In
high energy physics
Particle physics or high energy physics is the study of fundamental particles and forces that constitute matter and radiation. The fundamental particles in the universe are classified in the Standard Model as fermions (matter particles) and b ...
, a scalar meson is a
meson
In particle physics, a meson ( or ) is a type of hadronic subatomic particle composed of an equal number of quarks and antiquarks, usually one of each, bound together by the strong interaction. Because mesons are composed of quark subparticles ...
with
total spin 0 and even
parity
Parity may refer to:
* Parity (computing)
** Parity bit in computing, sets the parity of data for the purpose of error detection
** Parity flag in computing, indicates if the number of set bits is odd or even in the binary representation of the r ...
(usually noted as ''J
P''=0
+). Compare to ''
pseudoscalar meson
In high-energy physics, a pseudoscalar meson is a meson with total spin 0 and odd parity (usually notated as
Pseudoscalar mesons are commonly seen in proton-proton scattering and proton-antiproton annihilation, and include the pion (), k ...
''. 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.
Scalar mesons are most often observed in proton-antiproton annihilation, radiative decays of
vector mesons
In high energy physics, a vector meson is a meson with total spin 1 and odd parity (usually noted as ). Vector mesons have been seen in experiments since the 1960s, and are well known for their spectroscopic pattern of masses.
The vector meso ...
, and meson-meson scattering.
Groups
The light (unflavored) scalar mesons may be divided into three groups:
* mesons having a mass below 1 GeV/c
2
* mesons having a mass between 1 GeV/c
2 and 2 GeV/c
2
* other radially-excited unflavored scalar mesons above 2 GeV/c
2
Lower mass range
Since the late 1950s, the lightest scalar mesons were often interpreted within the framework of the
linear sigma model
In physics, a sigma model is a field theory that describes the field as a point particle confined to move on a fixed manifold. This manifold can be taken to be any Riemannian manifold, although it is most commonly taken to be either a Lie group ...
, and many theorists still choose this interpretation of the scalar mesons as the chiral partners of the pseudoscalar meson multiplet.
With the re-introduction of the σ 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.
Ever since Jaffe first suggested the existence of
tetraquark
A tetraquark, in particle physics, is an exotic meson composed of four valence quarks. A tetraquark state has long been suspected to be allowed by quantum chromodynamics, the modern theory of strong interactions. A tetraquark state is an examp ...
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
In theoretical physics, quantum chromodynamics (QCD) is the theory of the strong interaction between quarks mediated by gluons. Quarks are fundamental particles that make up composite hadrons such as the proton, neutron and pion. QCD is a type o ...
, 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
In particle physics, chiral symmetry breaking is the spontaneous symmetry breaking of a chiral symmetry – usually by a gauge theory such as quantum chromodynamics, the quantum field theory of the strong interaction. Yoichiro Nambu was award ...
and the possibility of a non-trivial vacuum state as suggested by Gribov.
Many attempts have been made to determine the
quark
A quark () is a type of elementary particle and a fundamental constituent of matter. Quarks combine to form composite particles called hadrons, the most stable of which are protons and neutrons, the components of atomic nuclei. All commonly o ...
content of the lighter scalar mesons; however, no consensus has yet been reached.
Intermediate range
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/c
2 and 2 GeV/c
2.
The scalar mesons in the mass range of 1 GeV/c
2 to 2 GeV/c
2 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
In particle physics, a glueball (also gluonium, gluon-ball) 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 ...
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/c
2 are much more controversial, and may be interpreted in a number of different ways.
Upper mass range
The heavier scalar mesons contain
charm
Charm may refer to:
Social science
* Charisma, a person or thing's pronounced ability to attract others
* Superficial charm, flattery, telling people what they want to hear
Science and technology
* Charm quark, a type of elementary particle
* Ch ...
and/or
bottom quarks
The bottom quark or b quark, also known as the beauty quark, is a third-generation heavy quark with a charge of − ''e''.
All quarks are described in a similar way by electroweak and quantum chromodynamics, but the bottom quark has exc ...
. All occur well over 2 GeV/c
2 and have well-separated masses which make them distinct and simplifies their analyses.
List
Confirmed
* K
0*(1430)
Candidates
* K
0*(800) or κ
* f
0(500) or σ
* f
0(980)
* a
0(980)
* f
0(1370)
* f
0(1500)
* f
0(1710)
* a
0(1450)
Unconfirmed resonances
* ''X''(1110)
* f
0(1200-1600)
* f
01790
* ''X''(1810)
See also
*
List of mesons
:''This list is of all known and predicted scalar, pseudoscalar and vector mesons. See list of particles for a more detailed list of particles found in particle physics.''
This article contains a list of mesons, unstable subatomic particles com ...
*
Pseudovector meson
In high energy physics, a pseudovector meson or axial vector meson is a meson with total spin 1 and even parity (+) (usually noted as
:Compare to a vector meson, which has a total spin 1 and odd parity
Charge parity (C) in addition ...
*
Scalar boson
A scalar boson is a boson whose spin equals zero. ''Boson'' means that the particle's wave function is symmetric under particle exchange and therefore follows Bose–Einstein statistics. The spin-statistics theorem implies that all bosons have a ...
References
{{DEFAULTSORT:Scalar Meson
Mesons
Bosons
Subatomic particles with spin 0