# Bonn 2020 – wissenschaftliches Programm

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# HK: Fachverband Physik der Hadronen und Kerne

## HK 13: Hadron Structure and Spectroscopy III

### HK 13.4: Vortrag

### Dienstag, 31. März 2020, 18:00–18:15, J-HS A

Final fit of the a_{1}(1420) as a triangle singularity — •Mathias Wagner^{1}, Mikhail Mikhasenko^{2}, and Bernhard Ketzer^{1} for the COMPASS collaboration — ^{1}Universität Bonn, Helmholtz-Institut für Strahlen- und Kernphysik, 53115 Bonn, Germany — ^{2}CERN, Geneva, Switzerland

In the recent past several new particle candidates (e.g. the X,Y,Z states in the charmed sector) were found which do not fit into the simple constituent-quark models for mesons and baryons. Different concepts were introduced in order to find an explanation for these exotic states. One of them is a rescattering effect in the three-particle system. Here, triangle diagrams can produce resonance-like signals.

One prominent example is the a_{1}(1420) signal, observed by the COMPASS experiment in the J^{PC}=1^{++} partial wave decaying to f_{0}(980)π in a P-wave.

By coupling the KKπ and the 3π final states we obtain a first order approximation to the Khuri-Treiman equation for the 1^{++} sector. We present the fit results of the finalized model, where we properly include all involved spins via a dispersion integral over a partial wave projection of the KKπ final state onto the 3π final state, using a general π K P-wave amplitude.

Many systematic studies were performed, including a bootstrap analysis and an investigation of other contributions to the decay amplitude. A comparison to a fit with a genuine-resonance model is done for all cases.

Supported by BMBF.