# Is it possible for different particle-antiparticl pair to annihilate with each other?

## Main Question or Discussion Point

I've been searching about particle-antiparticle annihilation and I've been wondering whether it was possible for particle to annihilate with antiparticle that is not its pair? Can annihilation occur with collision of different particle-antiparticle quarks(e.g. up antiquark and strange quark), with different particle-antiparticle leptons(e.g. electron antineutrino and muon) or with particle-antiparticle quark and lepton(e.g. bottom quark and antitau)?

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mfb
Mentor

Usually not, as this would violate some conserved quantum numbers (charge, lepton numbers and so on). In addition, the "classic" annihilation via the electromagnetic force requires particle + corresponding antiparticle.

However, the weak interaction can allow some processes which could be considered as annihilation of different particles: Neutral kaons (strange + anti-down quark or anti-strange + down-quark) can decay to two photons, for example.

In some cases yes, It depends on wheter the annihilation violates conservation laws.
For example, in the quark sector flavor conservation is violated by intercation with the W boson.

Therefore, for example

$u\bar{s}->W^{+}->e^{+}\nu_{e}$ is possible

In the lepton sector, in the limit of massless neutrinos, flavor is conserved and a muon can't annihilate with a electron neutrino( or anti electron neutrino).

quarks and leptons can't annihilate with eachother due to baryon and lepton number conservation. ( which is simply an outcome of other conservation laws (charge, color etc..) doesn't have to be assumed)

Usually not, as this would violate some conserved quantum numbers (charge, lepton numbers and so on). In addition, the "classic" annihilation via the electromagnetic force requires particle + corresponding antiparticle.

However, the weak interaction can allow some processes which could be considered as annihilation of different particles: Neutral kaons (strange + anti-down quark or anti-strange + down-quark) can decay to two photons, for example.
So, do you mean, without weak interaction, such annihilation is impossible?

mfb
Mentor

Some reactions "particle + different antiparticle -> 2 photons" are possible, but they have to include the weak interaction (together with the electromagnetic interaction).

One up-type quark (up, charm, top) plus a different up-type antiquark can annihilate to 2 photons.
One down-type quark (down, strange, bottom) plus a different down-type antiquark can annihilate to 2 photons.

Hmm.. I think if we neglect neutrino mixing, that was all.