Recent developments at HERMES

Abstract

Azimuthal single-spin asymmetries in inclusive and semi-inclusive electro-production of pions and charged kaons were measured on a transversely polarized hydrogen target. Evidence for a naive-T-odd, transverse-momentum-dependent parton distribution function is deduced from nonvanishing Sivers effects for π + , π 0 , and K ± , as well as in the difference of the π + and π − cross sections. Further azimuthal modulations of the semi-inclusive single-spin asymmetry were found to be consistent with zero except the one related to the Collins effect and one that is subleading in an expansion of the cross section in 1/Q. The single-spin asymmetries in inclusive electro-production of charged pions and kaons were found to be non-zero except for K − , and to qualitatively resemble the pattern of the Sivers amplitudes.

Extraction of the distribution functionh1T⊥from experimental data

“…We attempt the first extraction of pretzelosity from the latest experimental data [12][13][14][15][16][17][18][19] using extracted Collins fragmentation function from Ref. [20] for our analysis.…”

“…For the asymmetry as a function of x, z we are using our result of Eq. (17) in particular the value of experimental point's P hT . We also include a simplified scale dependence in the asymmetry by using Q 2 in the corresponding collinear distribution.…”

“…We will fit h + and h − data and on effective deuteron (LiD) [12] and proton (NH 3 ) [15] targets from COMPASS collaboration, π 0 , π + , and π − data on proton (H) target from preliminary HERMES [16][17][18], and JLab 6 data [19] on effective neutron ( 3 He) target. Note that COMPASS data [12,15] are presented in the following way:…”

“…(17). In our fitting procedure we take D N N ( y ) into account and use experimental values of y for each bin.…”

“…One can easily see that the modern experimental data do not allow for a precise extraction of pretzelosity as the errors reported in Table. III are [12], [15], HERMES [16][17][18], and JLab [19] . We then plot ∆χ 2 ≡ χ 2 − χ 2 min as a function of N u in Fig.…”