Trigonocephalie

Sivers effect in semiinclusive deeply inelastic scattering

“…In the transverse target SSA these effects can be distinguished by the different azimuthal angle distribution of the produced hadrons: Sivers effect ∝ sin(φ − φ S ), while Collins effect ∝ sin(φ + φ S ), where φ and φ S denote respectively the azimuthal angles of the produced hadron and the target polarization vector with respect to the axis defined by the hard virtual photon [15]. Both effects have been subject to intensive phenomenological studies in hadron-hadron-collisions [21][22][23][24][25] and in SIDIS [26][27][28][29][30][31][32][33][34][35]. For the longitudinal target SSA in SIDIS, which were observed first [4][5][6] but are dominated by subleading-twist effects [36], the situation is less clear and their description (presuming the factorization theorems [12][13][14] can be generalized to twist-3) is more involved [37,38].…”

“…However, the currently available final HERMES and COMPASS data [9,10] were analyzed without a transverse momentum weight, and can only be interpreted by resorting to some model for the distribution of the transverse parton momenta in the "unintegrated" [40] Sivers distribution and unpolarized fragmentation function. Different models have been explored in literature [33][34][35]. Here we approximate the distribution of transverse parton momenta in the Sivers function to be Gaussian.…”

Theoretical Overview on Recent Developments in Transverse Spin Physics

“…On the other hand, from the large transverse momentum quark Sivers function calculated in [13], we would already obtain the evolution equation for T F (x) (which is the transverse momentum moment of the quark Sivers function), since the collinear divergence in that calculation will lead to the splitting function of T F (x). This splitting function was confirmed by a complete calculation of next-to-leading order QCD correction to the transverse-momentum weighted spin asymmetry in Drell-Yan lepton pair production [17] and the derivations of the scale evolution equations directly [18,19]. In particular, the scale evolution for the quark-gluon correlation function T F (x) is found to be,…”

“…Most recently, there has been very exciting progress in studying the scale evolution equations for the quark-gluon and three-gluon correlation functions and their implications to the energy dependence of the relevant SSA observables [17,18,19]. General structure of the evolution equations for the twist-three quark-gluon correlation functions has been known in the literature [20].…”

“…Furthermore, the NLO perturbative-QCD correction to the transverse momentum weighted single spin asymmetry in Drell-Yan lepton pair production in hadronic collisions has engaged the transverse spin physics to a more solid theoretical ground [17]. It has been shown that the collinear divergences can be absorbed into the NLO twist-three quark-gluon correlation function of the transversely polarized nucleon and the unpolarized quark distribution of the unpolarized nucleon.…”