2026/07/15 by D. Winney, S. Gonzàlez-Solís, M. Mikhasenko +8
#hep-ph #hep-ex #nucl-th
We investigate the π1(1600) signal channel of 3π in the JPC = 1-+ [ππ]1--π~P-wave. This channel has recently been analyzed by the COMPASS collaboration using the ``freed-isobar" technique which provides direct experimental access to the lineshape modifications of the P-wave ππ sub-channel arising from both final state interactions and production processes such as the Deck mechanism. We motivate a unified formalism combining production effects and final state interactions using the Khuri-Treiman formalism, which is consistent with low-energy unitarity, analyticity, and crossing symmetry, seeded by an initial production amplitude. We demonstrate that the precise lineshape of the ππ mass spectrum can be used to determine the relative strength and complex phase of different production mechanisms contributing to the 3π final state. We conduct a global fit of the π1 freed-isobar data set, yielding a multi-dimensional parameterization of the π1→3π decay amplitude at COMPASS as a function of both decay and production variables. We identify the contributions from short-range production and the Deck mechanism and extract the total 3π invariant mass dependence which will be important for a future extraction of the π1 pole position in the ρπ channel.