Anti-matter and what it’s collisions create, are there only photons?

Question

Which fundamental particles are involved in an anti-matter collision and what is the result of this, only photons?

Answer 1

Any fundamental particle can in principle collide with its antiparticle and annihilate.

In practice this has been done on a reasonable scale only with positrons/electrons (as in PET scanning, as well as particle colliders) and protons/antiprotons(at the Fermilab Tevatron and the CERN Antiproton Decelerator, for example)

Such an annihilation can produce any particle-antiparticle $P \overline P$ pair, provided (1) there is enough energy, at least $2 M_P$ and (2) there is some sort of coupling, which rules out some proposed dark matter candidates.

The reason photons feature in descriptions ('matter and antimatter annihilate to gamma rays...') is that most of our experience is with low energy positrons and electrons, and the photon is the only particle lighter than the electron (apart from the neutrino, but see (2) above). Low energy positrons (from nuclear sources) annihilate with stationary electrons to produce photons but higher energy collisions, as happened at PETRA and LEP, can produce pairs of muons or pairs of quarks. Low energy proton-antiproton annihilation produces mostly pions.

Answer 2

The products of a matter-antimatter annihilation are constrained by all the various conservation laws: conservation of energy, momentum, charge, spin, etc. In principle, anything can come out of the annihilation that meets those constraints. In the case of electron-positron annihilation, without any energy added, there is not enough energy available to create any type of particle that has mass (other than, perhaps, neutrinos), so photons are the most likely product.