TMD Approach in High Energy Physics:Theory and Applications

This thesis advances the three–dimensional description of hadrons in momentum space within the Transverse Momentum Dependent (TMD) framework along two complementary directions: the study of azimuthal distributions of hadron pairs produced in photon–photon collisions, where we formulated the kinematics and helicity formalism and derived the pattern of azimuthal modulations proportional to the TMD fragmentation functions, and the use of the low-$q_T$ Higgs spectrum at the LHC to probe the gluon sector, where the colour–singlet final state and $q_T\ll m_H$ justify rigorous TMD factorisation and make the shape of the spectrum directly sensitive to the gluon distribution.