Search

The status of spin-exotic mesons is reviewed. There is now compelling evidence for at least three π1 states between one and two GeV. Preliminary results from the reaction π−p → π+π+π−π−π0p show structure in the exotic waves corresponding to IGJPC = 0−2+−.

This Letter reports results from the partial wave analysis of the π−π−π+η final state in π−p collisions at 18 GeV /c . Strong evidence is observed for production of two mesons with exotic quantum numbers of spin, parity and charge conjugation, JPC=1−+ in the decay channel f1(1285)π−. The mass M=1709±24±41 MeV /c 2 and width Γ=403±80±115 MeV /c 2 of the first state are consistent with the parameters of the previously observed π1(1600). The second resonance with mass M=2001±30±92 MeV /c 2 and width Γ=333±52±49 MeV /c 2 agrees very well with predictions from theoretical models. In addition, the presence of π2(1900) is confirmed with mass M=2003±88±148 MeV /c 2 and width Γ=306±132±121 MeV /c 2 and a new state, a1(2096), is observed with mass M=2096±17±121 MeV /c 2 and width Γ=451±41±81 MeV /c 2 . The decay properties of these last two states are consistent with flux tube model predictions for hybrid mesons with non-exotic quantum numbers.

An amplitude analysis of an exclusive sample of 5765 events from the reaction {pi}{sup -}p{yields}{eta}{prime}{pi}{sup -}p at 18 GeV/c is described. The {eta}{prime}{pi}{sup -} production is dominated by natural parity exchange and by three partial waves: those with J{sup PC}=1{sup -+} , 2{sup ++} , and 4{sup ++} . A mass-dependent analysis of the partial-wave amplitudes indicates the production of the a{sub 2}(1320) meson as well as the a{sub 4}(2040) meson, observed for the first time decaying to {eta}{prime}{pi}{sup -} . The dominant, exotic (non-q{bar q}) 1{sup -+} partial wave is shown to be resonant with a mass of 1.597{+-}0.010{sup +0.045}{sub -0.010} GeV/c{sup 2} and a width of 0.340{+-}0.040{+-}0.050 GeV /c{sup 2} . This exotic state, the {pi}{sub 1}(1600) , is produced with a t dependence which is different from that of the a{sub 2}(1320) meson, indicating differences between the production mechanisms for the two states.

The {eta}{pi}{sup {minus}} system has been studied in the reaction {pi}{sup {minus}}p{r_arrow}{eta}{pi}{sup {minus}}p at 18 GeV/c. A large asymmetry in the angular distribution is observed indicating interference between L-even and L-odd partial waves. The a{sub 2}(1320) is observed in the J{sup PC}=2{sup ++} wave, as is a broad enhancement between 1.2 and 1.6 GeV/c{sup 2} in the 1{sup {minus}+} wave. The observed phase difference between these waves shows that there is phase motion in addition to that due to a{sub 2}(1320) decay. The data can be fitted by interference between the a{sub 2}(1320) and an exotic 1{sup {minus}+} resonance with M=(1370{plus_minus}16{sup +50}{sub {minus}30}) MeV/c{sup 2} and {Gamma}=(385{plus_minus}40 {sup +65}{sub {minus}105}) MeV/c{sup 2} . {copyright} {ital 1997} {ital The American Physical Society}

A partial wave analysis (PWA) of the $\pi^- \pi^- \pi^+$ and $\pi^- \pi^0 \pi^0$ systems produced in the reaction $\pi^- p \to (3\pi)^-p$ at 18 GeV/$c$ was carried out using an \emph{isobar} model assumption. This analysis is based on 3.0M $\pi^- \pi^0 \pi^0$ events and 2.6M $\pi^- \pi^-\pi^+$ events and shows production of the $a_1(1260)$, $a_2(1320)$, $\pi_2(1670)$ and $a_4(2040)$ resonances. Results of detailed studies of the stability of partial wave fits are presented. An earlier analysis of 250K $\pi^- \pi^- \pi^+$ events from the same experiment showed possible evidence for a $J^{PC}=1^{-+}$ exotic meson with a mass of $\sim$1.6 GeV/$c^2$ decaying into $\rho \pi$. In this analysis of a higher statistics sample of the $(3\pi)^-$ system in two charged modes we find no evidence of an exotic meson.

A partial-wave analysis of the mesons from the reaction $\pi^{-}% p\to\pi^{+}\pi^{-}\pi^{-}\pi^{0}\pi^{0}p$ has been performed. The data show $b_{1}\pi$ decay of the spin-exotic states $\pi_{1}(1600)$ and \ $\pi_{1}(2000)$. Three isovector $2^{-+}$ states were seen in the $\omega\rho^{-}$ decay channel. In addition to the well known $\pi_{2}(1670)$, signals were also observed for $\pi_{2}(1880)$ and $\pi_{2}(1970)$.