Publications (221)661.28 Total impact
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ABSTRACT: New results for the double spin asymmetry $A_1^{\rm p}$ and the proton longitudinal spin structure function $g_1^{\rm p}$ are presented. They were obtained by the COMPASS collaboration using polarised 200 GeV muons scattered off a longitudinally polarised NH$_3$ target. The data were collected in 2011 and complement those recorded in 2007 at 160\,GeV, in particular at lower values of $x$. They improve the statistical precision of $g_1^{\rm p}(x)$ by about a factor of two in the region $x\lesssim 0.02$. A nexttoleading order QCD fit to the $g_1$ world data is performed. It leads to a new determination of the quark spin contribution to the nucleon spin, $\Delta \Sigma$ ranging from 0.26 to 0.36, and to a reevaluation of the first moment of $g_1^{\rm p}$. The uncertainty of $\Delta \Sigma$ is mostly due to the large uncertainty in the present determinations of the gluon helicity distribution. A new evaluation of the Bjorken sum rule based on the COMPASS results for the nonsinglet structure function $g_1^{\rm NS}(x,Q^2)$ yields as ratio of the axial and vector coupling constants $g_{\rm A}/g_{\rm V} = 1.22 \pm 0.05~({\rm stat.}) \pm 0.10~({\rm syst.})$, which validates the sum rule to an accuracy of about 9\%.  [Show abstract] [Hide abstract]
ABSTRACT: The COMPASS collaboration at CERN has measured diffractive dissociation of 190 GeV$/c$ pions into the $\pi^\pi^\pi^+$ final state using a stationary hydrogen target. A partialwave analysis (PWA) was performed in bins of $3\pi$ mass and fourmomentum transfer using the isobar model and the so far largest PWA model consisting of 88~waves. A narrow $J^{PC} = 1^{++}$ signal is observed in the $f_0(980)\,\pi$ channel. We present a resonancemodel study of a subset of the spindensity matrix selecting $3\pi$ states with $J^{PC} = 2^{++}$ and $4^{++}$ decaying into $\rho(770)\,\pi$ and with $J^{PC} = 1^{++}$ decaying into $f_0(980)\,\pi$. We identify a new $a_1$ meson with mass $(1414^{+15}_{13})$ MeV$/c^2$ and width $(153^{+8}_{23})$ MeV$/c^2$. Within the final states investigated in our analysis, we observe the new $a_1(1420)$ decaying only into $f_0(980)\,\pi$, suggesting its exotic nature. To our knowledge, such a state has never been predicted.  [Show abstract] [Hide abstract]
ABSTRACT: The main characteristics of the COMPASS experimental setup for physics with hadron beams are described. This setup was designed to perform exclusive measurements of processes with several charged and/or neutral particles in the final state. Making use of a large part of the apparatus that was previously built for spin structure studies with a muon beam, it also features a new target system as well as new or upgraded detectors. The hadron setup is able to operate at the high incident hadron flux available at CERN. It is characterised by large angular and momentum coverages, large and nearly flat acceptances, and good two and threeparticle mass resolutions. In 2008 and 2009 it was successfully used with positive and negative hadron beams and with liquid hydrogen and solid nuclear targets. This article describes the new and upgraded detectors and auxiliary equipment, outlines the reconstruction procedures used, and summarises the general performance of the setup.Nuclear Instruments and Methods in Physics Research Section A Accelerators Spectrometers Detectors and Associated Equipment 10/2014; 779. DOI:10.1016/j.nima.2015.01.035 · 1.32 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: Exclusive production of $\eta\pi^$ and $\eta'\pi^$ has been studied with a $191\,\textrm{GeV}/c$ $\pi^$ beam impinging on a hydrogen target at COMPASS (CERN). Partialwave analyses reveal different odd/even angular momentum ($L$) characteristics in the inspected invariant mass range up to $3\,\textrm{GeV}/c^2$. A striking similarity between the two systems is observed for the $L=2,4,6$ intensities (scaled by kinematical factors) and the relative phases. The known resonances $a_2(1320)$ and $a_4(2040)$ are in line with this similarity. In contrast, a strong enhancement of $\eta'\pi^$ over $\eta\pi^$ is found for the $L=1,3,5$ waves, which carry non$q\bar q$ quantum numbers. The $L=1$ intensity peaks at $1.7\,\textrm{GeV}/c^2$ in $\eta'\pi^$ and at $1.4\,\textrm{GeV}/c^2$ in $\eta\pi^$, the corresponding phase motions with respect to $L=2$ are different.  [Show abstract] [Hide abstract]
ABSTRACT: Measurements of the Collins and Sivers asymmetries for charged pions and charged and neutral kaons produced in semiinclusive deepinelastic scattering of high energy muons off transversely polarised protons are presented. The results were obtained using all the available COMPASS proton data, which were taken in the years 2007 and 2010. The Collins asymmetries exhibit in the valence region a nonzero signal for pions and there are hints of nonzero signal also for kaons. The Sivers asymmetries are found to be positive for positive pions and kaons and compatible with zero otherwise.  [Show abstract] [Hide abstract]
ABSTRACT: A search for the exclusive production of the $Z_c^{\pm}(3900)$ hadron by virtual photons has been performed in the channel $Z_c^{\pm}(3900)\rightarrow J/\psi \pi^{\pm}$. The data cover the range from 7 GeV to 19 GeV in the centreofmass energy of the photonnucleon system. The full set of the COMPASS data set collected with a muon beam between 2002 and 2011 has been used. An upper limit for the ratio $BR(Z_c^{\pm}(3900)\rightarrow J/\psi \pi^{\pm} )\times \sigma_{ \gamma~N \rightarrow Z_c^{\pm}(3900)~ N} /\sigma_{\gamma~N \rightarrow J/\psi~ N}$ of $3.7\times10^{3}$ has been established at the confidence level of 90%.Physics Letters B 07/2014; 742. DOI:10.1016/j.physletb.2015.01.042 · 6.02 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: Exclusive production of the isoscalar vector mesons $\omega$ and $\phi$ is measured with a 190 GeV$/c$ proton beam impinging on a liquid hydrogen target. Cross section ratios are determined in three intervals of the Feynman variable $x_{F}$ of the fast proton. A significant violation of the OZI rule is found, confirming earlier findings. Its kinematic dependence on $x_{F}$ and on the invariant mass $M_{p\mathrm{V}}$ of the system formed by fast proton $p_\mathrm{fast}$ and vector meson $V$ is discussed in terms of diffractive production of $p_\mathrm{fast}V$ resonances in competition with central production. The measurement of the spin density matrix element $\rho_{00}$ of the vector mesons in different selected reference frames provides another handle to distinguish the contributions of these two major reaction types. Again, dependences of the alignment on $x_{F}$ and on $M_{p\mathrm{V}}$ are found. Most of the observations can be traced back to the existence of several excited baryon states contributing to $\omega$ production which are absent in the case of the $\phi$ meson. Removing the lowmass $M_{p\mathrm{V}}$ resonant region, the OZI rule is found to be violated by a factor of eight, independently of $x_\mathrm{F}$.Nuclear Physics B 05/2014; 886. DOI:10.1016/j.nuclphysb.2014.07.020 · 3.95 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: The COMPASS collaboration at CERN has investigated pion Compton scattering, $\pi^\gamma\rightarrow \pi^\gamma$, at centreofmass energy below 3.5 pion masses. The process is embedded in the reaction $\pi^\mathrm{Ni}\rightarrow\pi^\gamma\;\mathrm{Ni}$, which is initiated by 190 GeV pions impinging on a nickel target. The exchange of quasireal photons is selected by isolating the sharp Coulomb peak observed at smallest momentum transfers, $Q^2<0.0015$ (GeV/$c$)$^2$. From a sample of 63000 events, the electric pion polarisability is determined to be $\alpha_\pi\ =\ (\,2.0\ \pm\ 0.6_{ stat}\ \pm\ 0.7_{ syst}\,)\ \times 10^{4}\,\mbox{fm}^3$ under the assumption $\alpha_\pi=\beta_\pi$ that relates the electric and magnetic dipole polarisabilities. This result is in tension with previous dedicated measurements, while it is found in agreement with the expectation from chiral perturbation theory. An additional measurement replacing pions by muons, for which the crosssection behavior is unambigiously known, was performed for an independent estimate of the systematic uncertainty.Physical Review Letters 05/2014; 114(6). DOI:10.1103/PhysRevLett.114.062002 · 7.51 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: The COMPASS Collaboration at CERN has investigated the reaction $\pi^ \gamma \rightarrow \pi^\pi^\pi^+$ embedded in the Primakoff reaction of $190~\textrm{GeV}$ pions scattering in the Coulomb field of a lead target, $\pi^ \text{Pb} \rightarrow \pi^\pi^\pi^+ \text{Pb}$. Exchange of quasireal photons is selected by isolating the sharp Coulomb peak observed at momentum transfer below $0.001~(\text{GeV}/c)^2$. Using a partialwave analysis the amplitudes and relative phases of the $a_2(1320)$ and $\pi_2(1670)$ mesons have been extracted, and the Coulomb and the diffractive contributions have been disentangled. Measuring absolute production cross sections we have determined the radiative width of the $a_2(1320)$ to be $\Gamma_0(a_2(1320) \rightarrow \pi\gamma) = (358 \pm 6_{\textrm{stat}} \pm 42_{\textrm{syst}})~\textrm{keV}$. As the first measurement, $\Gamma_0(\pi_2(1670) \rightarrow \pi\gamma) = (181 \pm 11_{\textrm{stat}} \pm 27_{\textrm{syst}})~\textrm{keV} \cdot (\textrm{BR}^{\textrm{PDG}}_{f_2 \pi}/\textrm{BR}_{f_2 \pi})$ is obtained for the radiative width of the $\pi_2(1670)$, where in this analysis the branching ratio $\textrm{BR}^{\textrm{PDG}}_{f_2 \pi}=0.56$ has been used. We compare these values to previous measurements and theoretical predictions.  [Show abstract] [Hide abstract]
ABSTRACT: A measurement of the azimuthal asymmetry in dihadron production in deepinelastic scattering of muons on transversely polarised proton (NH$_{3}$) targets are presented. They provide independent access to the transversity distribution functions through the measurement of the Collins asymmetry in single hadron production. The data were taken in the year $2010$ with the COMPASS spectrometer using a $160\,\mbox{GeV}/c$ muon beam of the CERN SPS, increasing by a factor of about three the available statistics of the previously published data taken in the year $2007$. The measured sizeable asymmetry is in good agreement with the published data. An approximate equality of the Collins asymmetry and the dihadron asymmetry is observed, suggesting a common physical mechanism in the underlying fragmentation.Physics Letters B 01/2014; 736. DOI:10.1016/j.physletb.2014.06.080 · 6.02 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: Spinaveraged asymmetries in the azimuthal distributions of positive and negative hadrons produced in deep inelastic scattering were measured using the CERN SPS muon beam at $160$ GeV/c and a $^6$LiD target. The amplitudes of the three azimuthal modulations $\cos\phi_h$, $\cos2\phi_h$ and $\sin\phi_h$ were obtained binning the data separately in each of the relevant kinematic variables $x$, $z$ or $p_T^{\,h}$ and binning in a threedimensional grid of these three variables. The amplitudes of the $\cos \phi_h$ and $\cos 2\phi_h$ modulations show strong kinematic dependencies both for positive and negative hadrons.Nuclear Physics B 01/2014; 886. DOI:10.1016/j.nuclphysb.2014.07.019 · 3.95 Impact Factor  Physical Review C 11/2013; 88(5). DOI:10.1103/PhysRevC.88.055208 · 3.88 Impact Factor
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ABSTRACT: Exclusive production of $\rho^0$ mesons was studied at the COMPASS experiment by scattering 160 GeV/$c$ muons off transversely polarised protons. Five singlespin and three doublespin azimuthal asymmetries were measured as a function of $Q^2$, $x_{Bj}$, or $p_{T}^{2}$. The $\sin \phi_S$ asymmetry is found to be $0.019 \pm 0.008(stat.) \pm 0.003(syst.)$. All other asymmetries are also found to be of small magnitude and consistent with zero within experimental uncertainties. Very recent calculations using a GPDbased model agree well with the present results. The data is interpreted as evidence for the existence of chiralodd, transverse generalized parton distributions.Physics Letters B 10/2013; 731. DOI:10.1016/j.physletb.2014.02.005 · 6.02 Impact Factor 
Dataset: 2007 PRC Cosy K+K

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ABSTRACT: Exclusive measurements of the quasifree $pn \to pp\pi^0\pi^$ reaction have been performed by means of $pd$ collisions at $T_p$ = 1.2 GeV using the WASA detector setup at COSY. Total and differential cross sections have been obtained covering the energy region $\sqrt s$ = (2.35  2.46) GeV, which includes the region of the ABC effect and its associated resonance structure. No ABC effect, {\it i.e.} lowmass enhancement is found in the $\pi^0\pi^$invariant mass spectrum  in agreement with the constraint from Bose statistics that the isovector pion pair can not be in relative swave. At the upper end of the covered energy region $t$channel processes for Roper, $\Delta(1600)$ and $\Delta\Delta$ excitations provide a reasonable description of the data, but at low energies the measured cross sections are much larger than predicted by such processes. Adding a resonance amplitude for the resonance at $m$=~2.37 GeV with $\Gamma$ =~70 MeV and $I(J^P)=~0(3^+)$ observed recently in $pn \to d\pi^0\pi^0$ and $pn \to d\pi^+\pi^$ reactions leads to an agreement with the data also at low energies.Physical Review C 06/2013; 88:055208. · 3.88 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: Multiplicities of charged hadrons produced in deep inelastic muon scattering off a $^6$LiD target have been measured as a function of the DIS variables $x_{Bj}$, $Q^2$, $W^2$ and the final state hadron variables $p_T$ and $z$. The $p_T^2$ distributions are fitted with a single exponential function at low values of $p_T^2$ to determine the dependence of $\langle p_T^2 \rangle$ on $x_{Bj}$, $Q^2$, $W^2$ and $z$. The $z$dependence of $\langle p_T^2 \rangle$ is shown to be a potential tool to extract the average intrinsic transverse momentum squared of partons, $\langle k_{\perp}^2 \rangle$, as a function of $x_{Bj}$ and $Q^2$ in a leading order QCD parton model.European Physical Journal C 05/2013; 73(8). DOI:10.1140/epjc/s1005201325316 · 5.44 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: An exclusive measurement of the dd > 3He n \pi 0 reaction was carried at a beam momentum of p = 1.2 GeV/c using the WASAatCOSY facility. For the first time data on the total cross section as well as differential distributions were obtained. The data are described with a phenomenological approach based on a combination of a quasifree model and a partial wave expansion for threebody reaction. The total cross section is found to be \sigma(tot) = (2.89 + 0.01(stat) + 0.06(sys) + 0.29(norm)) \mu b. The contribution of the quasifree processes (with the neutron being target or beam spectator) accounts for 38% of the total cross section and dominates the differential distributions in specific regions of the phase space. The remaining part of the cross section can be described within a partial wave decomposition indicating the significance of pwave contributions in the final state.Physical Review C 04/2013; 88:014004. DOI:10.1103/PhysRevC.88.014004 · 3.88 Impact Factor 
Article: Study of Σ(1385) and Ξ(1321) hyperon and antihyperon production in deep inelastic muon scattering
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ABSTRACT: Large samples of \Lambda, \Sigma(1385) and \Xi(1321) hyperons produced in deepinelastic muon scattering off a ^6LiD target were collected with the COMPASS experimental setup at CERN. The relative yields of \Sigma(1385)^+, \Sigma(1385)^, \bar{\Sigma}(1385)^, \bar{\Sigma}(1385)^+, \Xi(1321)^, and \bar{\Xi}(1321)^+ hyperons decaying into \Lambda(\bar{\Lambda})\pi were measured. The heavy hyperon to \Lambda and heavy antihyperon to \bar{\Lambda} yield ratios were found to be in the range 3.8% to 5.6% with a relative uncertainty of about 10%. They were used to tune the parameters relevant for strange particle production of the LEPTO Monte Carlo generator.European Physical Journal C 04/2013; 73(10). DOI:10.1140/epjc/s1005201325819 · 5.44 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: The presently world largest data sample of pi0 > gamma e+e decays containing nearly 5E5 events was collected using the WASA detector at COSY. A search for a dark photon U produced in the pi0 > gamma U > gamma e+e decay from the pp>pp\pi^0 reaction was carried out. An upper limit on the square of the Ugamma mixing strength parameter epsilon^2 of 5e6 at 90% CL was obtained for the mass range 20 MeV <M_U< 100 MeV. This result together with other recent experimental limits significantly reduces the M_U vs. \epsilon^2 parameter space preferred by the measured value of the muon anomalous magnetic moment.Physics Letters B 04/2013; 726:187. DOI:10.1016/j.physletb.2013.08.055 · 6.02 Impact Factor
Publication Stats
2k  Citations  
661.28  Total Impact Points  
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Institutions

1976–2014

University of Bonn
 HelmholtzInstitut für Strahlen und Kernphysik
Bonn, North RhineWestphalia, Germany


2005–2007

Bielefeld University
Bielefeld, North RhineWestphalia, Germany


1991–2003

University of Hamburg
 Institut für Experimentalphysik
Hamburg, Hamburg, Germany


1967

Max Planck Institute for Nuclear Physics
Heidelburg, BadenWürttemberg, Germany
