SiversEffekt

The Sivers effect is a phenomenon in quantum chromodynamics (QCD), the theory describing the strong interaction between quarks and gluons, which are the fundamental constituents of protons and neutrons. It refers to an asymmetry in the distribution of quarks within transversely polarized nucleons (such as protons or neutrons). This effect was first proposed by physicist Mikhail Sivers in 1991 to explain discrepancies observed in deep inelastic scattering experiments.

In deep inelastic scattering, a high-energy lepton (such as an electron or muon) collides with a nucleon, probing its internal structure. When the nucleon is polarized perpendicular to its motion (transverse polarization), the Sivers effect predicts an imbalance in the scattering rates for quarks moving parallel versus antiparallel to the nucleon’s spin direction. This asymmetry arises due to the interplay between the quark’s transverse momentum and the nucleon’s spin, mediated by gluon exchanges within the nucleon.

The Sivers effect is closely related to other transverse spin phenomena, such as the Collins effect, which describes asymmetries in hadron production in semi-inclusive deep inelastic scattering. Unlike the Collins effect, which involves final-state interactions, the Sivers effect is attributed to initial-state dynamics, specifically the correlation between the quark’s transverse momentum and the nucleon’s spin. This distinction is crucial for understanding the underlying mechanisms of spin structure in nucleons.

Experimental evidence for the Sivers effect has been observed in various high-energy physics experiments, including those conducted at the Jefferson Lab, the Relativistic Heavy Ion Collider (RHIC), and the Large Hadron Collider (LHC). These measurements help constrain theoretical models, such as those based on twist-3 QCD factorization, which describe the interplay between spin, momentum, and parton distributions in nucleons. The study of the Sivers effect contributes to a deeper understanding of the non-perturbative aspects of QCD and the role of gluons in mediating spin effects.