The reaction $\pi^{−}p \to \pi^{-}\pi^{-}\pi^{+}p$: Development of the analysis methods and selected results

We present the description of the analysis methods and results of applying them to the exclusive diffractive reaction π−p → π−π−π+p of 50 · 106 events measured with COMPASS detector. The large statistics of π−π−π+ events enables the two-dimensional partial-wave analysis independently in 100 bins of m(3π) with 0.5 < m(3π) < 2.5 GeV/c2 and in 11 intervals of squared momentum transfer with 0.1 < t′ < 1 GeV2/c2. The partial-wave analysis sub-density matrix is the subject to further mass-dependent fits describing the data in terms of resonances in 3π system and coherent background contributions. The novel approach of extracting JPC= 0++(π+π−)Sisobar amplitudes as model-free functions, different for several JPC3π states, is used. It demonstrates the presence of processes π(1800) → f0(980)π and π(1800) → f0(1500)π as well as π2(1880) → f0(980)π and new narrow signal a1(1420) → f0(980)π, without any established shapes used for (π+π−)Sisobars. The presented analysis is subject to further development and refinements which currently take place.