These mesons are most often observed in proton-antiproton annihilation, radiative decays of vector mesons, and meson-meson scattering. The first known scalar mesons have been observed since the late 1950s, with observations of numerous light states and heavier states proliferating since the 1980s. The light (unflavored) scalar mesons may be divided into three groups; those having a mass below 1 GeV/c2, those having a mass between 1 GeV/c2 and 2 GeV/c2, and other radially-excited unflavored scalar mesons above 2 GeV/c2. The heavier scalar mesons containing charm and/or bottom quarks all occur well over 2 GeV/c2. Many attempts have been made to determine the quark content of the lighter scalar mesons; however, no consensus has yet been reached.
The scalar mesons in the mass range of 1 GeV/c2 to 2 GeV/c2 are generally believed to be conventional quark-antiquark states with orbital excitation L = 1 and spin excitation S = 1, although they occur at a higher mass than one would expect in the framework of mass-splittings from spin-orbit coupling. The scalar glueball  is also expected to fall in this mass region, appearing in similar fashion to the conventional mesons but having very distinctive decay characteristics. The scalar mesons in the mass range below 1 GeV/c2 are much more controversial, and may be interpreted in a number of different ways.
Since the late 1950s, the lightest scalar mesons were often interpreted within the framework of the linear sigma model, and many theorists still choose this interpretation of the scalar mesons as the chiral partners of the pseudoscalar meson multiplet. Ever since Jaffe first suggested the existence of tetraquark multiplets in 1977, the lightest scalar mesons have been interpreted by some theorists to be possible tetraquark or meson-meson molecule states. The tetraquark interpretation works well with the MIT Bag Model of QCD, where the scalar tetraquarks are actually predicted to have lower mass than the conventional scalar mesons. This picture of the scalar mesons seems to fit experimental results well in certain ways, but often receives harsh criticism for ignoring unsolved problems with chiral symmetry breaking and the possibility of a non-trivial vacuum state as suggested by Gribov.
In-depth studies of the unflavored scalar mesons began with the Crystal Ball and Crystal Barrel experiments of the mid 1990s, focusing on the mass range between 1 GeV/c2 and 2 GeV/c2. With the re-introduction of the sigma meson as an acceptable candidate for a light scalar meson in 1996 by Tornqvist and Roos, in-depth studies into the lightest scalar mesons were conducted with renewed interest. The "Particle Data Group" provides current information on the experimental status of various particles, including the scalar mesons.
- confirmed: K0*(1430)
- candidates: K0*(800) or kappa, f0(500) or sigma, f0(980), a0(980), f0(1370), f0(1500), f0(1710), a0(1450)
- unconfirmed resonances: X(1110), f0(1200-1600), f01790, X(1810)
- W. –M. Yao et al. (Particle Data Group), J. Phys. G33, 1 (2006)
- F. E. Close, "An Introduction to Quarks and Partons", Academic Press: New York (1979), pgs. 88-89
- G. Bali et al. (UKQCD), Phys. Lett. B389, 378 (1993)
- M. Ishida, "hep-ph/9712231"
- R. L. Jaffe, Phys. Rev. D15, 267 (1977)
- K. Gottfried and V. Weisskopf, "Concepts of Particle Physics", Oxford University Press: New York (1986), Vol. II pgs. 409-419
- V. N. Gribov, "hep-ph/9902279"
- N. A. Tornqvist and M. Roos, Phys. Rev. Lett. 76, 1575 (1996)
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