In-medium omega-meson broadening and S-wave pion annihilation into e+e- pairs

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An ω-meson in motion with respect to a nuclear medium can couple to a σ-meson through a particle-hole excitation. This coupling is large. We investigate its consequences for the width of ω-mesons in matter and for the s-wave annihilation of pions into lepton pairs which can take place in relativistic heavy ion collisions. We find that the two pion decay of ω-mesons, resulting from the ω-σ transition and the subsequent 2π decay of the σ-meson, leads to a substantial broadening of ω-mesons in matter and possibly to an observable effect in experiments measuring the e+e- decay of vector mesons produced in nuclei and in relativistic heavy-ion collisions. The inverse process, the s-wave annihilation of pions into ω-mesons decaying into e+e- pairs, has in general a much smaller cross section than the corresponding p-wave annihilation through ρ-mesons and is expected to contribute rather little to the total e+e- pair production in relativistic heavy ion collisions.