First Results on Hadron Spectroscopy at COMPASS

TL;DR

The COMPASS experiment at CERN has collected extensive data on hadron structures, leading to the first observation of a spin-exotic signal consistent with the $ ext{π}_1(1600)$, advancing the search for exotic hadronic states.

Contribution

This paper reports the first results from COMPASS's hadron spectroscopy program, including the observation of a spin-exotic signal and the analysis of multiple decay modes using high-statistics data.

Findings

Observation of a significant $J^{PC}$ spin-exotic signal consistent with $ ext{π}_1(1600)$

Successful detection of new states in different decay modes, confirming experimental consistency

High-statistics data enabling detailed studies of hadron structures

Abstract

The COMPASS fixed-target experiment at the CERN SPS is dedicated to the study of hadron structure and dynamics. One goal of the physics programme using hadron beams is the search for new states, in particular the search for $J^{PC}$ exotic states and glueballs. After a short pilot run in 2004 (190 GeV/c $\pi^{-}$ beam, lead target), we started our hadron spectroscopy programme in 2008 by collecting unprecedented statistics using 190 GeV/c negative hadron beams on a liquid hydrogen target. A similar amount of data with 190 GeV/c positive hadron beams has been taken in 2009, as well as some data (negative beam) on nuclear targets. As a first result the observation of a significant $J^{PC}$ spin-exotic signal in the 2004 data -- consistent with the disputed $\pi_1(1600)$ -- was recently published. Our spectrometer features good coverage by electromagnetic calorimetry, crucial for the detection of final states involving $\pi^0$, $\eta$ or $\eta'$, and the 2008/09 data provide an excellent opportunity for the simultaneous observation of new states in different decay modes. The diffractively produced $(3\pi)^{-}$ system for example can be studied in $\pi^{-}\pi^{+}\pi^{-}$ and $\pi^{-}\pi^{0}\pi^{0}$ final states, respectively. Observation of new states in both modes provides important consistency checks within the same experiment as the reconstruction of charged and neutral modes rely on completely different parts of the apparatus. We present the first results and give an overview of the status on various ongoing analyses of the 2008/09 data.