Experimental review of the γ(1S, 2S, 3S) physics at e+e- colliders and the LHC

The three lowest-lying gamma states, i.e., gamma(1S), gamma(2S), and gamma(3S), composed of b (b) over bar pairs and below the B (B) over bar threshold, provide a good platform for the researches of hadronic physics and physics beyond the Standard Model. They can be produced directly in e(+)e(-) colliding experiments, such as CLEO, Babar, and Belle, with low continuum backgrounds. In these experiments, many measurements of the exclusive gamma(1S) and gamma(2S) decays into light hadrons, which shed light on the "80% rule" for the Okubo-Zweig-Iizuka suppressed decays in the bottomonium sector, were carried out. Meanwhile, many studies of the charmonium and bottomonium productions in gamma(1S, 2S, 3S) decays were performed, to distinguish different Quantum Chromodynamics (QCD) models. Besides, exotic states and new physics were also extensively explored in gamma(1S, 2S, 3S) decays at CLEO, BaBar, and Belle. The gamma(1S, 2S, 3S) states can also be produced in pp collisions and in collisions involving heavy ions. The precision measurements of their cross sections and polarizations at the large hadron collider (LHC), especially in the CMS, ATLAS, and LHCb experiments, help to understand gamma production mechanisms in pp collisions. The observation of the sequential gamma suppression in heavy ion collisions at CMS, LHCb, and ALICE is of great importance for verifying the quark-gluon plasma predicted by QCD. In this article, we review the experimental results on gamma(1S, 2S, 3S) at e(+)e(-) colliders and the LHC, and summarize their prospects at Belle II and the LHC.