Unraveling the rich proton structure requires studying its quark and gluon degrees of freedom including their transverse momenta. A quantum chromo dynamics (QCD)-based quantitative description of intrinsic transverse momentum degrees of freedom of quarks and gluons inside the proton ("TMDs") has emerged from intense interactions between experimentalists and theorists in the past one and half decades. Of particular interested are the Sivers quark distributions, which arise from correlations between proton spin and quark transverse momentum and thus appear connected to quark orbital motion inside the proton.
Very surprisingly the new QCD-TMD framework predicts that Sivers quark distributions are not universal, which is strikingly different from previously studied quark-momentum and quark-spin distributions in the nucleon. I will give an overview of existing experimental results from fixed-target and collider experiments and I will make some personal excursions to the enormous, later often hidden efforts to make these measurements. I will close with an outlook to future measurement campaigns.
