36 research outputs found
Measurement of azimuthal hadron asymmetries in semi-inclusive deep inelastic scattering off unpolarised nucleons
Adolph C, Akhunzyanov R, Alexeev MG, et al. Measurement of azimuthal hadron asymmetries in semi-inclusive deep inelastic scattering off unpolarised nucleons. Nuclear Physics B. 2014;886:1046-1077.Spin-averaged asymmetries in the azimuthal distributions of positive and negative hadrons produced in deep inelastic scattering were measured using the CERN SPS muon beam at GeV/c and a LiD target. The amplitudes of the three azimuthal modulations , and were obtained binning the data separately in each of the relevant kinematic variables , or and binning in a three-dimensional grid of these three variables. The amplitudes of the and modulations show strong kinematic dependencies both for positive and negative hadrons
Transverse target spin asymmetries in exclusive muoproduction
Adolph C, Alekseev MG, Alexakhin VY, et al. Transverse target spin asymmetries in exclusive muoproduction. Physics Letters B. 2014;731:19-26.Exclusive production of mesons was studied at the COMPASS experimentby scattering 160 GeV/ muons off transversely polarised protons. Fivesingle-spin and three double-spin azimuthal asymmetries were measured as afunction of , , or . The asymmetry isfound to be . All other asymmetriesare also found to be of small magnitude and consistent with zero withinexperimental uncertainties. Very recent calculations using a GPD-based modelagree well with the present results. The data is interpreted as evidence forthe existence of chiral-odd, transverse generalized parton distributions
High-statistics measurement of Collins and Sivers asymmetries for transversely polarised deuterons
New results are presented on a high-statistics measurement of Collins and Sivers asymmetries of charged hadrons produced in deep inelastic scattering of muons on a transversely polarised LiD target. The data were taken in 2022 with the COMPASS spectrometer using the 160 \gevv muon beam at CERN, balancing the existing data on transversely polarised proton targets. The first results from about two-thirds of the new data have total uncertainties smaller by up to a factor of three compared to the previous deuteron measurements. Using all the COMPASS proton and deuteron results, both the transversity and the Sivers distribution functions of the and quark, as well as the tensor charge in the measured -range are extracted. In particular, the accuracy of the quark results is significantly improved
Spin density matrix elements in exclusive meson muoproduction
We report on a measurement of Spin Density Matrix Elements (SDMEs) in hard exclusive meson muoproduction at COMPASS using 160~GeV/ polarised and beams impinging on a liquid hydrogen target. The measurement covers the kinematic range 5.0~GeV/ 17.0~GeV/, 1.0 (GeV/) 10.0 (GeV/) and 0.01 (GeV/) 0.5 (GeV/). Here, denotes the mass of the final hadronic system, the virtuality of the exchanged photon, and the transverse momentum of the 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 () indicate a violation of -channel helicity conservation. Additionally, we observe a dominant contribution of natural-parity-exchange transitions and a very small contribution of unnatural-parity-exchange transitions, which is compatible with zero within experimental uncertainties. The results provide important input for modelling Generalised Parton Distributions (GPDs). In particular, they may allow one to evaluate in a model-dependent way the role of parton helicity-flip GPDs in exclusive production.We report on a measurement of Spin Density Matrix Elements (SDMEs) in hard exclusive meson muoproduction at COMPASS using 160 GeV/c polarised and beams impinging on a liquid hydrogen target. The measurement covers the kinematic range 5.0 GeV/ 17.0 GeV/, 1.0 (GeV/c) 10.0 (GeV/c) and 0.01 (GeV/c) 0.5 (GeV/c). Here, W denotes the mass of the final hadronic system, the virtuality of the exchanged photon, and the transverse momentum of the 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 () indicate a violation of s-channel helicity conservation. Additionally, we observe a dominant contribution of natural-parity-exchange transitions and a very small contribution of unnatural-parity-exchange transitions, which is compatible with zero within experimental uncertainties. The results provide important input for modelling Generalised Parton Distributions (GPDs). In particular, they may allow one to evaluate in a model-dependent way the role of parton helicity-flip GPDs in exclusive production.We report on a measurement of Spin Density Matrix Elements (SDMEs) in hard exclusive meson muoproduction at COMPASS using 160~GeV/ polarised and beams impinging on a liquid hydrogen target. The measurement covers the kinematic range 5.0~GeV/ 17.0~GeV/, 1.0 (GeV/) 10.0 (GeV/) and 0.01 (GeV/) 0.5 (GeV/). Here, denotes the mass of the final hadronic system, the virtuality of the exchanged photon, and the transverse momentum of the 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 () indicate a violation of -channel helicity conservation. Additionally, we observe a dominant contribution of natural-parity-exchange transitions and a very small contribution of unnatural-parity-exchange transitions, which is compatible with zero within experimental uncertainties. The results provide important input for modelling Generalised Parton Distributions (GPDs). In particular, they may allow one to evaluate in a model-dependent way the role of parton helicity-flip GPDs in exclusive production