Vector meson dominance and the pi^0 transition form factor

It is shown that the pi^0 transition form factor F(Q_1^2,Q_2^2) differs substantially from its one-real-photon limit F(Q_1^2,0) even for rather small values of Q_2^2 (approx 0.1 GeV^2), which cannot be excluded in experiments with one "untagged" electron. It indicates that the comparison of data with theoretical calculations, which usually assume Q_2^2=0, may be untrustworthy. Our phenomenological model of the pi^0 transition form factor is based on the vector-meson-dominance hypothesis and all its parameters are fixed by using the experimental data on the decays of vector mesons. The model soundness is checked in the two-real-photon limit, where it provides a good parameter-free description of the pi^0 -> 2 gamma decay rate, and in the pi^0 Dalitz decay. The dependence of F(Q_1^2,Q_2^2) on Q_1^2 at several fixed values of Q_2^2 is presented and the comparison with existing data performed.Comment: A slightly abbreviated version has been published as a Brief Report in Physical Review

What to learn from dilepton transverse momentum spectra in heavy-ion collisions?

Recently the NA60 collaboration has presented high precision measurements of dimuon spectra double differential in invariant mass $M$ and transverse pair momentum $p_T$ in In-In collisions at $158 {\rm AGeV}$. While the $M$-dependence is important for an understanding of in-medium changes of light vector mesons and is $p_T$ integrated insensitive to collective expansion, the $p_T$-dependence arises from an interplay between emission temperature and collective transverse flow. This fact can be exploited to derive constraints on the evolution model and in particular on the contributions of different phases of the evolution to dimuon radiation into a given $M$ window. We present arguments that a thermalized evolution phase with $T > 170 {\rm MeV}$ leaves its imprint on the spectra.Comment: Contributed to 19th International Conference on Ultrarelativistic Nucleus-Nucleus Collisions: Quark Matter 2006 (QM 2006), Shanghai, China, 14- 20 Nov 200

Are the production and decay of a resonance always independent?

The widely accepted assumption that the decay of a resonance proceeds independently of its production, quantitatively expressed as a factorizing formula for the differential cross section in the invariant mass of unpolarized resonance debris, is put under scrutiny. It is shown that the factorization is always valid for scalar and pseudoscalar resonances, although the usual version of the formula is not entirely correct. For resonances with nonzero spins the factorization does not generally take place. We deal in more detail with the spin-one case, where we show a condition on the decay matrix element that ensures the validity of the same factorizing formula as in the spinless case. This condition is satisfied for rho --> pi pi but not, e.g., for K* --> pi K or a1 --> pi rho. The formalism is applied also to the case when the resonance is produced not in two-body collisions but in the decay of a heavier particle or resonance (chain decay). Application of our formulas to the e+e- production in one-photon approximation agrees with what is known from quantum electrodynamics and thus provides another test of their soundness