I – Experimental investigation of transverse spin asymmetries in μ-p SIDIS processes: Collins asymmetries

Abstract

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.

Results and perspective on TMDs and GPDs at COMPASS

“…1, as a function of the Bjorken variable x, the relative energy of the hadron z, and the hadron transverse momentum p h T . The pion asymmetries are very similar to the published unidentified hadron asymmetries [4]: at small x the asymmetries are compatible with zero, while in the valence region, they show an increasing signal, of opposite sign for + and − . The result for charged kaons, even if affected by large error bars, show a similar trend.…”

Transverse spin and transverse momentum structure of the nucleon from the COMPASS experiment

“…1, as a function of the Bjorken variable x, the fractional energy of the hadron z, and the hadron transverse momentum p h T . The pions asymmetries are very similar to the published unidentified hadron asymmetries [3]: at small x the asymmetries are compatible with zero, while in the valence region the asymmetries show an increasing signal, of opposite sign for π + and π − . This naively indicates that the unfavored and favored Collins FF have opposite sign.…”

“…SIDIS data have been collected using a 160 GeV longitudinally polarised muon beam and longitudinally or transversely polarised proton (NH 3 ) and deuteron ( 6 LiD) targets. The spectrometer comprises a variety of different tracking detectors, and allows to detect charged tracks in a broad momentum and angular range.…”

Transverse single-spin asymmetries of pion production in semi-inclusive DIS at subleading twist

“…A remarkable feature of the above mentioned asymmetries is that they appears at leading twist, thereby those effects should not be unsuppressed at high energies. Indeed significant asymmetries were measured by the HERMES Collaboration [10][11][12], the COMPASS Collaboration [13][14][15][16][17][18] and Jefferson Lab (JLab) Hall A Collaboration [19,20]. The corresponding data have been utilized to extract [21][22][23][24] the Sivers function and transversity distribution, within the TMD factorization [25].…”