Hadron Spectroscopy in COMPASS

TL;DR

COMPASS is a CERN experiment investigating hadron structures, aiming to discover exotic states like hybrids and glueballs, through detailed analysis of meson spectra and decay channels using high-resolution detection of various final states.

Contribution

This paper presents the first results from COMPASS data, focusing on the search for spin-exotic mesons and glueball candidates in multiple decay channels.

Findings

Evidence for spin-exotic mesons in diffractive reactions

Observation of scalar sector resonance structures

Data supporting the existence of glueball candidates

Abstract

The COmmon Muon and Proton Apparatus for Structure and Spectroscopy (COMPASS) is a multi-purpose fixed-target experiment at the CERN Super Proton Synchrotron (SPS) aimed at studying the structure and spectrum of hadrons. In the naive Constituent Quark Model (CQM) mesons are bound states of quarks and antiquarks. QCD, however, predict the existence of hadrons beyond the CQM with exotic properties interpreted as excited glue (hybrids) or even pure gluonic bound states (glueballs). One main goal of COMPASS is to search for these states. Particularly interesting are so called spin-exotic mesons which have J^{PC} quantum numbers forbidden for ordinary q\bar{q} states. Its large acceptance, high resolution, and high-rate capability make the COMPASS experiment an excellent device to study the spectrum of light-quark mesons in diffractive and central production reactions up to masses of about 2.5 GeV. COMPASS is able to measure final states with charged as well as neutral particles, so that resonances can be studied in different reactions and decay channels. During 2008 and 2009 COMPASS acquired large data samples using negative and positive secondary hadron beams on lH_2, Ni, and Pb targets. The presented overview of the first results from this data set focuses in particular on the search for spin-exotic mesons in diffractively produced \pi^-\pi^+\pi^-, \eta\pi, \eta'\pi, and \pi^-\pi^+\pi^-\pi^+\pi^- final states and the analysis of central-production of \pi^+\pi^- pairs in order to study glueball candidates in the scalar sector.