_____ P.J. Mulders/research overview _____

The focus of the research activities is Quantum Chromodynamics (QCD), the theory of the strong interactions. Specific topics to which contributions were made are the investigation of the role of multiquark hadrons, mesons with two quarks and two antiquarks, baryons with four quarks and one antiquark (nowadays also referred to as pentaquarks) and dibaryonic states built from six valence quarks. Multiquark configurations most likely don't appear as stable hadrons but they could very well dominate the short range part of the potential between hadrons (work with Johan de Swart and Ad Aerts). The richness of the spectra is a consequence of the color-magnetic spin-spin interactions, which also points to a possibly important role of diquarks (in particular an isosinglet u-d pair with spin 0). This role might be that of a constituent in exotic hadrons such as pentaquarks or connected to the formation of a color superconducting phase in QCD.

Another line of research has been the study of chiral symmetry in QCD and the corresponding key role that pions play. They affect spectra which can be studied in hybrid chiral quark models (work with Tony Thomas) or they can be considered as the essential degrees of freedom on which a full effective low-energy theory of QCD can be built, the Skyrme model (work with Jutta Kunz). A number of contributions in this field focus on the treatment of strangeness.

The fate of hadrons in a nuclear environment (work with Taber de Forest, Werenfried Spit and Lex Dieperink) was a topic closely related to experimental work at the medium energy electron accelerator at NIKHEF. Although or maybe because the richness in possible effects that can play a role, definite answers are still lacking.

An important part of the research focusses on hard (deep inelastic) scattering processes. Here quark and gluon degrees of freedom are directly accessible. This means that one through the field theoretical framework of QCD uses methods to translate observations into distribution and fragmentation functions for quarks and gluons or attribute them to more complex (higher twist) correlations (work with Paolo Castorina and Steve Pollock). One can even attempt to calculate these functions (work with Herman Meyer, Joao Rodrigues, Rainer Jakob, Andreas Metz and Rajen Kundu).

Important novel contributions have been made on the study of correlations between the momenta of the partons, their spins and the spin of the hadron they belong to or into which they fragment (initiated in work with Joachim Levelt and continued in work with Rik Tangerman, Daniel Boer, Rainer Jakob, Maria-Elena Boglione, Alessandro Bacchetta and Alex Henneman). An important ingredient is the possibility to connect particular correlations and observations on the basis of their behavior under time reversal. The origin of time reversal odd correlations is important for the understanding of single spin asymmetries in hard scattering processes (work with Cristian Pisano and Daniel Boer) and intimately connected to issues of color gauge invariance and the dependence of transverse momentum dependent functions (TMDs) on gauge links (investigated in work with Fetze Pijlman and Cedran Bomhof). There are important differences between distribution and fragmentation functions (work with Asmita Mukherjee and Leonard Gamberg).

The dependence on color flow for quarks as well as gluons may provide novel tools in the study of high-energy processes (work with Ted Rogers, Maarten Buffing and Asmita Mukherjee). This is also a main topic of the ERC Advanced Grant project QCD at Work (QWORK).

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