Extracting vector mesons magnetic dipole moment from radiative decays

The possibility that the magnetic dipole moment (MDM) of light charged vector mesons could be measured from radiative processes involving the production (τ → ρνγ) and decay (ρ → ππγ) of vector mesons is studied in a model independent way, via the soft-photon approximation. The angular and energy distribution of photons emitted at small angles respect to the final charged particle is found be sensitive to the effects of the MDM. We also show that model dependent contributions have a general structure, by gauge invariance requirements, that allows suppress them in the same kinematical region where the MDM effect is more important in the model independent approach. The electromagnetic structure of vector mesons (VM) offers an interesting window to strong interaction effects. The electric charge conservation accounts to determine its value for vector mesons. The next non-zero multipole is the magnetic dipole moment (MDM) which is not restricted to a particular value and is matter of study within phenomenological models of QCD [1]. However, the experimental determination of none of the vector mesons MDM has been made yet. The reason can be found in the extremely short lifetime of such particles (≈ 10−24 s.) and the requirements, for example, of knowing the initial and final polarizations for the method of spin precession in a magnetic field; rendering to be an impossible task. The same problems are faced by the W gauge boson whose MDM (here after denoted by μ) is predicted be μ = 2 in Bohr’s magneton units. Indeed, for VM this value seems to be the favored one to exhibit interesting features [2] and in general is considered as the canonical value. In the late 60’s Zakharov et al. [3] showed that radiative decays are the only viable option to study resonances, by observing the radiation distribution. In this work we propose the use of radiative production and/or decay of vector mesons to measure their MDM. First we show, in a model independent way, that the photon angular and energy distribution is sensitive to the MDM, in both radiative decay and production of VM, this is done by choosing a kinematical region to suppress the electric contribution leaving the MDM as the leading one. Then, the electric quadrupole moment (EQM) and model dependent contributions are shown to be suppressed in this same region. We base our analysis in the fact that the radiative decay amplitude expanded in powers of the photon energy (ω) M = Aω + Bω 0 + Cω + · · · has the two first terms completely determined by the non-radiative process and the electromagnetic properties To appear in the proceedings of the 5th Int. Conf. “Quark confinement and the hadron spectrum”, Gargnano, Garda Lake, Italy. 10-14th September 2002