–1– THE η(1405), η(1475), f1(1420), AND f1(1510)

The first observation of the η(1440) was made in pp annihilation at rest into η(1440)π+π−, η(1440) → KKπ [1]. This state was reported to decay through a0(980)π and K ∗(892)K with roughly equal contributions. The η(1440) was also observed in radiative J/ψ(1S) decay into KKπ [2–4] and γρ [5]. There is now evidence for the existence of two pseudoscalars in this mass region, the η(1405) and η(1475). The former decays mainly through a0(980)π (or direct KKπ) and the latter mainly to K∗(892)K. The simultaneous observation of two pseudoscalars is reported in three production mechanisms: π−p [6,7]; radiative J/ψ(1S) decay [8,9]; and pp annihilation at rest [11–14]. All of them give values for the masses, widths, and decay modes in reasonable agreement. However, Ref. [9] favors a state decaying into K∗(892)K at a lower mass than the state decaying into a0(980)π, although agreement with MARK-III is not excluded. In J/ψ(1S) radiative decay, the η(1405) decays into KKπ through a0(980)π, and hence a signal is also expected in the ηππ mass spectrum. This was indeed observed by MARK III in ηπ+π− [15], which reports a mass of 1400 MeV, in line with the existence of the η(1405) decaying into a0(980)π. BES [10] reports an enhancement in K+K−π0 around 1.44 GeV in J/ψ(1S) decay, recoiling against an ω (but not a φ) without resolving the presence of two states nor performing a spin-parity analysis, due to low statistics. This state could also be the f1(1420) (see below). The η(1405) is also observed in pp annihilation at rest into ηπ+π−π0π0, where it decays into ηππ [16]. The intermediate a0(980)π accounts for roughly half of the ηππ signal, in agreement with MARK III [15] and DM2 [4]. The η(1295) has been observed by four π−p experiments [7,17–19], and evidence is reported in pp annihilation [23–25]. In J/ψ(1S) radiative decay, an η(1295) signal is evident in the 0−+ ηππ wave of the DM2 data [9]. Also BaBar [20] reports evidence for a signal around 1295 MeV in B decays into ηππK.