Publications (93)56.64 Total impact
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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.08/2014;  [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.08/2014;  [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%.07/2014;  [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 $x_\mathrm{F}$.05/2014;  [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.05/2014;  [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.03/2014;  [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.01/2014;  [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.01/2014;  [Show abstract] [Hide abstract]
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.10/2013; 
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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.05/2013;  [Show abstract] [Hide abstract]
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.04/2013; 

Article: Leading order determination of the gluon polarisation from DIS events with highpTpT hadron pairs
Physics Letters B. 01/2013; 718(3):922–930.  Nuclear Physics B. 12/2012; 865(1):1–20.
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ABSTRACT: The gluon polarisation in the nucleon was measured using open charm production by scattering 160 GeV/c polarised muons off longitudinally polarised protons or deuterons. The data were taken by the COMPASS collaboration between 2002 and 2007. A detailed account is given of the analysis method that includes the application of neural networks. Several decay channels of D^0 mesons are investigated. Longitudinal spin asymmetries of the D meson production crosssections are extracted in bins of D^0 transverse momentum and energy. At leading order QCD accuracy the average gluon polarisation is determined as (Delta g/G)^LO=0.06 +/ 0.21 (stat.) +/ 0.08 (syst.) at the scale <mu^2> ~13 (GeV/c)^2 and an average gluon momentum fraction ~ 0.11. For the first time, the average gluon polarisation is also obtained at nexttoleading order QCD accuracy as (Delta g/G)^NLO = 0.13 +/ 0.15 (stat.) +/ 0.15 (syst.) at the scale <mu^2> ~ 13 (\GeV/c)^2 and ~ 0.20.11/2012;  [Show abstract] [Hide abstract]
ABSTRACT: The production of D* and D mesons in inelastic scattering of 160 GeV/c muons off a ^6LiD target has been investigated with the COMPASS spectrometer at CERN for 0.003 (GeV/c)^2 < Q^2 < 10 (GeV/c)^2 and 3x10^5< x_Bj < 0.1. The study is based on 8100 events where a D^0 or anti D^0 is detected subsequently to a D*+ or D* decay, and on 34000 events, where only a D^0 or anti D^0 is detected. Kinematic distributions of D*, D and K*_2 are given as a function of their energy E, transverse momentum p_T, energy fraction z, and of the virtual photon variables nu, Q^2 and x_Bj. Semiinclusive differential D* production crosssections are compared with theoretical predictions for D* production via photongluon fusion into open charm. The total observed production crosssection for D*+/ mesons with laboratory energies between 22 and 86 GeV is 1.9 nb. Significant crosssection asymmetries are observed between D*+ and D* production for nu<40 GeV and z>0.6.European Physical Journal C 11/2012; 72(12). · 5.25 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: The transverse target spin azimuthal asymmetry A_UT in hard exclusive production of rho^0 mesons was measured at COMPASS by scattering 160 GeV/c muons off transversely polarised protons and deuterons. The measured asymmetry is sensitive to the nucleon helicityflip generalised parton distributions E^q, which are related to the orbital angular momentum of quarks in the nucleon. The Q^2, x_B and p_t^2 dependence of A_UT is presented in a wide kinematic range. Results for deuterons are obtained for the first time. The measured asymmetry is small in the whole kinematic range for both protons and deuterons, which is consistent with the theoretical interpretation that contributions from GPDs E^u and E^d approximately cancel.07/2012;  [Show abstract] [Hide abstract]
ABSTRACT: The differential cross section for production of charged hadrons with high transverse momenta in scattering of 160\,GeV/$c$ muons off nucleons at low photon virtualities has been measured at the COMPASS experiment at CERN. The results, which cover transverse momenta from 1.1\,GeV/$c$ to 3.6\,GeV/$c$, are compared to a perturbative Quantum Chromodynamics (pQCD) calculation, in order to evaluate the applicability of pQCD to this process in the kinematic domain of the experiment. The shape of the calculated differential cross section as a function of transverse momentum is found to be in good agreement with the experimental data, but the absolute scale is underestimated by nexttoleading order (NLO) pQCD. The inclusion of allorder resummation of large logarithmic threshold corrections reduces the discrepancy from a factor of three to four to a factor of two. The dependence of the cross section on the pseudorapidity and on virtual photon energy fraction is investigated. Finally the dependence on the charge of the hadrons is discussed.07/2012; 
Article: Transverse spin effects in hadronpair production from semiinclusive deep inelastic scattering
Physics Letters B. 06/2012; 713(1):10–16.
Publication Stats
541  Citations  
56.64  Total Impact Points  
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Institutions

1998–2011

University of Bonn
 Physics Institute
Bonn, North RhineWestphalia, Germany
