New results on the search for spin-exotic mesons with COMPASS
Frank Nerling
备注：
6 pages, 5 figures, HEP 2011 Conference Proceedings, to appear in the proceedings of the XXI International Europhysics Conference on High Energy Physics: HEP 2011, Grenoble, France, 21-27 July 2011; v2: Corrected typos, added a reference
The COMPASS fixed-target experiment at the CERN-SPS studies the structure and spectrum of hadrons. One important goal using hadron beams is the search for new states, in particular spin-exotic mesons and glueballs. As a first input to the puzzle, COMPASS observed a significant $J^{PC}$ spin-exotic signal in the 2004 pilot run data (190\,GeV/$c$ $π^{-}$ beam, Pb target) in three charged pion final states consistent with the disputed $π_1(1600)$. We started our hadron spectroscopy programme in 2008 by collecting very high statistics using a 190 GeV/$c$ negative pion beam scattered off a liquid hydrogen (proton) target. The current status and new results from the 2008 data on the search for the $π_1(1600)$ resonance with exotic $J^{PC}=1^{-+}$ quantum numbers obtained from partial-wave analyses of the $ρπ$ and $η'π$ decay channels are presented.
We describe a hadron physics program attainable with a partially instrumented CERN COMPASS spectrometer, involving tracking detectors and moderate-size ECAL2/HCAL2 calorimeters. COMPASS can realize a state-of-the-art hadron beam physics program based on hadron polarizability, hybrid mesons, chiral anomaly, and meson radiative transition studies. We review here the physics motivation for this hadron beam program. We describe the beam, detector, trigger requirements, and hardware/software requirements for this program. The triggers for all this physics can be implemented for simultaneous data ta…
We examine the potential of the COMPASS experiment at CERN to study color transparency via exclusive coherent vector meson production in hard muon-nucleus scattering. It is demonstrated that COMPASS has high sensitivity to test this important prediction of perturbative QCD.
We describe a pion physics program attainable with the CERN COMPASS spectrometer, involving tracking detectors and an electromagnetic calorimeter. COMPASS can realize state-of-the-art pion beam hybrid meson and meson radiative transition studies. We review here the physics motivation for this program. We describe the beam, detector, trigger requirements, and hardware/software requirements for this program. The triggers for all this hybrid meson physics can be implemented for simultaneous data taking. We will investigate hybrid meson production via pion-photon Primakoff and pion-Pomeron diffrac…
A sample of 35,000 events of the type pi-p --> eta'pi-p_slow (eta' --> eta pi-pi+, eta --> gamma gamma) with -t>0.1 GeV^2/c^2 was selected from COMPASS 2008 data for a partial-wave analysis. We study the broad P_+ structure known from previous experiments at lower energies, in particular its phase motion relative to the D_+-wave near the a_2(1320) mass and relative to a broad D_+-wave structure at higher mass. We also find the a_4(2040). We compare kinematic plots for the eta'pi^- and eta pi- final states.
New results for the transverse target spin azimuthal asymmetry A_{UT}^{sin(ϕ-ϕ_S)} for hard exclusive ρ^0-meson production on a transversely polarised ^6LiD and NH_3 target will be presented. The measurement was performed with the COMPASS detector using the 160 GeV/c muon beam of the SPS at CERN. The asymmetry is sensitive to the nucleon helicity-flip generalised parton distribution E, which is related to the orbital angular momentum of quarks in the nucleon.
CERN COMPASS can investigate pion-photon interactions, to achieve a unique Primakoff Coulomb physics program, centered on pion polarizability and hybrid meson structure studies. COMPASS uses 100-280 GeV beams (muon, pion) and a virtual photon target, and magnetic spectrometers and calorimeters to measure the complete kinematics of pion-photon reactions. The COMPASS experiment is scheduled to begin data runs in 2001. Pion polarizabilities and hybrid mesons can be studied via the Primakoff reactions pion + gamma ---> pion + gamma and pion + gamma ---> Hybrid. The electric (alpha) and magnetic (b…
The COMPASS experiment at the CERN SPS features good charged particle tracking and coverage by electromagnetic calorimetry, and our data provide excellent opportunity for simultaneous observation of new states in two different decay modes within the same experiment. The existence of the spin-exotic $π_1(1600)$ resonance in the $ρπ$ decay channel is studied for the first time in COMPASS in both decay modes of the diffractively produced $(3π)^{-}$ system: $π^{-}p \rightarrow π^{-}π^{0}π^{0}p$ and $π^{-}p \rightarrow π^{-}π^{+}π^{-}p$. A preliminary partial-wave analysis (PWA) performed on the 20…
Narrow Xi-pi+- and Xi-bar+pi+- resonances produced by quasi-real photons have been searched for by the COMPASS experiment at CERN. The study was stimulated by the recent observation of an exotic baryonic state decaying into Xi-pi-, at a mass of 1862 MeV, interpreted as a pentaquark. While the ordinary hyperon states Xi(1530)^0 and Xi-bar(1530)^0 are clearly seen, no exotic baryon is observed in the data taken in 2002 and 2003.
COMPASS is a multi-purpose fixed-target experiment at CERN's M2 beam line aimed at studying the structure and spectrum of hadrons. It has collected the so far world's largest data set on diffractive production of the $K^-π^-π^+$ final state, which in principle gives access to all strange mesons. Based on this data set, we performed an elaborate partial-wave analysis. It reveals signals in the mass region of well-known states, such as the $K_2^*(1430)$. In addition, we found indications for a resonance-like signal in the mass region of the $K(1630)$. This state would be a supernumerary state an…
The COMPASS experiment at CERN SPS completed its data taking with hadron beams (p, π, K) in the years 2008 and 2009 by collecting a large set of data using different targets (H2, Pb, Ni, W). These data are dedicated to hadron spectroscopy, where the focus is directed to the search for exotic bound states of quarks and gluons (hybrids, glueballs). The production of such states is known to be favoured in glue-rich environments, e.g. so-called OZI-forbidden processes. The OZI rule postulates that processes with disconnected quark line diagrams are forbidden. On the one hand, the study of the degr…
Generalized parton distributions (GPDs) provide a new and powerful framework for a complete description of the nucleon structure. They can provide a three-dimensional picture of how the quarks and gluons form a nucleon. GPDs can be probed experimentally in hard exclusive meson production or deeply virtual Compton scattering (DVCS). The COMPASS experiment at CERN is a unique place to study these reactions. At COMPASS, a high energy polarized positive or negative muon beam is scattered off a polarized or unpolarized fixed target. First results for exclusive rho^0 meson production are shown. The …
COMPASS is a multi-purpose fixed-target experiment at the CERN Super Proton Synchrotron investigating the structure and spectrum of hadrons. One primary goal is the search for new hadronic states, in particular spin-exotic mesons and glueballs. After a short pilot run in 2004 with a 190 GeV/c $π^-$ beam on a Pb target, which showed a significant spin-exotic $J^{PC} = 1^{-+}$ resonance consistent with the controversial $π_1(1600)$, COMPASS collected large data samples with negative and positive hadron beams on H$_2$, Ni, W, and Pb targets in 2008 and 2009. We present results from a partial-wave…
We present first results of a partial wave analysis of the diffractive reaction $π- Pb \to π- π+ π- Pb$ based on data from the COMPASS experiment taken during a pilot run in 2004 using a 190 GeV/c $π-$ 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 $π- π+ π-$ 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 $ρ(770) π$ decay channel in natural parity exchange. The resonant nature of …
Exotic charmonium-like states have been targeted by various experiments in the last 15 years, but their nature still is unknown. Photo-(muo)production is a new promising instrument to study them. COMPASS, a fixed target experiment at CERN, analyzed the full set of the data collected with a muon beam between 2002 and 2011, covering the range from 7 GeV to 19 GeV in the centre-of-mass energy of the (virtual)photon-nucleon system. Production of the X(3872) state in the reaction $μ^+~N \rightarrow μ^+(J/ψπ^+π^- )π^{\pm} N'$ has been observed with a statistical significance of around 5 $σ$. The sha…
COMPASS is a fixed-target experiment at the CERN SPS that studies the spectrum of light-quark hadrons. In 2009, it collected a large dataset using a $190\,$GeV$/c$ positive hadron beam impinging on a liquid-hydrogen target in order to measure the central exclusive production of light scalar mesons. One of the goals is the search for so-called glueballs, which are hypothetical meson-like objects without valence-quark content. We study the decay of neutral resonances by selecting centrally produced pion pairs from the COMPASS dataset. The angular distributions of the two pseudoscalar mesons are …
The COMPASS experiment at CERN SPS will use hadron beams (pion, kaon and proton) and muons at 50-280 GeV/c and virtual photon targets to investigate, via Primakoff effect, important hadron properties: polarizability, chiral anomaly, radiative transitions and hybrid meson production. We present simulation studies to optimize the beam, detector setup and trigger for measuring with high statistics above topics.
One of the main goals of the COMPASS experiment at CERN is the determination of the gluon contribution to the nucleon spin. To achieve this goal COMPASS uses a naturally polarised muon beam with an energy of 160 GeV and a fixed polarised target. Two types of materials are used for the latter: $^{6}$LiD (polarised deuterons) during the years of 2002-2006 and NH$_{3}$ (polarised protons) in 2007. The gluons in the nucleon can be accessed directly via the Photon Gluon Fusion (PGF) process. Among the channels studied by COMPASS, the production of charmed mesons is the one that tags a PGF interacti…
COMPASS is a multi-purpose fixed-target experiment at CERN aimed at studying the structure and spectrum of hadrons. It has collected the so far world's largest data set on diffractive production of the $K^-π^-π^+$ decay, which in principle gives access to all kaon states. We performed an elaborate partial-wave analysis, using model-selection techniques to select the wave set based on a large systematically constructed pool of allowed partial waves. The partial-wave decomposition reveals signals in the mass region of well-known states, such as $K_1(1270)$ and $K_1(1400)$. In addition, we observ…
The COMPASS fixed-target experiment at CERN SPS is dedicated to the study of hadron structure and dynamics. In the physics programme using hadron beams, the focus is on the detection of new states, in particular the search for $J^{PC}$ exotic states and glueballs. After a short pilot run in 2004 (190 GeV/c negative pion beam, lead target), we started our hadron spectroscopy programme in 2008 by collecting an unprecedented statistics with a negative hadron beam (190 GeV/c) on a liquid hydrogen target. A similar amount of data with positive hadron beam (190 GeV/c) has been taken in 2009, as well…
We calculate the two leading-twist transverse momentum dependent distribution functions of the pion meson, the unpolarized distribution $f_{1π}(x,\bm{k}^2_T)$ and the Boer-Mulders function $h_{1π}^\perp (x,\bm{k}^2_T)$, using the pion wave functions derived from a light-cone approach. The evolution effect of the first $\bm k_T$-moment of the pion Boer-Mulders function is studied by employing an approximate evolution kernel. Using the model resulting distributions, we predict the transverse momentum weighted $\cos2ϕ$ azimuthal asymmetry in the unpolarized $π^- p$ Drell-Yan process which can be …
The COMPASS experiment at the CERN SPS investigates the structure and spectrum of hadrons by scattering high energetic hadrons and polarised muons off various fixed targets. During the years 2002-2007, COMPASS focused on nucleon spin physics using 160 GeV/c polarised $μ^+$ beams on polarised deuteron and proton targets, including measurements of the gluon contribution to the nucleon spin using longitudinal target polarisation as well as studies of transverse spin effects in the nucleon on a transversely polarised target. One major goal of the physics programme using hadron beams is the search …
The goal of the COMPASS experiment at CERN is to study the structure and spectroscopy of hadrons. The two-stage spectrometer has large acceptance and covers a wide kinematic range for charged as well as neutral particles allowing to access a wide range of reactions. Light mesons are studied with negative (mostly $π^-$) and positive ($p$, $π^+$) hadron beams with a momentum of $190\,\text{GeV}/c$. 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\,(\text{GeV}/…
Diffractive dissociation reactions studied at the COMPASS experiment at CERN provide access to the light-meson spectrum. During a pilot run in 2004, using a pion beam and a lead target, 420k π- π- π+ final-state events with masses below 2.5 GeV/c2 were recorded, yielding a significant spin-exotic signal for the controversial π1(1600) resonance. After a significant upgrade of the spectrometer in 2007, the following two years were dedicated to meson spectroscopy. Using again a pion beam, but now with a liquid hydrogen target, an unique statistics of ~60M events of the same final state was gather…
The measurement of transverse spin effects in semi-inclusive deep-inelastic scattering is an important part of the COMPASS physics program. From the analysis of the 2002-2004 data, new results for the transverse target spin asymmetry of z-ordered identified pion and kaon pairs are presented. In addition, a first result for the transverse target spin asymmetry of exclusively produced rho^0 mesons on the deuteron is shown.
The COMPASS experiment at CERN's SPS investigates the structure and excitations of strongly interacting systems. Using reactions of 190 GeV/c pions with protons and nuclear targets, mediated by the strong and electromagnetic interaction, an unprecedented statistical precision has been reached allowing new insight into the properties of light mesons. For the first time the diffractively produced 3pi final state has been analyzed simultaneously in bins of invariant mass and four-momentum transfer using a large set of 88 waves up to a total angular momentum of 6. In addition to a precise determin…
The aim of the COMPASS hadron programme is to study the light-quark hadron spectrum, and in particular, to search for evidence of hybrids and glueballs. COMPASS is a fixed-target experiment at the CERN SPS and features a two-stage spectrometer with high momentum resolution, large acceptance, particle identification and calorimetry. A short pilot run in 2004 resulted in the observation of a spin-exotic state with $J^{PC} = 1^{-+}$ consistent with the debated $\pi1(1600)$. In addition, Coulomb production at low momentum transfer data provide a test of Chiral Perturbation Theory. During 2008 and …
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 targ…
The COMPASS experiment at CERN is dedicated to light hadron spectroscopy with emphasis on the detection of new states, in particular the search for spin exotic states and glueballs. After a short pilot run in 2004 (190 GeV/c $π^{-}$ beam, Pb target) showing significant production strength of an exotic $J^{PC}=1^{-+}$ state at 1.66\,GeV/${\rm c^2}$, we have collected data with 190 GeV/c hadron beams on a liquid hydrogen target in 2008/09. The spectrometer features good coverage by calorimetry and our data provide excellent opportunity for simultaneous observation of new states in different deca…
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 $π^-$) and positive ($p$, $π^+$) hadron beams with a momentum of 190 GeV/$c$. The spectrum of light mesons is investigated in various final states produced in di…
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 wh…
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 oppo…
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 (3π)^- and π^-η' final states based on a large data set of diffractive dissociation of a 190 GeV/c π^- 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^-π^+…
The COMPASS experiment at the CERN-SPS studies the spectrum and the structure of hadrons by scattering high energy hadrons and polarised muons off various fixed targets. Recent results for the hadron programme comprise highlights from different topics. A selective overview is given and, among others, the following results are discussed. The precise determination of the pion polarisability, a long standing puzzle that has been solved now, is presented as well as measurements of radiative widths. The observation of a new narrow axial-vector state, the $a_1(1420)$, as well as deeper insights into…
We present results from a Partial-Wave Analysis (PWA) of diffractive dissociation of 190 GeV/c $π^-$ into $π^-π^+π^-$ 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 $π_1(1600)$, which is considered to be a candidate for a non-$q\bar{q}$ mesonic state. In 2008 COMPASS collected a large diffractive $π^-π^+π^-$ data sample using a hydrogen target. A first comparison with the 2004 data shows a strong target dependence of the production st…
Based on the largest event sample of diffractively produced $π^-π^-π^+$, obtained by a pion beam of $190~\rm{GeV/c}$ momentum, the COMPASS collaboration has performed the most advanced partial wave analysis on multi-body final states, using the isobar model. The large number of waves included in the analysis reduces truncation effects. We have used fourteen out of a total of eighty-eight waves, to extract resonance parameters for eleven light meson candidates, most of them observed previously. The coherence of the analysis and the large variety of systematic studies has allowed to determine ma…
COMPASS is a fixed-target experiment at the CERN SPS which focused on light-quark hadron spectroscopy during the data taking periods in 2008 and 2009. A world-leading data set was collected with a 190GeV/c hadron beam impinging on a liquid hydrogen target in order to study, inter alia, the central exclusive production of glueball candidates in the light-meson sector. Especially the double-Pomeron exchange mechanism is well suited for the production of mesons without valence quark content. We select centrally produced systems with two pseudo-scalar mesons in the final state from the COMPASS dat…
COMPASS is a fixed-target experiment at CERN SPS which focused on light-quark meson spectroscopy during the data-taking periods in 2008 and 2009. The central exclusive production of glueball candidates is studied with a 190GeV/c proton beam impinging on a liquid hydrogen target. We select centrally produced systems with two pseudo-scalar mesons in the final state. The decay of this system is decomposed in terms of partial waves, with particular attention paid to the inherent mathematical ambiguities of the amplitude analysis. We show that simple parametrisation are able to describe the mass de…
The 160 GeV polarised muon beam available at CERN, with positive or negative charge, makes COMPASS a unique place for GPD studies. The first GPD related COMPASS results come from exclusive vector meson production on transversely polarised protons and deuterons. The data were taken in 2003-2010 with large solid-state polarised targets, although without detection of recoil particles. Results on various transverse target spin dependent azimuthal asymmetries are presented and their relations to GPDs are discussed. The dedicated COMPASS GPD program started in 2012 with commissioning of a new long l…
Modern hadron-spectroscopy experiments such as COMPASS collect data samples of unprecedented size, so that novel analysis techniques become possible and necessary. One such technique is the freed-isobar partial-wave analysis (PWA). In this approach, fixed parametrizations for the amplitudes of intermediate states commonly modeled using Breit-Wigner shapes are replaced by sets of step-like functions that are determined from the data. This approach not only reduces the model dependence of partial-wave analyses, but also allows us to study the amplitudes of the intermediate states and their depen…
The COMPASS experiment at CERN is well designed for light-hadron spectroscopy with emphasis on the detection of new states, in particular the search for $J^{PC}$-exotic states and glueballs. We have collected data with 190 GeV/c charged hadron beams on a liquid hydrogen and nuclear targets in 2008/09. The spectrometer features good coverage by electromagnetic calorimetry and a RICH detector further provides $π$ / $K$ separation, allowing for studying final states involving neutral particles like $π^0$ or $η$ as well as hidden strangeness, respectively. We discuss the status of ongoing analyses…
Quantum Chromodynamics (QCD), the theory of strong interactions, in principle describes the interaction of quark and gluon fields. However, due to the self-coupling of the gluons, quarks and gluons are confined into hadrons and cannot exist as free particles. The quantitative understanding of this confinement phenomenon, which is responsible for about 98\% of the mass of the visible universe, is one of the major open questions in particle physics. The measurement of the excitation spectrum of hadrons and of their properties gives valuable input to theory and phenomenology. In the Constituent Q…
COMPASS is a fixed-target experiment at CERN SPS which investigates the structure and spectroscopy of hadrons. During nine weeks in 2008 and 2009, a 190 GeV/c proton beam impinging on a liquid hydrogen target was used in order to study the production of exotic mesons and glueball candidates at central rapidities. As no bias on the production mechanism was introduced by the trigger system, the contribution from diffractive dissociation of the beam proton poses a challenge. We select a centrally produced sample by kinematic cuts and introduce a model to describe the data in terms of partial wave…
I will review the current status of exotic hadrons in the meson sector. There is currently strong evidence that a scalar glueball mixed into the normal scalar mesons has been found. There is also an interesting candidate for the tensor glueball state. Finally, I will discuss hybrid mesons with explicitly exotic quantum numbers.
The COMPASS fixed-target experiment at CERN/SPS is dedicated to the study of hadron structure and spectroscopy, especially the search for spin-exotic states. After having started to study the existence of the spin-exotic $π_1(1600)$ resonance in the 2004 pilot-run data, the new 2008/09 data will enable us to further clarify the situation. Apart from the $π_1(1600)$ resonance, also a spin-exotic $π_1(2000)$ was reported in the past in the $f_1(1285)π$ decay channel by the E852/BNL experiment, however, this state still lacks confirmation. We present a first event selection of the diffractively p…
The longitudinal double spin asymmetry A_1^rho for exclusive leptoproduction of rho^0 mesons, mu + N -> mu + N + rho, is studied using the COMPASS 2002 and 2003 data. The measured reaction is incoherent exclusive rho^0 production on polarised deuterons. The Q^2 and x dependence of A_1^rho is presented in a wide kinematical range: 3x10^-3 < Q^2 < 7 (GeV/c)^2 and 5x10^-5 < x < 0.05. The presented results are the first measurements of A_1^rho at small Q2 (Q2 < 0.1 (GeV/c)^2) and small x (x < 3x10^-3). The asymmetry is in general compatible with zero in the whole kinematical range.
The COMPASS experiment at CERN SPS features charged particle tracking as well as good coverage by electromagnetic calorimetry, and our data provide an excellent opportunity for simultaneous observation of new states in different decay modes by the same experiment. The existence of the spin-exotic $π_1(1600)$ resonance in the $ρπ$ decay channel is studied for the first time at COMPASS in both decay modes of the diffractively produced $(3π)^{-}$ system: $π^{-}p \rightarrow π^{-}π^{+}π^{-}p$ and $π^{-} p \rightarrow π^{-}π^{0}π^{0} p$. A preliminary partial-wave analysis performed on the 2008 pro…
The main goal of the spectroscopy program at COMPASS is to explore the light-meson spectrum below about $2\,\text{GeV}/c^2$ in diffractive production. Our flagship channel is the decay into three charged pions: $p + π^-\to π^-π^-π^+ + p_\text{recoil}$, for which COMPASS has acquired the so far world's largest dataset of roughly $50\,\text{M}$ exclusive events using an $190\,\text{GeV}/c$ $π^-$ beam. Based on this dataset, we performed an extensive partial-wave analysis. In order to extract the resonance parameters of the $π_J$ and $a_J$ states that appear in the $π^-π^-π^+$ system, we performe…
One of the key objectives of the COMPASS experiment at CERN is the determination of the gluon contribution to the nucleon spin. The gluon polarization is measured via photon-gluon fusion in deep-inelastic scattering of 160 GeV/c polarized muons on a polarized 6LiD solid-state target. Photon-gluon fusion is tagged by the observation of charmed mesons or the production of hadron pairs with large transverse momenta p_t. The status of the analysis of the D0 and D0* events and of the high-p_t hadron pairs is shown. The gluon polarization ΔG/G has been determined from the asymmetry of high-p_t hadro…
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 $π^{-}$ 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 (…
COMPASS is a fixed-target experiment at CERN SPS which focused on light-quark hadron spectroscopy during the data taking in 2008 and 2009. A world-leading data set was collected with a 190GeV/c hadron beam impinging on a liquid hydrogen target in order to study the central production of glueball candidates. In this report, we motivate double-Pomeron exchange as a relevant production process for mesons without valence quark content. We select a centrally produced sample from the COMPASS data set recorded with a proton beam and introduce a decomposition into partial waves. Particular attention i…
COMPASS is a fixed-target experiment at the CERN SPS for the investigation of the structure and the dynamics of hadrons. The experimental setup features a large acceptance and high momentum resolution spectrometer including particle identification and calorimetry and is therefore ideal to access a broad range of different final states. Following the promising observation of a spin-exotic resonance during an earlier pilot run, COMPASS focused on light-quark hadron spectroscopy during the years 2008 and 2009. A data set, world leading in terms of statistics and resolution, has been collected wit…
A partial-wave analysis of the mesons from the reaction $π^{-}% p\toπ^{+}π^{-}π^{-}π^{0}π^{0}p$ has been performed. The data show $b_{1}π$ decay of the spin-exotic states $π_{1}(1600)$ and \ $π_{1}(2000)$. Three isovector $2^{-+}$ states were seen in the $ωρ^{-}$ decay channel. In addition to the well known $π_{2}(1670)$, signals were also observed for $π_{2}(1880)$ and $π_{2}(1970)$.
COMPASS is a new fixed target experiment presently in operation at CERN. It has the goal to investigate hadron structure and hadron spectroscopy by using either muon or hadron beams. From measurements of various hadron asymmetries in polarized muon - nucleon scattering it will be possible to determine the contribution of the gluons to the nucleon spin. Main objective of the hadron program is the search of exotic states, and glueballs in particular. This physics programme is carried out with a two-stage magnetic spectrometer, with particle identification and calorimetry in both stages, which ha…
We report on the results of a novel partial-wave analysis based on $50\cdot 10^6$ events from the reaction $π^- + p \to π^-π-^π^+ + p_\text{recoil}$ at 190 $\text{GeV/c}$ incoming beam momentum using the COMPASS spectrometer. A separated analysis in bins of $m_{3π}$ and four-momentum transfer $t'$ reveals the interference of resonant and non-resonant particle production and allows their spectral separation. Besides well known resonances we observe a new iso-vector meson $a_1(1420)$ at a mass of 1420 $\text{MEV\c}^2$ in the $f_0(980)π$ final state only, the origin of which is unclear. We have a…
