Single-Spin Asymmetries in Inclusive Charged Pion Production by Transversely Polarized Antiprotons

Polarization analysis of p̄ produced in pA collisions

A quite simple procedure for the generation of a polarized antiproton beam could be worked out if antiprotons are produced with some polarization. In order to investigate this possibility measurements of the polarization of produced antiprotons have been started at a CERN/PS test beam. The polarization will be determined from the asymmetry of the elastic antiproton scattering at a liquid hydrogen target in the CNI region for which the analyzing power is well known. The data are under analysis and an additional measurement is done in 2018. Details on the experiment and the ongoing data analysis will be given.

Partonic pole matrix elements for fragmentation

A model-independent analysis of collinear three-parton correlation functions for fragmentation is performed. By investigating their support properties it is shown, in particular, that the so-called partonic pole matrix elements vanish. This sheds new light on the understanding of transverse single spin asymmetries in various hard semi-inclusive reactions. Moreover, it gives additional strong evidence for the universality of transverse-momentum-dependent fragmentation functions.

Forward neutral-pion transverse single-spin asymmetries in p + p collisions at sqrt[s] = 200 GeV

We report precision measurements of the Feynman x (xF) dependence, and first measurements of the transverse momentum (pT) dependence, of transverse single-spin asymmetries for the production of pi0 mesons from polarized proton collisions at sqrt[s] = 200 GeV. The xF dependence of the results is in fair agreement with perturbative QCD model calculations that identify orbital motion of quarks and gluons within the proton as the origin of the spin effects. Results for the pT dependence at fixed xF are not consistent with these same perturbative QCD-based calculations.

Single-transverse-spin asymmetries of identified charged hadrons in polarized pp collisions at sqrt[s]=62.4 GeV

The first measurements of xF-dependent single-spin asymmetries of identified charged hadrons, pi+/-, K+/-, and protons, from transversely polarized proton-proton collisions at 62.4 GeV at RHIC are presented. Large asymmetries are seen in the pion and kaon channels. The asymmetries in inclusive pi+ production, AN(pi+), increase with xF from 0 to approximately 0.25 and AN(pi-) decrease from 0 to approximately -0.4. Observed asymmetries for K- unexpectedly show positive values similar to those for K+, increasing with xF, whereas proton asymmetries are consistent with zero over the measured kinematic range. Comparisons of the data with predictions of QCD-based models are presented.

First measurement of the transverse spin asymmetries of the deuteron in semi-inclusive deep inelastic scattering

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.

Cross sections and transverse single-spin asymmetries in forward neutral-pion production from proton collisions at sqrt[s]=200 GeV

Measurements of the production of forward high-energy pi(0) mesons from transversely polarized proton collisions at sqrt[s]=200 GeV are reported. The cross section is generally consistent with next-to-leading order perturbative QCD calculations. The analyzing power is small at x(F) below about 0.3, and becomes positive and large at higher x(F), similar to the trend in data at sqrt[s]< or =20 GeV. The analyzing power is in qualitative agreement with perturbative QCD model expectations. This is the first significant spin result seen for particles produced with p(T)>1 GeV/c at a polarized proton collider.

Evidence for a single-spin azimuthal asymmetry in semi-inclusive pion electroproduction

Single-spin asymmetries for semi-inclusive pion production in deep-inelastic scattering have been measured for the first time. A significant target-spin asymmetry of the distribution in the azimuthal angle straight phi of the pion relative to the lepton scattering plane was formed for pi(+) electroproduction on a longitudinally polarized hydrogen target. The corresponding analyzing power in the sinstraight phi moment of the cross section is 0.022+/-0.005+/-0.003. This result can be interpreted as the effect of terms in the cross section involving chiral-odd spin distribution functions in combination with a chiral-odd fragmentation function that is sensitive to the transverse polarization of the fragmenting quark.