Diffractive pion production at COMPASS -- First results on 3$π$ final states - neutral mode
Frank Nerling
备注：
7 pages, 7 figures, to appear in the AIP Conf. Proc., Hadron 2009, XIII Intern. Conference on Hadron Spectroscopy, Nov 29th - Dec 4th 2009, Florida State University
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 opportunity for simultaneous observation of new states in two different decay modes in the same experiment. The diffractively produced $(3π)^{-}$ system for example can be studied in both modes $π^{-}p \rightarrow π^{-}π^{+}π^{-}p$ and $π^{-}~p \rightarrow π^{-}π^{0}π^{0}~p$. Charged and neutral mode rely on completely different parts of the spectrometer. Observing a new state in both modes provides important cross-check. First results of a preliminary PWA performed on the 2008 data are presented.
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…
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 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…
The COMPASS spectrometer at CERN has collected a large data set for diffractive three-pion production of $46\times10^6$ exclusive events. Based on previous conventional Partial-Wave Analyses (PWA), we performed a `freed-isobar PWA' on the same data, removing model assumptions on the dynamic isobar amplitudes for dominating waves. In this analysis, we encountered continuous mathematical ambiguities, which we were able to identify and resolve. This analysis gives an unprecedented insight in the interplay of $2π$ and $3π$ dynamics in the process. As an example we show results for a spin-exotic wa…
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^+…
In order to study diffractive dissociation reactions, COMPASS has taken data with a 190 GeV/$c$ pion beam impinging on a liquid hydrogen target in 2008 and 2009. At squared four-momentum transfers to the target t' between 0.1 GeV$^2/c^2$ and 1.0 GeV$^2/c^2$ the number of events with three pions in the final state is about an order of magnitude larger than that acquired by any previous experiment. In COMPASS, the three-pion final state can be studied in the two channels $π^-π^-π^+$ and $π^-π^0π^0$. The large data sample in particular for the $π^-π^-π^+$ channel allows to find even small signals…
The COMPASS experiment at CERN has collected a large data sample of 50 million diffractively produced $π^-π^+π^-$ events using a $190\,$GeV$/c$ negatively charged hadron beam. The partial-wave analysis (PWA) of these high-precision data reveals previously unseen details. The PWA, which is currently limited by systematic uncertainties, is based on an isobar model, where multi-particle decays are described as subsequent two-body decays and where a prior-knowledge parametrization for the intermediate two-pion resonances has to be assumed -- usually a Breit-Wigner amplitude -- thus increasing syst…
At the COMPASS experiment, diffractive dissociation of the beam proton is one of the dominant processes for the 190GeV/c positive hadron beam impinging on a liquid hydrogen target. The status of the analysis of the reactions pp -> p_f pi^+ pi^- p_s and pp -> p_f K^+ K^- p_s is presented, where dominant features of the light-baryon spectrum become clearly visible. Furthermore, partial-wave analysis techniques to disentangle these spectra are discussed.
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 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 value of the pion polarizability is predicted with high precision by Chiral Perturbation Theory. However, the existing experimental values are at tension with this prediction as well as among themselves. The COMPASS experiment at the CERN SPS accesses pion-photon reactions via the Primakoff effect, where high-energetic pions react with the quasi-real photon field surrounding the target nuclei. Flagship channel is the Primakoff reaction in which a single real photon is produced, giving access to pion Compton scattering. Using this process the pion polarizability is extracted from the measur…
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…
The electric ($α_π$) and the magnetic ($β_π$) polarisabilities are fundamental properties of the pion characterising the rigidity of its internal structure. They have been precisely measured at the COMPASS experiment at CERN with a $π^{-}$ beam of 190~GeV/c assuming $α_π+β_π=0$. Muons of the same momentum were used for controlling of systematic effects. The obtained result $α_π=-β_π=(2.0\pm 0.6_{stat.}\pm 0.7_{syst.})\times 10^{-4} fm^3$ is in agreement with the prediction of the Chiral Perturbation Theory.
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…
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 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…
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…
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…
We investigate the Sivers asymmetry in the pion-induced single polarized Drell-Yan process in the theoretical framework of the transverse momentum dependent factorization up to next-to-leading logarithmic order of QCD. Within the TMD evolution formalism of parton distribution functions, the recently extracted nonperturbative Sudakov form factor for the pion distribution functions as well as the one for the Sivers function of the proton are applied to numerically estimate the Sivers asymmetry in the $π^- p$ Drell-Yan at the kinematics of the COMPASS at CERN. In the low $b$ region, the Sivers fu…
We investigate the theoretical framework of the $\cos 2ϕ$ azimuthal asymmetry contributed by the coupling of two Boer-Mulders functions in the dilepton production unpolarized $πp$ Drell-Yan process by applying the transverse momentum dependent factorization at leading order. We adopt the model calculation results of the unpolarized distribution function $f_1$ and Boer-Mulders function $h_1^\perp$ of pion meson from the light-cone wave functions. We take into account the transverse momentum evolution effects for both the distribution functions of pion and proton by adopting the existed extracti…
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 h…
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 one of the leading experiments studying the nucleon spin structure. Until 2012 the Parton Distribution Functions and the Transverse Momentum Dependent Parton Distribution Functions (TMDs) were extensively studied at COMPASS using Semi-Inclusive Deep Inelastic Scattering measurements. In 2015, the Drell-Yan measurements with a negative pion beam interacting with a transversely polarized ammonia target have started and will be continued through 2018. The goal is to access the TMDs of both pions and protons without any prior knowledge about fragmentation function…
Modern Determination of Pion and Kaon Fragmentation Functions from SIA and High-Precision COMPASS SIDIS Multiplicities
School of Particles and Accelerators, Institute for Research in Fundamental Sciences (IPM)（粒子与加速器学院，基础科学研究院）；AGH University, Faculty of Physics and Applied Computer Science（AGH科技大学，物理与应用计算机科学系）；PRISMA ++ Cluster of Excellence, Institut für Physik, Johannes Gutenberg-Universität Mainz（PRISMA ++ 卓越集群，物理学研究所，约翰内斯·古腾堡美因茨大学）；Institut für Theoretische Physik, Universität Münster（理论物理研究所，明斯特大学）
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 …
The study of transverse spin and transverse momentum effects is an important part of the scientific program of COMPASS, a fixed target experiment at the CERN SPS taking data since 2002. The studies are carried on by measuring the hadrons produced in deep inelastic scattering (DIS) of 160 GeV/c muons off different targets. Among the possible asymmetries in the hadron azimuthal distributions, particularly interesting are the Collins and Sivers asymmetries which the COMPASS Collaboration has measured using transversely polarised deuteron and proton targets. The new results for charged pions and k…
The COMPASS experiment is a fixed target experiment at the CERN SPS using muon and hadron beams for the investigation of the spin structure of the nucleon and hadron spectroscopy. The main objective of the muon physics program is the study of the spin of the nucleon in terms of its constituents, quarks and gluons. COMPASS has accumulated data during 6 years scattering polarized muons off a longitudinally or a transversely polarized deuteron (6LiD) or proton (NH3) target. Results for the gluon polarization are obtained from longitudinal double spin cross section asymmetries using two different …
COMPASS is a fixed target experiment at CERN investigating the spin structure of the nucleon and performing hadron spectroscopy. The transverse spin structure of the nucleon is studied in semi-inclusive deep-inelastic scattering of 160 GeV/c muons off a transversely polarized proton or deuteron target. In 2002-2005, a transversely polarized 6LiD, and in 2007 a transversely polarized NH3 target were used. To get access to the transversity distribution, different single-spin asymmetries have been measured: the Collins asymmetry, the hadron-pair asymmetry and the transverse lambda polarization. I…
In the polarised Drell-Yan experiment at the COMPASS facility at CERN the beam of negatively-charged pions with 190 GeV/c momentum and intensity about $10^8$ pions/s interacted with transversely polarised NH$_3$ target. Muon pairs produced in Drell-Yan process (DY) were detected. Recently, the first ever Transverse Spin Asymmetries (TSAs) measurement in DY has been presented by COMPASS. A complementary analysis of the TSAs weighted by powers of the dimuon transverse momentum $q_T$ are presented. In the Transverse Momentum Dependent (TMD) PDF formalism, the $q_T$-weighted TSAs can be written in…
The COMPASS experiment at the CERN SPS has studied the diffractive dissociation of negative pions into the pi- pi- pi+ final state using a 190 GeV/c pion beam hitting a lead target. A partial wave analysis has been performed on a sample of 420000 events taken at values of the squared 4-momentum transfer t' between 0.1 and 1 GeV^2/c^2. The well-known resonances a1(1260), a2(1320), and pi2(1670) are clearly observed. In addition, the data show a significant natural parity exchange production of a resonance with spin-exotic quantum numbers J^PC = 1-+ at 1.66 GeV/c^2 decaying to rho pi. The resona…
Muon pairs created in the Drell-Yan process of a negative pion beam of 190 GeV/c momentum impinging on a transversely polarised NH$_3$ target were detected with the COMPASS spectrometer at CERN in 2015. In addition to the extraction of the transverse spin asymmetries (TSAs) from these data, a complementary analysis of the TSAs weighted by powers of the dimuon momentum $q_T$ has been performed and is presented in this paper. In the transverse momentum dependent parton distribution functions (TMD PDFs) formalism, the $q_T$-weighted TSAs can be interpreted in terms of products of the TMD PDFs of …
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 is thus able to measure 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 light-meson spectrum is investigated in various final states produced in diff…
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}/…
The electric $α_π$ and magnetic $β_π$ charged pion Compton polarizabilities are of fundamental interest in the low-energy sector of quantum chromodynamics (QCD).They are directly linked to the phenomenon of spontaneously broken chiral symmetry within QCD and to the dynamics of the pion-photon interaction.The combination ($α_π-β_π$) was measured by:(1) CERN COMPASS via radiative pion Primakoff scattering (Bremsstrahlung) in the nuclear Coulomb field, $π^- Z \rightarrow π^-Z γ$, (2) SLAC PEP Mark-II via two-photon production of pion pairs, $γγ\rightarrowπ^+π^-$, and (3) Mainz Microtron via radia…
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 …
COMPASS at CERN is preparing for a new series of measurements on the nucleon structure comprising deep virtual Compton scattering and hard exclusive meson production using muon beams, as well as Drell-Yan reactions using a polarised proton target and a negative pion beam. The former will mainly constrain the generalised parton distribution H and determine the transverse size of the nucleon, while the latter measurements will provide information on transverse-momentum dependent parton distribution functions. The projected results of the programme and the necessary hardware upgrades are discusse…
Fragmentation functions represent a key ingredient to address the proton spin structure in semi-inclusive deep-inelastic scattering and proton-proton collisions. They can not be determined from perturbative Quantum Chromodynamics and have to be extracted from experimental data in different processes. The COMPASS experiment at CERN provides a large data sample and covers a wide kinematic range for precise measurement of hadron multiplicities, directly connected to fragmentation functions. Recent, full-differential results on pion and kaon multiplicities are presented and discussed
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 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…
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 electric $α_π$ and magnetic $β_π$ charged pion Compton polarizabilities are of fundamental interest in the low-energy sector of quantum chromodynamics (QCD). They characterize the induced dipole moments of the pion during $γπ$ Compton scattering. Pion polarizabilities affect the shape of the $γπ$ Compton scattering angular distribution.The combination $(α_π - β_π)$ was measured by: (1) CERN COMPASS via $π Z \rightarrow π Z γ$ radiative pion Primakoff scattering in the nuclear Coulomb field, (2) SLAC PEP Mark-II via two-photon production of pion pairs, $γ γ \rightarrow π^+ π^-$, and (3) Mai…
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…
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…
In the polarised Drell-Yan experiment at the COMPASS facility in CERN pion beam with momentum of 190 GeV/c and intensity about $10^8$ pions/s interacted with transversely polarised NH$_3$ target. Muon pairs produced in Drel-Yan process were detected. The measurement was done in 2015 as the 1st ever polarised Drell-Yan fixed target experiment. The hydrogen nuclei in the solid-state NH$_3$ were polarised by dynamic nuclear polarisation in 2.5 T field of large-acceptance superconducting magnet. Large helium dilution cryostat was used to cool the target down below 100 mK. Polarisation of hydrogen …
