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International audience; For a large fraction of its already twenty year long existence, the COMPASS collaboration hasstudied the polarised structure of the nucleon via muon-nucleon scattering. This paper summa-rizes the final results obtained in longitudinally polarised DIS, either inclusive or semi-inclusive.For inclusive DIS, these cover the measurements of structure function $g_1$, and the information ex-tracted from them in terms of polarised PDFs (pPDFs) and Bjorken sum rule. From $g_1$ measure-ments alone, combining COMPASS with world data, the quark spin contribution to the nucleonspin is determined to be 0.26 < ∆Σ < 0.36. And from COMPASS data alone, the Bjorken sumrule is verified to 9% accuracy. In semi-inclusive DIS, more information can be gained on the pPDFs of sea quarks and gluons. A selection of our achievements along this line are reported.

The European physical journal / C 78(8), 621 (2018). doi:10.1140/epjc/s10052-018-6090-8, Fits to the final combined HERA deep-inelastic scattering cross-section data within the conventional DGLAP framework of QCD have shown some tension at low x and low $Q^2$ . A resolution of this tension incorporating $\ln (1/x)$ -resummation terms into the HERAPDF fits is investigated using the xFitter program. The kinematic region where this resummation is important is delineated. Such high-energy resummation not only gives a better description of the data, particularly of the longitudinal structure function $F_L$ , it also results in a gluon PDF which is steeply rising at low x for low scales, $Q^2 \simeq 2.5\,\hbox {GeV}^2$ , contrary to the fixed-order NLO and NNLO gluon PDF., Published by Springer, Heidelberg

Universität Hamburg, Diss., 1999; 91 pp., (1999). doi:10.3204/DESY-THESIS-1999-024, We analyse gluonic twist-four contributions to inclusive deep inelastic structure functions. For this, we estimate these contributions numerically in the first part. The results are based on the DLA contributions of gluon-operators which are expected to dominate at small XB. We, first, summarize relations of Ute two-e, three- and four-gluon amplitude and show how to extract the relevant twistfour. parts. Moreover, we are using experimental data on diffractive final states in order to fix free parameters within the initial distribution of the four-gluon amplitude. Therefore, it is also necessary to study relations between diffractive and inclusive amplitudes. This relation can be established with help of the AGK rules. Finally, we present our numerical results for three different choices of the input distributions. In the second part we consider gluonic higher-twist contributions from a more general point of view. Using a graphical method we can find expressions for gluonic twist-four contributions at leading order. For thls we consider diagrams with up to four t-channel gluons. The used scheme is an extension of the methods developed by R.K. Ellis, W. Furmanski, R. Petronzio (EFP). Making systematic use of the axial gauge, Ward identities and symmetry relations, we can write the results as explicitly gauge invariant expressions. Moreover, we can perform the factorization of matrix-elements of gluon-operators and coefficient functions.

129 pp. (1998).

The gluon contribution to the svuclure function FL(x. Q2) is calculated using kifactorizalion. A generalization of this factorization is given, which allows the expression ofshucture functions and hard CIOSS sections in term of quantities that are well defined withinperturbalive QCD.

24 pp. (1985).