1 Studying the ω properties in pA collisions via the ω→π 0 γ decay ∗

1 Studying the ω properties in pA collisions via the ω→π 0 γ decay * Within transport calculations we study the production and decay of ω-mesons in pA reactions at COSY energies including elastic and inelastic ωN rescattering, the ω→π 0 γ Dalitz decay as well as π 0 N rescattering. The resulting invariant π 0 γ mass distributions indicate that in-medium modifications of the ω-meson may be observed experimentally. The modification of the ω-meson properties [1– 7] in nuclear matter have become a challenging subject in dilepton physics from π − A, pA and AA collisions. Here the dilepton (e + e −) radiation from ω's propagating in finite density nuclear matter is directly proportional to the ω spectral function which becomes distorted in the medium due to the interactions with nucleons [8– 13]. Apart from the vacuum width (3 pion decay) these modifications are described by the real and imaginary part of the retarded ω self energy Σ ω (P, X), where the real part ℜΣ ω yields a shift of the ω-meson mass pole and the additional imaginary part ℑΣ ω (half) the collisional broadening of the vector meson in the medium. We recall that the ω-meson self energy in the t − ρ approximation is proportional to the complex forward ωN scattering amplitude f ω (P, 0) and the nuclear density ρ(X), i.e. Σ ω (P, X) = −4πρ(X)f ωN (P, 0). The scattering amplitude itself , furthermore, obeys dispersion relations between the real and imaginary parts [14–16] while the imaginary part can be determined from the total ωN cross section according to the optical theorem. Thus the ω meson spectral function can be constructed once the ωN elastic and inelastic cross sections are known. Note that in (1) all quantities * Supported by Forschungszentrum Jülich depend on space-time X and four-momentum P. As mentioned before, the in-medium ω-spectral function can be measured directly by the leptonic decay ω→l + l − or the Dalitz decay ω→π 0 γ experimentally. The advantage of the ω→π 0 γ mode is due to an isolation of the ω-signal, while the dilep-tonic mode always has to fight with a background from ρ 0 decays that are as well produced in pA collisions with compatible abundance. The ω-production in pA collisions can be considered as a natural way [17] to study the ω-properties at normal nuclear density under rather well …