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 interaction between themselves. Glueballs are extremely difficult to identify in particle accelerators, because they mix with ordinary meson states. 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. 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. 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. 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. All glueballs would have an electric charge of zero as gluons themselves do not have an electric charge.