The nature of the light scalar mesons from their radiative decays

The nature of the light scalar mesons is one of the most intriguing open challenges in hadronic spectroscopy. It is argued that radiative decays involving these scalars can serve as an important decisive tool in establishing their nature. In particular, special emphasis is made on the radiative decays of the scalars themselves (in addition to the radiative decays of the phi-meson with the scalars appearing in the final state), including their two-photon decays. All the above mentioned processes are considered in detail in the (point-like) kaon molecule model of the scalars and explicit predictions for the decay widths are made. In addition, finite-range corrections to the point-like results are investigated, with a special attention payed to gauge invariance of the decay amplitude. Finally, the conclusion is made that experimental data on the radiative decays with the light scalar mesons involved strongly support the molecule assignment for the latter.Comment: Invited talk at the International Symposium on Meson-Nucleon Physics and the Structure of Hadrons (MENU2007), September 10-14, 2007, Forschungszentrum Juelich, Germany; LaTeX, 4 pages, uses conference.cls, to appear in Proceeding

Light-light and heavy-light mesons in the model of QCD string with quarks at the ends

The variational einbein field method is applied to the model of the QCD string with quarks at the ends for the case of light-light and heavy-light mesons. Special attention is payed to the proper string dynamics. The correct string slope of the Regge trajectories is reproduced for light-light states which comes out from the picture of rotating string. Masses of several low-lying orbitally and radially excited states in the D, D_s, B, and B_s meson spectra are calculated and a good agreement with the experimental data as well as with recent lattice calculations is found. The role of the string correction to the interquark interaction is discussed at the example of the identification of D*'(2637) state recently claimed by DELPHI Collaboration. For the heavy-light mesons the standard constants used in Heavy Quark Effective Theory are extracted and compared to the results of other approaches.Comment: 4 pages, 3 EPS figures, uses epsfig.sty, specialized style files included. Talk given at the 9th International Conference On Hadron Spectroscopy (Hadron 2001), Protvino, Russia, 25 August - 1 September 2001, to appear in Proceeding

Can X(3915) be the tensor partner of the X(3872)?

It has been proposed recently (Phys. Rev. Lett. 115 (2015), 022001) that the charmoniumlike state named X(3915) and suggested to be a $0^{++}$ scalar, is just the helicity-0 realisation of the $2^{++}$ tensor state $\chi_{c2}(3930)$. This scenario would call for a helicity-0 dominance, which were at odds with the properties of a conventional tensor charmonium, but might be compatible with some exotic structure of the $\chi_{c2}(3930)$. In this paper, we investigate, if such a scenario is compatible with the assumption that the $\chi_{c2}(3930)$ is a $D^*\bar D^*$ molecular state - a spin partner of the $X(3872)$ treated as a shallow bound state. We demonstrate that for a tensor molecule the helicity-0 component vanishes for vanishing binding energy and accordingly for a shallow bound state a helicity-2 dominance would be natural. However, for the $\chi_{c2}(3930)$, residing about 100 MeV below the $D^*\bar D^*$ threshold, there is no a priori reason for a helicity-2 dominance and thus the proposal formulated in the above mentioned reference might indeed point at a molecular structure of the tensor state. Nevertheless, we find that the experimental data currently available favour a dominant contribution of the helicity-2 amplitude also in this scenario, if spin symmetry arguments are employed to relate properties of the molecular state to those of the X(3872). We also discuss what research is necessary to further constrain the analysis.Comment: LaTeX2e, 23 pages, 2 figures, version to appear in JHE