Extraction of the $\pi^+\pi^-$ Subsystem in Diffractively Produced $\pi^-\pi^+\pi^-$ at COMPASS

TL;DR

This paper introduces a new method for extracting isobar amplitudes directly from high-precision diffractive three-pion data, reducing systematic uncertainties and providing clearer insights into the scalar $pb1pb1$ subsystem and the $a_1(1420)$ state.

Contribution

A novel data-driven approach for extracting isobar amplitudes in partial-wave analysis, minimizing bias from prior parametrizations.

Findings

Identification of scalar pb1pb1$ amplitudes directly from data.

Confirmation of the $a_1(1420)$ signal decaying into $f_0(980)pb1pb1$.

Reduction of systematic uncertainties in partial-wave analysis.

Abstract

The COMPASS experiment at CERN has collected a large data sample of 50 million diffractively produced $\pi^-\pi^+\pi^-$ events using a $190\,$GeV$/c$ negatively charged hadron beam. The partial-wave analysis (PWA) of these high-precision data reveals previously unseen details. The PWA, which is currently limited by systematic uncertainties, is based on an isobar model, where multi-particle decays are described as subsequent two-body decays and where a prior-knowledge parametrization for the intermediate two-pion resonances has to be assumed -- usually a Breit-Wigner amplitude -- thus increasing systematic uncertainties, due to the concrete choice of the parametrization. We present a novel method, which allows to extract isobar amplitudes directly from the data in a less biased way. The focus lies on the scalar $\pi^+\pi^-$ subsystem, where a previous analysis found a signal for a new axial-vector state $a_1(1420)$ decaying into $f_0(980)\pi$.