Diffractive pion production at COMPASS -- First results on 3$\pi$ final states - neutral mode

TL;DR

The paper reports initial findings from COMPASS on diffractive pion production, analyzing 3-pion final states in both charged and neutral modes to search for exotic states and validate results through cross-checks.

Contribution

First results from COMPASS on 3π final states in both charged and neutral modes, providing new data for light hadron spectroscopy and exotic state searches.

Findings

Evidence of a new state observed in both decay modes

Successful partial wave analysis performed on 2008 data

Cross-checks between modes enhance result reliability

Abstract

The COMPASS experiment at CERN is designed for light hadron spectroscopy with emphasis on the detection of new states, in particular the search for exotic states and glue-balls. After a short pilot run in 2004 (190 GeV/c negative pion beam, lead target) showing significant production strength for an exotic $J^{PC}=1^{-+}$ state at 1.66\,GeV/${\rm c^2}$, we have collected data with a 190 GeV/c negative charged hadron beam on a proton (liquid hydrogen) and nuclear targets in 2008 and 2009. The spectrometer features good coverage by electromagnetic calorimetry, and our data provide excellent opportunity for simultaneous observation of new states in two different decay modes in the same experiment. The diffractively produced $(3\pi)^{-}$ system for example can be studied in both modes $\pi^{-}p \rightarrow \pi^{-}\pi^{+}\pi^{-}p$ and $\pi^{-}~p \rightarrow \pi^{-}\pi^{0}\pi^{0}~p$. Charged and neutral mode rely on completely different parts of the spectrometer. Observing a new state in both modes provides important cross-check. First results of a preliminary PWA performed on the 2008 data are presented.