Evidence for a Single-Spin Azimuthal Asymmetry in Semi-inclusive Pion Electroproduction

Abstract

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 φ of the pion relative to the lepton scattering plane was formed for π+ electroproduction on a longitudinally polarized hydrogen target. The corresponding analyzing power in the sinφ 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

Sum rules for theT-odd fragmentation functions

“…The resulting asymmetries should be much smaller for non-leading hadrons than for leading ones. This behaviour actually shows up in a comparison of the double semi-inclusive π + and π − asymmetries as measured by HERMES [11] with the π + and π − azimuthal single spin asymmetries, measured by the same collaboration [7,12].…”

“…One should note the measurement of longitudinal handedness [4] and the correlation of the Collins fragmentation function [5] in e + e − -annihilation by the DELPHI collaboration. At the same time, the recent measurements of azimuthal asymmetries in DIS by SMC [6] and HERMES [7] show the need for an extended theoretical treatment. In a complete analysis the effects are represented as convolutions of various distribution and fragmentation functions [8] (and possibly, T-odd fracture functions [9]), for which one needs some phenomenological inputs.…”

Three-dimensional imaging of the nucleon and semi-inclusive high-energy reactions

“…At DESY, the single-spin asymmetries for SIDIS were first measured with a longitudinally polarized target by HERMES [66] for production of charged pions and then measured for the first time with a transversely polarized target in [67]. They found nonzero Sivers and Collins asymmetries sin(φ h − φ S ) UT and sin(φ h + φ S ) UT .…”

“…For Drell-Yan processes, data are available on the azimuthal asymmetries in, e.g., reactions using pion beams [90][91][92][93] and pp or pD collisions [94,95]. [66] e + N → e + π ± X A Siv , A Coll [67] e + N → e + π ±,0 (K ± )X A Siv [68] e + N → e + π ±,0 (K ± )X A Coll [69] [84] Although the data are still far from sufficiently abundant to give precise control of the TMDs involved, different sets of TMD parameterizations have already been extracted from them. We briefly sort them out as follows.…”

“…Here, we will just briefly summarize the main data available and then try to sort out the available TMD parameterizations. For SIDIS, measurements have been made by the HERMES Collaboration [66][67][68][69][70] at DESY, the COM-PASS Collaboration [71][72][73][74][75][76][77][78] at CERN, CLAS [79,80], and the Hall A Collaboration [81][82][83][84] at Jefferson Laboratory (JLab). We list these SIDIS experiments in Table 5 and briefly summarize the results as follows.…”