Exotic states X ±(1600, I G (J PC)=2+ (2++)) in photoproduction reactions

Abstract

It is shown that the list of unusual mesons that are planned to be studied in photoproduction reactions can be supplemented with I G (J PC)=2+ (2++) exotic states X ±(1600), which are natural to seek as manifestations of the ρ±ρ0 decay channels in the reactions γN → ρ±ρ0 N and γN → ρ±ρ0Δ. A classification of the ρ±ρ0 states according to their quantum numbers is presented. A model for the spin structure of the amplitudes for the reactions γp → f 2(1270)p, γp → a 2 0 (1320)p, and γN → X ±(N, Δ) is proposed, and estimates are obtained for the corresponding cross sections. At E γ≈6 GeV, it is found that σ(γP → f 2(1270)p)≈0.12 µb, σ(γp → a 2 0 (1320)p)≈0.25 µb, σ(γN → X ± N → ρ±ρ0 N) ≈ 0.018 µb, and σ(γp → X −Δ++ → ρ−ρ0Δ++≈0.031 µb. The problem of isolating signals from X ± states against the natural background that is associated with other channels of π±π0π+π− production is discussed. It is deduced that searches for exotic states X ±(2+ (2++)) in experiments at JLAB will be quite efficient—for example, the yield of about 2.8×106 events per month is expected to correspond to the estimated cross sections for the reactions γN → X ± N → ρ±ρ0 N.