Abstract NarrowΞπ andΞ + π ± resonances produced by quasi-real photons have been search ed for by the COMPASS experiment at CERN. The study was stimulat ed by the recent observation of an exotic baryonic state decaying into Ξπ−, at a mass of 1862 MeV, interpreted as a pentaquark. While the ordinary hyperon states Ξ(1530)0 andΞ(1530)0 are clearly seen, no exotic baryon is observed in the data taken in 2002 an d 003.

We present the results of the analysis of diffractive reaction π−Be → π−π0π0Be measured with upgraded VES detector with the statistics of about 16 · 106 events. The mass-independent partial-wave analysis is performed using set of 70 partial waves corresponding to natural-parity exchange mechanism and restricting the rank of spin-density matrix Nr = 1. The data is divided into 40 MeV m(3π) bins and in 10 intervals of momentum transfer squared t′. The sub-density-matrix of mass independent PWA is subject to further mass dependent fits using Breit-Wigner resonant terms and coherent non-resonant backgrounds.

We present the results of the analysis of diffractive reaction pi(-) Be -> pi(-) pi(0) pi(0) B measured with upgraded VES detector with the statistics of about 16 . 10(6) events. The mass -independent partial-wave analysis is performed using set of 70 partial waves corresponding to natural-parity exchange mechanism and restrictim, the rank of spin -density matrix N-r = 1. The data is divided into 40 MeV m(3 pi) bins and in 10 intervals of momentum transfer squared t'. The sub-density-matrix of mass independent PWA is subject to further mass dependent fits using Breit-Wigner resonant terms and coherent non-resonant backgrounds.

Partial wave analysis of the π–A π+π–π– system produced by 29 GeV/cπ– beam on a beryllium target is presented. About 30 × 106 |events in the wide t′|range 0–0.8 GeV2/c 2 are collected with upgraded VES setup. The size of the data sample is 2.5 times larger than one previously analyzed by VES. Data are analyzed using formalism of the density matrix with unlimited rank. We discuss status of the a 1(1420) a 2(1700) a 3(1875) states and a structure of exotic ρ(770)π P-wave with J PC = 1-+. Parameters of a 3(1875) are estimated as M = 1985 ± 20 MeV/c 2, Γ = 200 ± 50 MeV/c 2 (preliminary).

Partial wave analysis of the pi(+)pi(-)pi(-) system produced by 29 GeV/c pi(-) beam on a beryllium target is presented. About 30 x 10(6) events in the wide vertical bar t'vertical bar range 0-0.8 GeV2/c(2) are collected with upgraded VES setup. The size of the data sample is 2.5 times larger than one previously analyzed by VES. Data are analyzed using formalism of the density matrix with unlimited rank. We discuss status of the a(1)(1420), a(2)(1700), a(3)(1875) states and a structure of exotic rho(770)pi P-wave with J(PC) = 1(-+). Parameters of a3(1875) are estimated as M = 1985 +/- 20 MeV/c(2), Gamma = 200 +/- 50 MeV/c(2) (preliminary).

7 × 106 events of the reaction π−N → π−π0N′ were collected at the VES experiment. Mass independent angular analysis is performed to study production mechanism of ρ-meson. t-channel ω-exchange and π-exchange are isolated. The analysis is done for 8 bins of a four momentum transfer squared to extract cross-section dependence of t′. Novel method of waves extraction is applied to deal with a significant irremovable background.

Reactions of pion-nuclear interaction with beryllium target at 28 GeV with a single track of fast charged particle in the final state (one-prong topology) became accessible for the study with the upgraded VES setup. Studies of two of them are presented. The Partial Wave Analysis (PWA) of the π−π0π0 system was performed. The preliminary results on the resonance structure of this system are presented. The highest statistics in the world for this system and the advanced PWA technique permit us to study detailed dependence of the waves on the momentum transfer squared |t|. New parameterization of the π0π0 S-wave amplitude was extracted from the data. The very high statistics on the π−π0 system was obtained, which exceeds predecessors by three orders of magnitude. It will be used for a study of the |t| dependence of the ρ(770) production density matrix. The feasibility of the corresponding PWA is evaluated. The results of PWA of the π−π0π0 system were used to estimate the major background for π−π0 production.

Status of the VES setup upgrade is shortly reviewed. A number of one-prong multi-photon final states not (or badly) accessible with the VES before, such as the π−3π0, π−ηπ0 in background favorable mode π−4γ and others, becomes promising for the study. An overview of new data set is presented. The preliminary results of PWA of the π−2π0, system show a clear signal of the f0(980) in the π(1800) decay.

The isospin symmetry-breaking (ISB) decay f 1(1285) → π+π−π0 has been studied at the VES facility. The branching ratio is measured: \(\frac{{BR\left( {f_1 \to \pi ^ + \pi ^ - \pi ^0 } \right)}}{{BR\left( {f_1 \to \eta \pi ^ + \pi ^ - } \right)}} = 0.86 \pm 0.16_{ - 0.20}^{ + 0.70} \%\). An upper limit for the f 1 decay into ρ(770)π is obtained: BR(f 1(1285) → ρ +−π−+) < 0.31% at 95% CL.

Results obtained in pi(-) Be interactions at 27, 36.6 and 41 GeV/c are presented. Decay f(1)(1285) -> pi(+)pi(-)pi(0) is observed; the dominant mechanism of the isospin symmetry violation in this decay is the a(0)(980) - f(0)(980) mixing. The intensity of this mixing is measured. An upper limit on possible f(1)(1285) - a(1)(1260) mixing is obtained. Preliminary results of a study of (omega phi) system are shown. Plans for coming upgrade of the detector are presented.

The isospin-violating decay f 1 (1285) → π + π - π 0 has been studied at the VES facility. This study is based on the statistics acquired in π - Be interactions at 27, 36.6 and 41 GeV/ c in the diffractive reaction π - N → ( f 1 π - ) N . The f 1 (1285) → π + π - π 0 decay is observed. The preliminary ratio of decay probabilities BR ( f 1 (1285) → π + π - π 0 ) to BR ( f 1 (1285) → ηπ + π - ) . BR (η → γγ) is ∼ 2% .

A partial-wave analysis of the reaction π−p→ηηπ−p at 18 GeV/c has been performed on a data sample of approximately 4000 events obtained by Brookhaven experiment E852. The JPC=0−+π(1800) state is observed in the a0(980)η and f0(1500)π decay modes. It has a mass of 1876±18±16 MeV/c2 and a width of 221±26±38 MeV/c2. The JPC=2−+π2(1880) meson is observed decaying through a2(1320)η. It has a mass of 1929±24±18 MeV/c2 and a width of 323±87±43 MeV/c2. Both states are potential candidates for non-exotic hybrid mesons.

P. Eugenio, A. I. Ostrovidov, G. S. Adams, T. Adams, Z. Bar-Yam, J. M. Bishop, V. A. Bodyagin, ∗ D. S. Brown, † N. M. Cason, S. U. Chung, J. P. Cummings, K. Danyo, S. P. Denisov, V. Dorofeev, J. P. Dowd, X. L. Fan, A. M. Gribushin, R. W. Hackenburg, M. Hayek, ‡ J. Hu, § E. I. Ivanov, D. Joffe, W. Kern, E. King, O. L. Kodolova, V. L. Korotkikh, M. A. Kostin, J. Kuhn, ¶ V. V. Lipaev, J. M. LoSecco, M. Lu, ∗∗ J. J. Manak, J. Napolitano, M. Nozar, § C. Olchanski, § T. K. Pedlar, †† A. V. Popov, D. I. Ryabchikov, L. I. Sarycheva, K. K. Seth, N. Shenhav, ‡ X. Shen, ‡‡ W. D. Shephard, N. B. Sinev, D. L. Stienike, J. S. Suh, §§ S. A. Taegar, A. Tomaradze, I. N. Vardanyan, D. P. Weygand, D. B. White, H. J. Willutzki, ∗ M. Witkowski, and A. A. Yershov

The Dalitz plot of the η′→ηπ+π− decay is studied using the data collected with the VES spectrometer in two different exclusive reactions, π−p→η′n (charge-exchange) and π−N→η′π−N (diffractive-like production). The η mesons from η′ decays were detected in the η→γγ mode. The coefficients for the matrix element squared decomposition are measured on the largest statistics of η′ decays reported so far; there are ∼13.6×103 and ∼6.5×103 decays after background subtraction in charge-exchange and diffractive-like production, respectively.

A partial wave analysis (PWA) of the ${\ensuremath{\pi}}^{\ensuremath{-}}{\ensuremath{\pi}}^{\ensuremath{-}}{\ensuremath{\pi}}^{+}$ and ${\ensuremath{\pi}}^{\ensuremath{-}}{\ensuremath{\pi}}^{0}{\ensuremath{\pi}}^{0}$ systems produced in the reaction ${\ensuremath{\pi}}^{\ensuremath{-}}p\ensuremath{\rightarrow}(3\ensuremath{\pi}{)}^{\ensuremath{-}}p$ at $18\text{ }\text{ }\mathrm{GeV}/c$ was carried out using an isobar model assumption. This analysis is based on 3.0 M ${\ensuremath{\pi}}^{\ensuremath{-}}{\ensuremath{\pi}}^{0}{\ensuremath{\pi}}^{0}$ events and 2.6 M ${\ensuremath{\pi}}^{\ensuremath{-}}{\ensuremath{\pi}}^{\ensuremath{-}}{\ensuremath{\pi}}^{+}$ events and shows production of the ${a}_{1}(1260)$, ${a}_{2}(1320)$, ${\ensuremath{\pi}}_{2}(1670)$, and ${a}_{4}(2040)$ resonances. Results of detailed studies of the stability of partial wave fits are presented. An earlier analysis of 250 K ${\ensuremath{\pi}}^{\ensuremath{-}}{\ensuremath{\pi}}^{\ensuremath{-}}{\ensuremath{\pi}}^{+}$ events from the same experiment showed possible evidence for a ${J}^{PC}={1}^{\ensuremath{-}+}$ exotic meson with a mass of $\ensuremath{\sim}1.6\text{ }\text{ }\mathrm{GeV}/{c}^{2}$ decaying into $\ensuremath{\rho}\ensuremath{\pi}$. In this analysis of a higher statistics sample of the $(3\ensuremath{\pi}{)}^{\ensuremath{-}}$ system in two charged modes we find no evidence of an exotic meson.