The COMPASS Collaboration at CERN has measured the transverse spin azimuthal asymmetry of\ud charged hadrons produced in semi-inclusive deep inelastic scattering using a 160 GeV μ+ beam and a\ud transversely polarised NH3 target. The Sivers asymmetry of the proton has been extracted in the Bjorken\ud x range 0.003 < x < 0.7. The new measurements have small statistical and systematic uncertainties of\ud a few percent and confirm with considerably better accuracy the previous COMPASS measurement. The\ud Sivers asymmetry is found to be compatible with zero for negative hadrons and positive for positive\ud hadrons, a clear indication of a spin–orbit coupling of quarks in a transversely polarised proton. As\ud compared to measurements at lower energy, a smaller Sivers asymmetry for positive hadrons is found\ud in the region x > 0.03. The asymmetry is different from zero and positive also in the low x region,\ud where sea-quarks dominate. The kinematic dependence of the asymmetry has also been investigated and\ud results are given for various intervals of hadron and virtual photon fractional energy. In contrast to the\ud case of the Collins asymmetry, the results on the Sivers asymmetry suggest a strong dependence on the\ud four-momentum transfer to the nucleon, in agreement with the most recent calculations

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First measurements of the Collins and Sivers asymmetries of charged hadrons produced in deep-inelastic scattering of muons on a transversely polarized 6LiD target are presented. The data were taken in 2002 with the COMPASS spectrometer using the muon beam of the CERN SPS at 160 GeV/c. The Collins asymmetry turns out to be compatible with zero, as does the measured Sivers asymmetry within the present statistical errors.

We present a new measurement of the longitudinal spin asymmetry A1\ud and the spin-dependent structure function g1 of the deuteron in the range 1

The Collins and Sivers asymmetries for charged hadrons produced in deeply inelastic scattering on transversely polarised protons have been extracted from the data collected in 2007 with the CERN SPS muon beam tuned at 160 GeV/c. At large values of the Bjorken x variable non-zero Collins asymmetries are observed both for positive and negative hadrons while the Sivers asymmetry for positive hadrons is slightly positive over almost all the measured x range. These results nicely support the present theoretical interpretation of these asymmetries, in terms of leading-twist quark distribution and fragmentation functions.

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 µ + beam and a transversely polarised NH 3 target. The Collins asymmetry of the proton was extracted in the Bjorken x range 0.003 < x < 0.7. These new measurements confirm with higher accuracy previous measurements from the COMPASS and HERMES collaborations, which exhibit a definite effect in the valence quark region. The asymmetries for negative and positive hadrons are similar in magnitude and opposite in sign. They are compatible with model calculations in which the u-quark transversity is opposite in sign and somewhat larger than the d-quark transversity distribution function. The asymmetry is extracted as a function of Bjorken x, the relative hadron energy z and the hadron transverse momentum p h T . The high statistics and quality of the data also allow for more detailed investigations of the dependence on the kinematic variables. These studies confirm the leading-twist nature of the Collins asymmetry.