Sector of the $2^{++}$ mesons: observation of the tensor glueball

Abstract

Data of the Crystal Barrel and L3 collaborations clarified essentially the situation in the $2^{++}$ sector in the mass region up to 2400 MeV, demonstrating the linearity of $(n,M^2)$ trajectories, where $n$ is the radial quantum number of a quark-antiquark state with mass $M$. We discuss these data and show that there exists a superfluous state for the $(n,M^2)$ trajectories: a broad resonance $f_2(2000)$. We pay special attention to the reactions $p\bar p\to\pi\pi,\eta\eta,\eta\eta'$ in the mass region 1990--2400 MeV where, together with $f_2(2000)$, four relatively narrow resonances are seen: $f_2(1920)$, $f_2(2020)$, $f_2(2240)$, $f_2(2300)$. We analyse the branching ratios of all these resonances and show that only the decay couplings of the broad state $f_2(2000)\to\pi^0\pi^0,\eta\eta, \eta\eta'$ satisfy relations inherent in the glueball decay.