[en] Short communication

[en] We report on a measurement of Spin Density Matrix Elements (SDMEs) in hard exclusive $\mathrm{\omega <!--\; ??\; -->}$ meson muoproduction on the proton at COMPASS using 160 GeV/c polarised µ${}^{+}$ and µ${}^{-<!--\; ???\; -->}$ beams impinging on a liquid hydrogen target. The measurement covers the range 5.0 GeV/c${}^2$< W < 17.0 GeV/c${}^2$, with the average kinematics $⟨<!--\; ???\; -->$Q${}^2$$⟩<!--\; ???\; -->$ = 2.1 (GeV/c)${}^2$, $⟨<!--\; ???\; -->$W$⟩<!--\; ???\; -->$ = 7.6 GeV/c${}^2$, and $⟨<!--\; ???\; -->$p${}_T^2$$⟩<!--\; ???\; -->$ = 0.16 (GeV/c)${}^2$. Here, Q${}^2$ denotes the virtuality of the exchanged photon, W the mass of the final hadronic system and p${}_T$ the transverse momentum of the $\mathrm{\omega <!--\; ??\; -->}$ meson with respect to the virtual-photon direction. The measured non-zero SDMEs for the transitions of transversely polarised virtual photons to longitudinally polarised vector mesons (${\mathrm{\gamma <!--\; ??\; -->}}_T^{\ast <!--\; ???\; -->}$$\to <!--\; ???\; -->$ V${}_L$) indicate a violation of s-channel helicity conservation. Additionally, we observe a sizeable contribution of unnatural-parity-exchange (UPE) transitions that decreases with increasing W. The results provide important input for modelling Generalised Parton Distributions (GPDs). In particular, they may allow to evaluate in a model-dependent way the contribution of UPE transitions and assess the role of parton helicity-flip GPDs in exclusive $\mathrm{\omega <!--\; ??\; -->}$ production.

[en] A new-generation high-granularity Shashlyk EM calorimeter read out by micropixel avalanche photodiodes (MAPD) with precision thermostabilization based on the Peltier element was designed, constructed and tested. MAPD-3N with a superhigh pixel density of 1.5·10⁴ mm^-2 and an area of 3x3 mm manufactured by the Zecotek Company were used in the photodetector unit

[en] Based on the simulation and R and D results the JINR project - to supplement AMS with a finely granulated scintillator calorimeter (TCAL) - is discussed. The project cost is about 1 million USD. TCAL would essentially increase the AMS potential in the studies of antimatter, matter and missing matter in the experiments in outer space

[en] Short communication. 1 fig

[en] ECAL0 is a new electromagnetic calorimeter designed for studying generalized parton distributions at the COMPASS II experiment at CERN. It will be located next to the target and will cover larger photon angles (up to 30 degrees). It is a modular high-granularity Shashlyk device with total number of individual channels of approx. 1700 and readout based on wavelength shifting fibers and micropixel avalanche photodiodes. Characterization of the calorimeter includes tests of particular sub-components, tests of complete modules and module arrays, as well as a pilot run of a fully-functional, quarter-size prototype in the COMPASS experiment. The main goals of the tests on low-intensity electron beam at the ELSA accelerator in Bonn were: to provide energy calibration using electrons, to measure angular response of the calorimeter and to perform an energy scan to cross-check previously collected data. A dedicated measurement setup was prepared for the tests, including a 3x3 array of the ECAL0 modules, a scintillating-fibre hodoscope and a remotely-controlled motorized movable platform. The measurements were performed using three electron energies: 3.2 GeV, 1.6 GeV and 0.8 GeV. They include a calibration of the whole detector array with a straight beam and multiple angular scans

[en] The new-generation high-granularity Shashlyk EM calorimeter readout by micropixel avalanche photodiodes (MAPD) with precision thermostabilization based on the Peltier element is designed, constructed end tested. MAPD-3N with superhigh pixel density 1.5×10⁴ mm⁻² and area 3×3 mm² manufactured by the Zecotek Company were used in the photodetector unit

[en] A measurement of the azimuthal asymmetry in dihadron production in deep-inelastic scattering of muons on transversely polarised proton (NH_3) 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.

[en] Exclusive production of ρ⁰ 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 Q², x_Bj, or p_T². The sinϕ_S 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.

[en] Measurements of the Collins and Sivers asymmetries for charged pions and charged and neutral kaons produced in semi-inclusive deep-inelastic 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 non-zero signal for pions and there are hints of non-zero signal also for kaons. The Sivers asymmetries are found to be positive for positive pions and kaons and compatible with zero otherwise