MULTICHANNEL APPROACH TO STUDYING SCALAR RESONANCES

The N-channel formalism (in more detail, 2- and 3-channel) is presente d which is based on such general principles, as analyticity and unitar ity, and realizes an idea of the consistent account of the nearest (to the considered physical region) singularities on all sheets of the Ri emann surface of the S-matrix, thus giving a chance to obtain model-in dependent information on multichannel resonances from the analysis of data on the coupled processes. The resonance representations by pairs of complex-conjugate clusters of poles and zeros on the Riemann surfac e and the conception of standard clusters as a model-independent test of the resonance presence are discussed. The role of closed channels i n forming resonances and fictitious states is investigated. The method is exemplified with the isoscalar s-wave channel of coupled processes pi pi --> pi pi, <K(K)over bar>, eta eta and with analysing pi pi sca ttering up to 1.9 GeV and scalar resonances.