– 1– the Ρ(1450) and the Ρ(1700)

In our 1988 edition, we replaced the ρ(1600) entry with two new ones, the ρ(1450) and the ρ(1700), because there was emerging evidence that the 1600-MeV region actually contains two ρ-like resonances. Erkal [1] had pointed out this possibility with a theoretical analysis on the consistency of 2π and 4π electromagnetic form factors and the ππ scattering length. Donnachie [2], with a full analysis of data on the 2π and 4π final states in e + e − annihilation and photoproduction reactions, had also argued that in order to obtain a consistent picture, two resonances were necessary. The existence of ρ(1450) was supported by the analysis of ηρ 0 mass spectra obtained in photoproduction and e + e − annihilation [3], as well as that of e + e − → ωπ [4]. The analysis of [2] was further extended by [5,6] to include new data on 4π-systems produced in e + e − annihilation, and in τ-decays (τ decays to 4π, and e + e − annihilation to 4π can be related by the Conserved Vector Current assumption). These systems were successfully analyzed using interfering contributions from two ρ-like states, and from the tail of the ρ(770) decaying into two-body states. While specific conclusions on ρ(1450) → 4π were obtained, little could be said about the ρ(1700). Independent evidence for two 1 − states is provided by [7] in 4π electroproduction at Q 2 = 1 (GeV/c) 2 , and by [8] in a high-statistics sample of the ηππ system in π − p charge exchange. This scenario with two overlapping resonances is supported by other data. Bisello [9] measured the pion form factor in the interval 1.35–2.4 GeV, and observed a deep minimum around 1.6 GeV. The best fit was obtained with the hypothesis of ρ-like resonances at 1420 and 1770 MeV, with widths of about 250 MeV. Antonelli [10] found that the e + e − → η π + π − cross section is better fitted with two fully interfering Breit-Wigners, with parameters in fair agreement with those of [2] and [9]. These results can be considered as a confirmation of the ρ(1450).