On the role of Collins effect in the single spin asymmetry A_N in p(transv. polarized) p --> h X processes

TL;DR

The paper revisits the role of the Collins effect in the single-spin asymmetry A_N for p^↑ p → π X within a TMD factorization framework, correcting a sign error and updating with transversity and Collins fragmentation data from SIDIS and e^+e^- experiments. It introduces a scan-based approach to probe the large-x behavior of transversity and constructs a Collins-only prediction band across BRAHMS, STAR, and E704 kinematics. The results show that Collins can account for some low-to-moderate x_F data but cannot explain the large x_F STAR π^0 asymmetries, indicating that additional effects, such as the Sivers mechanism, are needed and that TMD factorization in hadronic collisions remains unsettled. The study also explores different fragmentation-function sets and Collins evolution schemes, highlighting significant uncertainties from poorly constrained large-x transversity and data-driven constraints from SIDIS/e^+e^- measurements.

Abstract

The much debated issue of the transverse single spin asymmetry A_N observed in the inclusive large P_T production of a single hadron in pp interactions, p(transv. polarized) p --> pion X, is considered in a TMD factorization scheme. A previous result [1,2] stating that the maximum contribution of the Collins effect is strongly suppressed, is revisited, correcting a numerical error. New estimates are given, adopting the Collins functions recently extracted from SIDIS and e+e- data, and phenomenological consequences are discussed.

Figures (5)

• Figure 1: Scan band ( i.e. the envelope of possible values) for the Collins contribution to the charged pion single spin asymmetries $A_N$, as a function of $x_F$ at two different scattering angles, compared with the corresponding BRAHMS experimental data Lee:2007zzh. The shaded band is generated, adopting the GRV98 and GRSV2000 sets of collinear PDFs, the Kretzer FF set and an "unpolarized-like" evolution for the Collins function, following the procedure explained in the text.
• Figure 2: Scan band ( i.e. the envelope of possible values) for the Collins contribution to the neutral pion single spin asymmetry $A_N$, as a function of $x_F$ at two different rapidity values, compared with the corresponding STAR experimental data Abelev:2008af. The shaded band is generated, adopting the GRV98 and GRSV2000 sets of collinear PDFs, the Kretzer FF set and an "unpolarized-like" evolution for the Collins function, following the procedure explained in the text.
• Figure 3: The same as for Fig. \ref{['fig:an-star']}, but with the STAR data plotted vs. the pion transverse momentum, $P_T$, for different bins in $x_F$, $x_F =$ 0.28, 0.37, 0.43 and 0.50.
• Figure 4: The Collins contribution to the neutral pion single spin asymmetry $A_N$, compared with the corresponding STAR experimental data at two fixed pion rapidities Abelev:2008af. The shaded statistical error band is generated, adopting the GRV98 and GRSV2000 sets of collinear PDFs, the Kretzer FF set and an "unpolarized-like" evolution for the Collins function, starting from the 7-parameter fit in the grid procedure that maximizes the neutral pion SSA in the large $x_F$ region and applying the error estimate procedure described in Appendix A of Ref. Anselmino:2008sga. See text for more details.
• Figure 5: The same as for Fig. \ref{['fig:an-star-free7']}, but this time with the statistical error band generated starting from the scan procedure with 11 free parameters. See text for more details.