Abstract NarrowΞπ andΞ + π ± resonances produced by quasi-real photons have been search ed for by the COMPASS experiment at CERN. The study was stimulat ed by the recent observation of an exotic baryonic state decaying into Ξπ−, at a mass of 1862 MeV, interpreted as a pentaquark. While the ordinary hyperon states Ξ(1530)0 andΞ(1530)0 are clearly seen, no exotic baryon is observed in the data taken in 2002 an d 003.

C. Adolph h, R. Akhunzyanov g, M.G. Alekseev x, V.Yu. Alexakhin g, Yu. Alexandrov o,1, G.D. Alexeev g, A. Amoroso aa,ab, V. Andrieux v, V. Anosov g, A. Austregesilo j,q, B. Badełek ae, F. Balestra aa,ab, J. Barth d, G. Baum a, R. Beck c, Y. Bedfer v, A. Berlin b, J. Bernhard m, R. Bertini aa,ab, K. Bicker j,q, J. Bieling d, R. Birsa x, J. Bisplinghoff c, M. Bodlak s, M. Boer v, P. Bordalo l,2, F. Bradamante y,j, C. Braun h, A. Bravar x, A. Bressan y,x,∗, M. Büchele i, E. Burtin v, L. Capozza v, M. Chiosso aa,ab, S.U. Chung q,3, A. Cicuttin z,x, M.L. Crespo z,x, Q. Curiel v, S. Dalla Torre x, S.S. Dasgupta f, S. Dasgupta x, O.Yu. Denisov ab, S.V. Donskov u, N. Doshita ag, V. Duic y, W. Dünnweber p, M. Dziewiecki af, A. Efremov g, C. Elia y,x, P.D. Eversheim c, W. Eyrich h, M. Faessler p, A. Ferrero v, A. Filin u, M. Finger s, M. Finger Jr. s, H. Fischer i, C. Franco l, N. du Fresne von Hohenesche m,j, J.M. Friedrich q, V. Frolov j, R. Garfagnini aa,ab, F. Gautheron b, O.P. Gavrichtchouk g, S. Gerassimov o,q, R. Geyer p, M. Giorgi y,x, I. Gnesi aa,ab, B. Gobbo x, S. Goertz d, M. Gorzellik i, S. Grabmüller q, A. Grasso aa,ab, B. Grube q, R. Gushterski g, A. Guskov g, T. Guthörl i,4, F. Haas q, D. von Harrach m, D. Hahne d, R. Hashimoto ag, F.H. Heinsius i, F. Herrmann i, C. Heß b, F. Hinterberger c, Ch. Höppner q, N. Horikawa r,5, N. d’Hose v, S. Huber q, S. Ishimoto ag,6, A. Ivanov g, Yu. Ivanshin g, T. Iwata ag, R. Jahn c, V. Jary t, P. Jasinski m, P. Joerg i, R. Joosten c, E. Kabuß m, D. Kang m, B. Ketzer q, G.V. Khaustov u, Yu.A. Khokhlov u,7, Yu. Kisselev g, F. Klein d, K. Klimaszewski ad, J.H. Koivuniemi b, V.N. Kolosov u, K. Kondo ag, K. Königsmann i, I. Konorov o,q, V.F. Konstantinov u, A.M. Kotzinian aa,ab, O. Kouznetsov g, Z. Kral t, M. Krämer q, Z.V. Kroumchtein g, N. Kuchinski g, F. Kunne v,∗, K. Kurek ad, R.P. Kurjata af, A.A. Lednev u, A. Lehmann h, S. Levorato x, J. Lichtenstadt w, A. Maggiora ab, A. Magnon v, N. Makke y,x, G.K. Mallot j, C. Marchand v, A. Martin y,x, J. Marzec af, J. Matousek s, H. Matsuda ag, T. Matsuda n, G. Meshcheryakov g, W. Meyer b, T. Michigami ag, Yu.V. Mikhailov u, Y. Miyachi ag, A. Nagaytsev g, T. Nagel q, F. Nerling i, S. Neubert q, D. Neyret v, V.I. Nikolaenko u, J. Novy t, W.-D. Nowak i, A.S. Nunes l, I. Orlov g, A.G. Olshevsky g, M. Ostrick m, R. Panknin d, D. Panzieri ac,ab, B. Parsamyan aa,ab, S. Paul q, M. Pesek s, D. Peshekhonov g, G. Piragino aa,ab, S. Platchkov v, J. Pochodzalla m, J. Polak k,x, V.A. Polyakov u, J. Pretz d,8, M. Quaresma l, C. Quintans l, S. Ramos l,2, G. Reicherz b, E. Rocco j, V. Rodionov g, E. Rondio ad, N.S. Rossiyskaya g, D.I. Ryabchikov u, V.D. Samoylenko u, A. Sandacz ad, M.G. Sapozhnikov g, S. Sarkar f, I.A. Savin g, G. Sbrizzai y,x, P. Schiavon y,x, C. Schill i, T. Schlüter p, A. Schmidt h, K. Schmidt i,4,∗, L. Schmitt q,9, H. Schmïden c, K. Schönning j, S. Schopferer i, M. Schott j, O.Yu. Shevchenko g, L. Silva l, L. Sinha f, S. Sirtl i, M. Slunecka g, S. Sosio aa,ab, F. Sozzi x, A. Srnka e, L. Steiger x, M. Stolarski l, M. Sulc k, R. Sulej ad, H. Suzuki ag,5, A. Szabelski ad, T. Szameitat i, P. Sznajder ad, S. Takekawa ab, J. Ter Wolbeek i,4, S. Tessaro x, F. Tessarotto x,

Exclusive production of rho(0) mesons was studied at the COMPASS experiment by scattering 160 GeV/c muons off transversely polarised protons. Five single-spin and three double-spin azimuthal asymmetries were measured as a function of Q(2), x(Bj), or p(T)(2). The sin Os asymmetry is found to be -0.019 +/- 0.008(stat.) +/- 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 GPD-based model agree well with the present results. The data is interpreted as evidence for the existence of chiral-odd, transverse generalized parton distributions. (C) 2014 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org!licenses/by/3.0/).

Large samples of Λ, Σ(1385) and Ξ(1321) hyperons produced in the deep-inelastic muon scattering off a 6LiD target were collected with the COMPASS experimental setup at CERN. The relative yields of Σ(1385)+, Σ(1385)−, \(\bar{\varSigma}(1385)^{-}\), \(\bar{\varSigma}(1385)^{+}\), Ξ(1321)−, and \(\bar{\varXi}(1321)^{+}\) hyperons decaying into \(\varLambda(\bar{\varLambda})\pi\) were measured. The ratios of heavy-hyperon to Λ and heavy-antihyperon to \(\bar{\varLambda}\) 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.

Multiplicities of charged hadrons produced in deep inelastic muon scattering off a ^6LiD 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 ⟨ p_T^2 ⟩ on x_Bj, Q^2, W^2 and z. The z-dependence of ⟨ p_T^2 ⟩ is shown to be a potential tool to extract the average intrinsic transverse momentum squared of partons, ⟨ k_⊥^2 ⟩, as a function of x_Bj and Q^2 in a leading order QCD parton model.

The gluon polarisation in the nucleon was measured using open charmproduction by scattering 160 GeV/c polarised muons off longitudinally polarisedprotons or deuterons. The data were taken by the COMPASS collaboration between2002 and 2007. A detailed account is given of the analysis method that includesthe application of neural networks. Several decay channels of D^0 mesons areinvestigated. Longitudinal spin asymmetries of the D meson productioncross-sections are extracted in bins of D^0 transverse momentum and energy. Atleading 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 ~13(GeV/c)^2 and an average gluon momentum fraction ~ 0.11. For the first time,the average gluon polarisation is also obtained at next-to-leading order QCDaccuracy as (Delta g/G)^NLO = -0.13 +/- 0.15 (stat.) +/- 0.15 (syst.) at thescale ~ 13 (\GeV/c)^2 and ~ 0.20.

The differential cross section for the production of charged hadrons with high transverse momenta in the 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 next-to-leading order pQCD. The inclusion of all-order resummation of large logarithmic threshold corrections reduces the discrepancy from a factor of 3 to 4 to a factor of 2. The dependence of the cross section on the pseudorapidity and on the virtual photon energy fraction is investigated. Finally the dependence on the charge of the hadrons is discussed.

We present a determination of the gluon polarisation Delta g/g in the nucleon, based on the longitudinal double-spin asymmetry of DIS events with a pair of large transverse-momentum hadrons in the final state. The data were obtained by the COMPASS experiment at CERN using a 160 GeV/c polarised muon beam scattering off a polarised ^6LiD target. The gluon polarisation is evaluated by a Neural Network approach for three intervals of the gluon momentum fraction x_g covering the range 0.04 < x_g < 0.27. The values obtained at leading order in QCD do not show any significant dependence on x_g. Their average is Delta g/g = 0.125 +/- 0.060 (stat.) +/- 0.063 (syst.) at x_g=0.09 and a scale of mu^2 = 3 (GeV/c)^2.

The COMPASS Collaboration at CERN has measured the transverse spin azimuthal asymmetry of charged hadrons produced in semi-inclusive deep inelastic scattering using a 160 GeV positive muon beam and a transversely polarised NH_3 target. The Sivers asymmetry of the proton has been extracted in the Bjorken x range 0.003 0.03. The asymmetry is different from zero and positive also in the low x region, where sea-quarks dominate. The kinematic dependence of the asymmetry has also been investigated and results are given for various intervals of hadron and virtual photon fractional energy. In contrast to the case of the Collins asymmetry, the results on the Sivers asymmetry suggest a strong dependence on the four-momentum transfer to the nucleon, in agreement with the most recent calculations.

A measurement of the azimuthal asymmetry in dihadron production in deep-inelastic scattering of muons on transversely polarised proton (NH3) targets is 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 GeV/c muon beam of the CERN SPS, increasing by a factor of about four the overall statistics with respect to 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.

The COMPASS Collaboration at CERN has investigated the pi(-)gamma -> pi(-)pi(-)pi(+) reaction at center-of-momentum energy below five pion masses, root s < 5m(pi), embedded in the Primakoff reaction of 190 GeV pions impinging on a lead target. Exchange of quasireal photons is selected by isolating the sharp Coulomb peak observed at smallest momentum transfers, t' < 0.001 GeV2/c(2). Using partial-wave analysis techniques, the scattering intensity of Coulomb production described in terms of chiral dynamics and its dependence on the 3 pi-invariant mass m(3 pi) = root s were extracted. The absolute cross section was determined in seven bins of root s with an overall precision of 20%. At leading order, the result is found to be in good agreement with the prediction of chiral perturbation theory over the whole energy range investigated.

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"0.6."

Analyzing DISTO data of pp → pΛK + at T p = 2.50 and 2.85GeV to populate a previously reported X(2265) -resonance with M x = 2267 MeV/c2 and Γ x = 118 MeV at 2.85GeV, we found that the yield of X(2265) at 2.50GeV is much less than that at 2.85GeV (less than 10%), though it is expected from a kinematical consideration to be produced as much as 33% of that at 2.85GeV. The small population of X(2265) at 2.50GeV is consistent with the very weak production of Γ(1405) at the same incident energy toward its production threshold, thus indicating that Γ(1405) plays an important role as a doorway state for the formation of X(2265).

We have analyzed data of the DISTO experiment on the exclusive pp rightarrow p Lambda K+ reaction at 2.50 GeV and 2.85 GeV incident energy and found that a previously reported X = K-pp rightarrow p Lambda with M_X = 2267 MeV/c2 and a width Gamma_X = 118$ MeV, in the reaction channel pp rightarrow K+ + X at 2.85 GeV, is not populated at 2.50 GeV, though it is kinematically allowed and is expected to be produced as much as 50 % of that at 2.85 GeV. The absence of X at 2.50 GeV is in line with the very weak production of Lambda (1405) at 2.50 GeV toward its production threshold, thus giving a direct evidence that Lambda (1405) is the doorway state for the formation of K-pp with a high p-Lambda(1405) sticking probability at 2.85 GeV.

Analyzing DISTO data of $pp -> p Lambda K^+$ at $T_p = 2.50$ and 2.85 GeV to populate a previously reported X(2265) resonance with $M_X = 2267$ MeV/$c^2$ and $Gamma_X = 118$ MeV at 2.85 GeV, we found that the production of X(2265) at 2.50 GeV is much less than that at 2.85 GeV (less than 10%), though it is expected from a kinematical consideration to be produced as much as 33% of that at 2.85 GeV. The small population of X(2265) at 2.50 GeV is consistent with the very weak production of Lambda (1405) at the same incident energy toward its production threshold, thus indicating that Lambda (1405) plays an important role as a doorway state for the formation of X(2265).

We have analyzed data of the DISTO experiment on the exclusive pp -> p Lambda K+ reaction at 2.85 GeV to search for a strongly bound compact K-pp (= X) state to be formed in the pp -> K+ + X reaction. The observed spectra of the K+ missing-mass and the p Lambda invariant-mass with high transverse momenta of p and K+ revealed a broad distinct peak with a mass M_X = 2265 +- 2 (stat) +- 5 (syst) MeV/c2 and a width Gamma_X = 118 +- 8 (stat) +- 10 (syst) MeV.

The COMPASS experiment at the CERN SPS has studied the diffractive dissociation of negative pions into the π- π- π+ final state using a 190  GeV/c pion beam hitting a lead target. A partial wave analysis has been performed on a sample of 420,000 events taken at values of the squared 4-momentum transfer t' between 0.1 and 1  GeV2/c2. The well-known resonances a1(1260), a2(1320), and π2(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/c2 decaying to ρπ. The resonant nature of this wave is evident from the mass-dependent phase differences to the J(PC)=2-+ and 1++ waves. From a mass-dependent fit a resonance mass of (1660±10(-64)(+0))  MeV/c2 and a width of (269±21(-64)(+42))  MeV/c2 are deduced, with an intensity of (1.7±0.2)% of the total intensity.

The data from the DISTO Collaboration on the exclusive pp -> pK(+)Lambda production acquired at T-p = 2.85 GeV have been re-analysed in order to search for a deeply bound K(-)pp( X) state, to be formed in the binary process pp -> K+X. The preliminary spectra of the Delta MK+, missing-mass and of the M-p Lambda invariant-mass show, for large transverse-momenta of protons and kaons, a distinct broad peak with a mass M-X = 2265 +/- 2 MeV/c(2) and a width Gamma(X) = 118 +/- 8 MeV/c(2).