摘要
I. Introduction： In contrast to the vector and tensor mesons, the identification of the scalar mesons is a long-standing puzzle. Scalar resonances are difficult to resolve because some of them have large decay widths which cause a strong overlap between resonances and background. In addition, several decay channels sometimes open up within a short mass interval （e.g. at the KK and ηη thresholds）, producing cusps in the line shapes of the near-by resonances. Furthermore, one expects non-qq scalar objects, such as glueballs and multiquark states in the mass range below 2 GeV （for reviews see, e.g., Refs. [1-5] and the mini-review on non-qq states in this Review of Particle Physics （RPP））.
I. Introduction： In contrast to the vector and tensor mesons, the identification of the scalar mesons is a long-standing puzzle. Scalar resonances are difficult to resolve because some of them have large decay widths which cause a strong overlap between resonances and background. In addition, several decay channels sometimes open up within a short mass interval （e.g. at the KK and ηη thresholds）, producing cusps in the line shapes of the near-by resonances. Furthermore, one expects non-qq scalar objects, such as glueballs and multiquark states in the mass range below 2 GeV （for reviews see, e.g., Refs. [1-5] and the mini-review on non-qq states in this Review of Particle Physics （RPP））.
