Speaker
Description
High-energy peripheral reactions provide an excellent opportunity to study the excitation spectrum of hadrons. The COMPASS experiment at CERN has collected $5\times10^6$ events of the diffractive scattering of pions to the 3-pion final state.
Partial wave analysis techniques have been employed to obtain an expansion of the
reaction cross section in terms of partial waves with quantum numbers $J^{PC}\,M^\varepsilon$.
Since around hundred of observed resonances in light sector are coupled to pionic systems
the spin-density matrices from COMPASS PWA are extremely valuable data to identify ordinary and, possibly, exotic mesons.
The aim of our analysis is to extract three pion scattering amplitudes
from mass-dependence of the COMPASS spin-density matrices using analyticity and unitarity constraints.
Three-body unitarity problem is very difficult and has not been completely solved.
Our approximation of those constraints is reduced in the isobar model by quasi-two-body
($\pi\pi$-subchannnel resonance + pion) unitarity requirements.
K-matrix approach is used to parametrize scattering matrix.
We invoke an unitarization procedure to incorporate
non-resonant long-range production processes via pion exchange, i.e. ``Deck''-like processes.
The developed theoretical framework is applied to the COMPASS data.
We discuss resonant pole positions of $\pi_2$ mesons located at the unphysical sheet, spurious poles caused by the parametrization and stability of solutions with respect to the changes in the model.
