Scalar meson
Encyclopedia
In high energy physics, a scalar meson is a meson with total spin 0 and even parity
(usually noted as JP=0+). Compare to pseudoscalar meson
.
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.
Parity (physics)
In physics, a parity transformation is the flip in the sign of one spatial coordinate. In three dimensions, it is also commonly described by the simultaneous flip in the sign of all three spatial coordinates:...
(usually noted as JP=0+). Compare to pseudoscalar meson
Pseudoscalar meson
In high energy physics, a pseudoscalar meson is a meson with total spin 0 and odd parity . Compare to scalar meson.Pseudoscalar mesons are commonly seen in proton-proton scattering and proton-antiproton annihilation...
.
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
Glueball
In particle physics, a glueball is a hypothetical composite particle. It consists solely of gluon particles, without valence quarks. Such a state is possible because gluons carry color charge and experience the strong interaction...
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
Pseudoscalar meson
In high energy physics, a pseudoscalar meson is a meson with total spin 0 and odd parity . Compare to scalar meson.Pseudoscalar mesons are commonly seen in proton-proton scattering and proton-antiproton annihilation...
multiplet. Ever since Jaffe first suggested the existence of tetraquark
Tetraquark
In particle physics a tetraquark is a hypothetical meson composed of four valence quarks. In principle, a tetraquark state may be allowed in quantum chromodynamics, the modern theory of strong interactions. However, there has been no confirmed report of a tetraquark state to date...
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
Chiral symmetry breaking
In particle physics, chiral symmetry breaking is an example of spontaneous symmetry breaking affecting the chiral symmetry of gauge theory such as Quantum Chromodynamics. The origin may be described as a fermion condensate...
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.
Examples
- confirmed: K0*(1430)
- candidates: K0*(800) or kappa, f0(600) or sigma, f0(980), a0(980), f0(1370), f0(1500), f0(1710), a0(1450)
- unconfirmed resonances: X(1110), f0(1200-1600), f01790, X(1810)