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 interaction
between themselves.
Glueballs are extremely difficult to identify
in particle accelerators, because they mix
with ordinary meson states.Theoretical calculations
show that glueballs should exist at energy
ranges accessible with current collider technology.
However, due to the aforementioned difficulty
(among others), they have so far not been
observed and identified with certainty, although
phenomenological calculations have suggested
that an experimentally identified glueball
candidate, denoted
f
0
(
1710
)
{\displaystyle f_{0}(1710)}
, has properties consistent with those expected
of a Standard Model glueball.The prediction
that glueballs exist is one of the most important
predictions of the Standard Model of particle
physics that has not yet been confirmed experimentally.
Glueballs are the only particles predicted
by the Standard Model with total angular momentum
(J) (sometimes called "intrinsic spin") that
could be either 2 or 3 in their ground states.
== Properties ==
In principle, it is theoretically possible
for all properties of glueballs to be calculated
exactly and derived directly from the equations
and fundamental physical constants of quantum
chromodynamics (QCD) without further experimental
input.
So, the predicted properties of these hypothetical
particles can be described in exquisite detail
using only Standard Model physics which have
wide acceptance in the theoretical physics
literature.
But, there is considerable uncertainty in
the measurement of some of the relevant key
physical constants, and the QCD calculations
are so difficult that solutions to these equations
are almost always numerical approximations
(reached by several very different methodologies).
This can lead to variation in theoretical
predictions of glueball properties like mass
and branching ratios in glueball decays.
=== Constituent particles and color charge
===
Theoretical studies of glueballs have focused
on glueballs consisting of either two gluons
or three gluons, by analogy to mesons and
baryons that have two and three quarks respectively.
As in the case of mesons and baryons, glueballs
would be QCD color charge neutral.
The baryon number of a glueball is zero.
=== Total angular momentum ===
Two gluon glueballs can have total angular
momentum (J) of 0 (which are scalar or pseudo-scalar)
or 2 (tensor).
Three gluon glueballs can have total angular
momentum (J) of 1 (vector boson) or 3.
All glueballs have integer total angular momentum
which implies that they are bosons rather
than fermions.
Glueballs are the only particles predicted
by the Standard Model with total angular momentum
(J) (sometimes called "intrinsic spin") that
could be either 2 or 3 in their ground states,
although mesons made of two quarks with J=0
and J=1 with similar masses have been observed
and excited states of other mesons can have
these values of total angular momentum.
Fundamental particles with ground states having
J=0 or J=2 are easily distinguished from glueballs.
The hypothetical graviton, while having a
total angular momentum J=2 would be massless
and lack color charge, and so would be easily
distinguished from glueballs.
The Standard Model Higgs boson for which an
experimentally measured mass of about 125–126
GeV/c2 has been determined is the only fundamental
particle with J=0 in the Standard Model.
It also lacks color charge and hence does
not engage in strong force interactions.
But the Higgs boson is about 25–80 times
as heavy as the mass of the various glueball
states predicted by the Standard Model.
=== Electric charge ===
All glueballs would have an electric charge
of zero as gluons themselves do not have an
electric charge.
=== Mass and parity ===
Glueballs are predicted by quantum chromodynamics
to be massive, notwithstanding the fact that
gluons themselves have zero rest mass in the
Standard Model.
Glueballs with all four possible combinations
of quantum numbers P (parity) and C (c-parity)
for every possible total angular momentum
have been considered, producing at least fifteen
possible glueball states including excited
glueball states that share the same quantum
numbers but have differing masses with the
lightest states having masses as low as 1.4
GeV/c2 (for a glueball with quantum numbers
J=0, P=+, C=+), and the heaviest states having
masses as great as almost 5 GeV/c2 (for a
glueball with quantum numbers J=0, P=+, C=-).These
masses are on the same order of magnitude
as the masses of many experimentally observed
mesons and baryons, as well as to the masses
of the tau lepton, charm quark, bottom quark,
some hydrogen isotopes, and some helium isotopes.
=== Stability and decay channels ===
Just as all Standard Model mesons and baryons,
except the proton, are unstable in isolation,
all glueballs are predicted by the Standard
Model to be unstable in isolation, with various
QCD calculations predicting the total decay
width (which is functionally related to half-life)
for various glueball states.
QCD calculations also make predictions regarding
the expected decay patterns of glueballs.
For example, glueballs would not have radiative
or two photon decays, but would have decays
into pairs of pions, pairs of kaons, or pairs
of eta mesons.
== Practical impact on macroscopic low energy
physics ==
Because Standard Model glueballs are so ephemeral
(decaying almost immediately into more stable
decay products) and are only generated in
high energy physics, glueballs only arise
synthetically in the natural conditions found
on Earth that humans can easily observe.
They are scientifically notable mostly because
they are a testable prediction of the Standard
Model, and not because of phenomenological
impact on macroscopic processes, or their
engineering applications.
== Lattice QCD simulations ==
Lattice QCD provides a way to study the glueball
spectrum theoretically and from first principles.
Some of the first quantities calculated using
lattice QCD methods (in 1980) were glueball
mass estimates.
Morningstar and Peardon computed in 1999 the
masses of the lightest glueballs in QCD without
dynamical quarks.
The three lowest states are tabulated below.
The presence of dynamical quarks would slightly
alter these data, but also makes the computations
more difficult.
Since that time calculations within QCD (lattice
and sum rules) find the lightest glueball
to be a scalar with mass in the range of about
1000–1700 MeV.
== Experimental candidates ==
Particle accelerator experiments are often
able to identify unstable composite particles
and assign masses to those particles to a
precision of approximately 10 MeV/c2, without
being able to immediately assign to the particle
resonance that is observed all of the properties
of that particle.
Scores of such particles have been detected,
although particles detected in some experiments
but not others can be viewed as doubtful.
Some of the candidate particle resonances
that could be glueballs, although the evidence
is not definitive, include the following:
=== Vector, pseudo-vector, or tensor glueball
candidates ===
X(3020) observed by the BaBar collaboration
is a candidate for an excited state of the
2−+, 1+− or 1−− glueball states with
a mass of about 3.02 GeV/c2.
=== Scalar glueball candidates ===
f0(500) also known as σ – the properties
of this particle are possibly consistent with
a 1000 MeV or 1500 MeV mass glueball.
f0(980) – the structure of this composite
particle is consistent with the existence
of a light glueball.
f0(1370) – existence of this resonance is
disputed but is a candidate for a glueball-meson
mixing state
f0(1500) – existence of this resonance is
undisputed but its status as a glueball-meson
mixing state or pure glueball is not well
established.
f0(1710) – existence of this resonance is
undisputed but its status as a glueball-meson
mixing state or pure glueball is not well
established.
=== Other candidates ===
Gluon jets at the LEP experiment show a 40%
excess over theoretical expectations of electromagnetically
neutral clusters which suggests that electromagnetically
neutral particles expected in gluon rich environments
such as glueballs are likely to be present.Many
of these candidates have been the subject
of active investigation for at least eighteen
years.
The GlueX experiment has been specifically
designed to produce more definitive experimental
evidence of glueballs.
== See also ==
Exotic meson
GlueX
Gluon
Yang–Mills theory
