Study of the $\pi^-\pi^+$ Subsystem with $J^{PC}=1^{--}$ in the Diffractively Produced $\pi^-\pi^+\pi^-$ Final State at COMPASS [Master's Colloquium]

by Martin Bartl (TUM)

Thursday 14 Dec 2023, 09:30 → 11:00 Europe/Berlin

E18 Seminar Room

Applying perturbation theory to quantum chromodynamics (QCD) describes the strong interaction in the high-energy limit with high precision. However, this ansatz fails at low energies since the coupling constant of the strong interaction $\alpha_S$ becomes large in this regime. Therefore, one must rely on other methods, i.e., simulations using lattice QCD or effective theories, such as chiral perturbation theory, to study QCD at low energies and to explain experimental findings. Light-meson spectroscopy, i.e., the study of the excitation spectrum of mesons composed of $u$, $d$, or $s$ quarks, plays a crucial part in probing low-energy QCD.

One way to produce excited light mesons is diffraction of a high-energy meson beam on a nucleon or nuclear target. One realization of such an inelastic scattering reaction is the process $\pi^- p \rightarrow \pi^- \pi^+ \pi^- p$, for which the COMPASS experiment at CERN acquired a world-leading data set with more than $10^8$ exclusive events. In this reaction, COMPASS so far studied mainly the appearing intermediate $3\pi$ resonances by performing a so-called partial-wave analysis (PWA). This approach disentangles the various short-lived $3\pi$ resonances and determines their quantum numbers, masses, and widths. However, this analysis relies on the isobar model, which assumes that the decay of the excited mesons into $3\pi$ can be completely described as a series of two two-body decays. The decay amplitudes of the appearing intermediate $\pi^-\pi^+$ resonances have to be known a priori, giving rise to potential model bias.

To reduce this model bias and to extract information about the intermediate $\pi^-\pi^+$ resonances, an extension of the PWA approach, the so-called freed-isobar PWA, was developed. In this work, we perform for the first time a high-precision study of the $\pi^-\pi^+$ isobar amplitudes obtained from a freed-isobar PWA of the full COMPASS data set on $\pi^- p \rightarrow \pi^- \pi^+ \pi^- p$. We analyze the most dominant $\pi^-\pi^+$ resonance, i.e., the $\rho(770)$, in eight different partial waves of the $3\pi$ system and study the $\rho(770)$ resonance parameters as a function of the squared four-momentum transfer $t'$ and the invariant mass $m_{3\pi}$ of the $3\pi$ system. To extract the $\rho(770)$ resonance parameters, we use a simple Breit-Wigner amplitude and a more elaborate model based on the Gounaris-Sakurai description of the pion form factor. The latter model allows us to measure the pole position of the $\rho(770)$ in the complex energy plane.

 

Zoom room:

https://tum-conf.zoom-x.de/j/98354294090?pwd=Q210VFp3cjgvL0M2NE1PcVBESWt3QT09
Meeting-ID: 983 5429 4090
Kenncode: 1881