Meson spectroscopy at COMPASS

Boris Grube

doi:10.1051/epjconf/201613001024

Abstract

The goal of the COMPASS experiment at CERN is to study the structure and dynamics of hadrons. The two-stage spectrometer used by the experiment has large acceptance and covers a wide kinematic range for charged as well as neutral particles and can therefore measure a wide range of reactions. The spectroscopy of light mesons is performed with negative (mostly π−) and positive (p, π+) hadron beams with a momentum of 190 GeV/c. The light-meson spectrum is measured in different final states produced in diffractive dissociation reactions with squared four-momentum transfer t to the target between 0.1 and 1.0 (GeV=c)2. The flagship channel is the π−π−π+ final state, for which COMPASS has recorded the currently world’s largest data sample. These data not only allow to measure the properties of known resonances with high precision, but also to observe new states. Among these is a new axial-vector signal, the a1(1420), with unusual properties. Novel analysis techniques have been developed to extract also the amplitude of the π−π+ subsystem as a function of 3π mass from the data. The findings are confirmed by the analysis of the π−π0π0 final state.

Highlights

The COMPASS experiment [1] has recorded large data sets of the diffractive dissociation reaction π− + p → (3π)− + precoil using a 190 GeV/c pion beam on a liquid-hydrogen target
Diffractive reactions are known to exhibit a rich spectrum of intermediate states X− and are a good place to search for states beyond the naive constituent-quark model
Several candidates for so-called spin-exotic mesons, which have JPC quantum numbers that are forbidden in the non-relativistic quark model, have been reported in pion-induced diffraction [2, 3]

Summary

Introduction

The COMPASS experiment [1] has recorded large data sets of the diffractive dissociation reaction π− + p → (3π)− + precoil using a 190 GeV/c pion beam on a liquid-hydrogen target. In this process, the beam hadron is excited to some intermediate three-pion state X− via t-channel Reggeon exchange with the target. The 3π data samples consist of 46 × 106 π−π−π+ and 3.5 × 106 π−π0π0 exclusive events in the analyzed kinematic region of three-pion mass, 0.5 < m3π < 2.5 GeV/c2. The known pattern of resonances a1(1260), a2(1320), and π2(1670) is seen in the 3π system along with ρ(770), f0(980), f2(1270), and ρ3(1690) in the π−π+ subsystem

Partial-wave decomposition

Findings

Extraction of ππ S-wave isobar amplitudes from data