Hadron Spectroscopy in COMPASS

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.Comment: 8 pages, 5 figures, proceedings of the "Xth Quark Confinement and the Hadron Spectrum" (ConfX), TUM Campus Garching, Munich, Germany, 8 - 12 October 201

Diffractive Dissociation of 190 GeV/c π−\pi^- into π−π+π−\pi^-\pi^+\pi^- Final States at COMPASS

We present results from a Partial-Wave Analysis (PWA) of diffractive dissociation of 190 GeV/c $\pi^-$ into $\pi^-\pi^+\pi^-$ final states on nuclear targets. A PWA of the data sample taken during a COMPASS pilot run in 2004 on a Pb target showed a significant spin-exotic $J^{PC} = 1^{-+}$ resonance consistent with the controversial $\pi_1(1600)$, which is considered to be a candidate for a non-$q\bar{q}$ mesonic state. In 2008 COMPASS collected a large diffractive $\pi^-\pi^+\pi^-$ data sample using a hydrogen target. A first comparison with the 2004 data shows a strong target dependence of the production strength of states with spin projections $M = 0$ and 1.Comment: 3 pages, 3 figures, proceedings of the "11th International Workshop on Meson Production, Properties, and Interaction" (MESON2010), Krakow, Poland, June 10 - 15, 201

Precision Meson Spectroscopy at COMPASS

We present first results of a partial wave analysis of the diffractive reaction $\pi- Pb \to \pi- \pi+ \pi- Pb$ based on data from the COMPASS experiment taken during a pilot run in 2004 using a 190 GeV/c $\pi-$ beam on a lead target. The analysis was performed in the region of squared four-momentum transfer $t'$ between 0.1 and 1.0 (GeV/c)^2. The $\pi- \pi+ \pi-$ final state shows a rich spectrum of well-known resonances. In addition a spin-exotic $J^{PC} = 1^{-+}$ state with significant intensity was observed at 1.66 GeV/c^2 in the $\rho(770) \pi$ decay channel in natural parity exchange. The resonant nature of this state is manifest in the mass dependence of its phase difference to $J^{PC} = 1^{++}$ and $2^{-+}$ waves. The measured resonance parameters are consistent with the disputed $\pi_1(1600)$. An outlook on the analyses of the much larger data set taken during 2008 and 2009 is given.Comment: 10 pages, 18 figures, proceedings of "19th International IUPAP Conference on Few-Body Problems in Physics" (FB19), Bonn, Germany, Aug. 31 - Sept. 5, 200

Meson Spectroscopy at COMPASS

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. The two-stage spectrometer has a good acceptance for charged as well as neutral particles over a wide kinematic range and thus allows to access a wide range of reactions. Light mesons are studied with negative (mostly $\pi^-$) and positive ($p$, $\pi^+$) hadron beams with a momentum of 190 GeV/$c$. The spectrum of light mesons is investigated in various final states produced in diffractive dissociation reactions at squared four-momentum transfers to the target between 0.1 and 1.0 $(\text{GeV}/c)^2$. The flagship channel is the $\pi^-\pi^+\pi^-$ final state, for which COMPASS has recorded the currently largest data sample. These data not only allow to measure the properties of known resonances with high precision, but also to search for new states. Among these is a new resonance-like signal, the $a_1(1420)$, with unusual properties. Of particular interest is also the resonance content of the partial wave with spin-exotic $J^{PC} = 1^{-+}$ quantum numbers, which are forbidden for quark-antiquark states.Comment: 6 pages, 11 figures, proceedings of the "21th International Workshop on Photon-Photon Collisions and the International Workshop on High Energy Photon Colliders" (PHOTON 2015), Budker Institute of Nuclear Physics (BINP), Novosibirsk, Russia, 15-19 June, 201

Hadron Spectroscopy with COMPASS

COMPASS is a multi-purpose fixed-target experiment at the CERN Super Proton Synchrotron aimed at studying the structure and spectrum of hadrons. One primary goal is the search for new hadronic states, in particular spin-exotic mesons and glueballs. We present recent results of partial-wave analyses of (3\pi)^- and \pi^-\eta' final states based on a large data set of diffractive dissociation of a 190 GeV/c \pi^- beam on a proton target in the squared four-momentum-transfer range 0.1 < t' < 1 (GeV/c)^2. We also show first results from a partial-wave analysis of diffractive dissociation of K^- into K^-\pi^+\pi^- final states are presented.Comment: 5 pages, 6 figures, proceedings of the "19th Particles and Nuclei International Conference" (PANIC11), MIT, Cambridge, MA, USA, July 24 - 29, 201