Inparticle physics, aglueball (alsogluonium,gluon-ball) is a hypothetical compositeparticle.[1] It consists solely ofgluon particles, without valencequarks. Such a state is possible because gluons carrycolor charge and experience thestrong interaction between themselves. Glueballs are extremely difficult to identify inparticle accelerators, because theymix with ordinarymeson states.[2][3] In puregauge theory, glueballs are the only states of the spectrum and some of them are stable.[4]
Theoretical calculations show that glueballs should exist at energy ranges accessible with currentcollider technology. However, due to the aforementioned difficulty (among others), they have so far not been observed and identified with certainty,[5] although phenomenological calculations have suggested that an experimentally identified glueball candidate, denotedf0(1710), has properties consistent with those expected of aStandard Model glueball.[6]
The prediction that glueballs exist is an essential prediction ofQCD as part of theStandard Model of particle physics that has not yet been unambiguously confirmed experimentally.[7]
Experimental evidence was announced in 2021, by the TOTEM collaboration at theLHC in collaboration with the DØ collaboration at the formerTevatron collider atFermilab, ofodderon (a composite gluonic particle with oddC-parity) exchange. This exchange, associated with a quarkless three-gluon vector glueball, was identified in the comparison of proton–proton and proton–antiproton scattering.[8][9][10] In 2024, the X(2370) particle was determined to have mass and spin parity consistent with that of a glueball.[11] However, other exotic particle candidates such as atetraquark could not be ruled out.[12]
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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 ofquantum 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 that 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 (calculated using several very different methods). This can lead to variation in theoretical predictions of glueball properties, like mass and branching ratios in glueball decays.
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Theoretical studies of glueballs have focused on glueballs consisting of either two gluons or three gluons, by analogy tomesons andbaryons that have two and threequarks respectively. As in the case of mesons and baryons, glueballs would beQCD color charge neutral. Thebaryon number of a glueball is zero.
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Double-gluon glueballs can havetotal angular momentumJ = 0 (which are eitherscalar orpseudo-scalar) orJ = 2 (tensor). Triple-gluon glueballs can have total angular momentumJ = 1 (vector boson) orJ = 3 (third-order tensorboson). All glueballs have integer total angular momentum that implies that they arebosons rather thanfermions.
Glueballs are the only particles predicted by theStandard 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 withJ = 0 andJ = 1 with similar masses have been observed and excited states of other mesons can have these values of total angular momentum.
All glueballs would have anelectric charge of zero, as gluons themselves do not have an electric charge.[citation needed]
Glueballs are predicted by quantum chromodynamics to be massive, despite the fact that gluons themselves have zero rest mass in the Standard Model. Glueballs with all four possible combinations of quantum numbersP (spatial parity) andC (charge 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 as1.4 GeV/c2 (for a glueball with quantum numbersJ = 0, P = +1, C = +1, or equivalentlyJPC = 0++), and the heaviest states having masses as great as almost5 GeV/c2 (for a glueball with quantum numbersJ = 0, P = +1, C = -1, orJPC = 0+-).[5]
These masses are on the same order of magnitude as the masses of many experimentally observedmesons andbaryons, as well as to the masses of thetau lepton,charm quark,bottom quark, somehydrogen isotopes, and somehelium isotopes.[citation needed]
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 variousQCD 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.[13][14] For example, glueballs would not have radiative or two photon decays, but would have decays into pairs ofpions, pairs ofkaons, or pairs ofeta mesons.[13]
Standard Model glueballs are extremely ephemeral (decaying almost immediately into more stable decay products) and are only generated in high energy physics. Thus in the natural conditions found on Earth that humans can easily observe, glueballs arise only synthetically. 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 theirengineering applications.
Lattice QCD provides a way to study the glueball spectrum theoretically and from first principles. Some of the first quantities calculated using latticeQCD methods (in 1980) were glueball mass estimates.[16] Morningstar and Peardon computed the masses of the lightest glueballs in QCD without dynamical quarks in 1999.[17] 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 about1000–1700 MeV/c2.[5] Lattice predictions for scalar and pseudoscalar glueballs, including their excitations, were confirmed by Dyson–Schwinger/Bethe–Salpeter equations inYang–Mills theory.[18]
| JPC | mass |
|---|---|
| 0++ | 1730±80 MeV/c2 |
| 2++ | 2400±120 MeV/c2 |
| 0−+ | 2590±130 MeV/c2 |
Particle accelerator experiments are often able to identify unstable composite particles and assign masses to those particles to a precision of approximately10 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.
Many of these candidates have been the subject of active investigation for at least eighteen years.[13] TheGlueX experiment has been specifically designed to produce more definitive experimental evidence of glueballs.[19]
Some of the candidate particle resonances that could be glueballs, although the evidence is not definitive, include the following:
Various candidates for scalar glueballs were identified by Ochs:[5]
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