New insights into the 3D nucleon structure with deep inelastic semi-inclusive electron scattering with CLAS12 at JLAB

• Applicant: Dr. Stefan Diehl
• Subject Area: Nuclear and Elementary Particle Physics, Quantum Mechanics, Relativity, Fields
• Term: since 2022
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 508107918

Exploring the three-dimensional structure of the nucleon can help to understand several fundamental questions of nature, such as the origin of the nucleon spin and the charge and density distributions inside the nucleon. As one aspect, the momentum distribution of the partons can be accessed by transverse momentum dependent parton distribution functions (TMDs), measured in semi-inclusive deep inelastic scattering (SIDIS). Remarkable theoretical advances over the past decade have led to a rigorous framework where information on the confined motion of the partons inside a fast-moving nucleon is matched to TMDs which encode information about the orbital motion of partons in the parent nucleon and correlations between the motion of partons and their spin. SIDIS cross sections as well as spin asymmetries are directly related to TMDs and fragmentation functions (FFs), and are the subject of intense theoretical and experimental studies. The goal of this project is to use the high integrated luminosity on an extended kinematic range, which is available with the new CLAS12 spectrometer at JLAB, to perform a fully multi-dimensional analysis with high precision for the single terms of the SIDIS cross section. The first objective of the project is to extend the extraction of the structure function ratio FLUsinϕ/(FUU,T + ε FUU,L) based on spin asymmetries which is already done for π+ to π0 and π- as well as K+ and K- and to increase the statistics to a point where measurements in finer multi-dimensional bins with smaller statistical uncertainties become feasible. As a second objective, the unpolarized structure functions (F_UU,T + ε F_UU,L), F_UU^cosϕ and F_UU^cos2ϕ will be extracted based on measurements of the total cross section. Detailed multi-dimensional studies will be performed for pions and kaons. Here, the measurement of the cos2ϕ azimuthal moment of the cross section will probe the convolution of the Collins fragmentation function with the leading-twist Boer-Mulders TMD h1┴. Furthermore, the measurement of the cos2ϕ and cosϕ moments will enable us to isolate the flavor-dependent Boer-Mulders effect. The cosϕ and sinϕ moments will be also used to probe different TMD models and to study the influence of higher-twist contributions. Altogether, a global fit of the different parts of the cross section will enable a more reliable extraction of TMDs and FFs with a much higher precision than the current fits based on already existing data in the valence quark regime.

• DFG Programme: Research Grants
• International Connection: USA
• Co-Investigator: Professor Dr. Kai-Thomas Brinkmann
• Cooperation Partner: Professor Kyungseon Joo