin Nov 29 '15 at 20:36
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    $\begingroup$ @Bobson I suspect that having a positron captured will certainly be unlikely, because the energy scales involved in annihilation are so large compared to atomic binding energies. $\endgroup$ – Gremlin Nov 29 '15 at 20:37
  • $\begingroup$ @Eoin: Two masses of nucleon (about 1850 MeV) are also over 800 times larger then deuteron binding energy (2.225 MeV) so for the same reason I do not expect it to often stay bound. $\endgroup$ – BartekChom Nov 30 '15 at 6:15
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For this reaction, you might get different results for different experiments. You might have a nice even annihilation reaction and produce an antideuterium atom and lots of energy in the form of gamma rays.

However, it is also possible that one of the gamma rays is absorbed by a proton which gives it enough energy to spontaneously split into a neutron, positron and neutrino, then the neutron (still excited) spits out another gamma ray which hits something else etc.

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