First results from an extended freed-isobar analysis at COMPASS

TL;DR

This paper presents initial results from an extended freed-isobar analysis at COMPASS, replacing fixed amplitudes with piecewise functions to directly extract $oldsymbol{\pi^+\pi^- ext{-}}$ subsystem amplitudes, and discusses methods to address resulting ambiguities.

Contribution

It introduces an extended freed-isobar method for light-meson spectroscopy analysis, allowing direct amplitude extraction for multiple $oldsymbol{\pi^+\pi^- ext{-}}$ states, and proposes solutions to mathematical ambiguities.

Findings

Successful extension of freed-isobar analysis to multiple $oldsymbol{\pi^+\pi^- ext{-}}$ states

First results demonstrating feasibility of the extended method

Methods developed to resolve ambiguities in the model

Abstract

One of the goals of the COMPASS experiment is the precision study of light-meson spectroscopy with data for various final states. With $46\times10^6$ exclusive events, the process $\pi^- p\to\pi^-\pi^+\pi^- p$ constitutes the flagship channel. Based on this data set, the results of an extensive partial-wave analysis (PWA), using a total of 88 partial waves in the model, were published. Along with it, results of a first study of the so-called freed-isobar method were shown. Here, the fixed amplitudes for appearing $\pi^+\pi^-$ intermediate states used in the conventional analysis were replaced by sets of piecewise constant functions to extract the amplitudes of the $\pi^+\pi^-$ subsystems directly from the data. In this first study, this was one for three $\pi$ partial waves with $J^{PC}=0^{++}$ intermediate $\pi^+\pi^-$ states. The promising results inspired further extension of this method, by also freeing intermediate $\pi^+\pi^-$ states with $J^{PC} = 1^{--}$ and $2^{++}$. With this extension of the set of freed waves, mathematical ambiguities in the model arise due to the much higher flexibility of the model. We will present first results of these extended studies on COMPASS data along with methods to resolve the arising ambiguities.