Research topics
Electromagnetic production of kaons
The investigation of kaon-hyperon photo- and electroproduction off nucleons in the nucleon resonance region provides us with crucial knowledge on the baryon resonance spectrum and interactions in the hyperon-nucleon systems which arise from QCD. Except for studying the reaction mechanism, one can learn more about the existence and properties of the so-called missing resonances that were predicted by quark models but couple only weakly to the πN state and thus have not been observed in the pion production nor in the πN scattering processes. Moreover, we need a correct description of the elementary production for obtaining reliable predictions of the excitation spectra in electroproduction of hypernuclei.
Many theoretical studies of the hyperon production have been performed in previous decades. The analyses performed before the year 2004 suffered heavily from a lack of high-quality experimental data. In the last more than a decade, however, after the new high-duty-factor accelerators were constructed at JLab (CEBAF) and Bonn University (ELSA), the number of good quality data increased more than tenfold and today we have at our hand data on various response functions with a good level of precision and in a wide kinematic region.
The models that are used for describing the kaon-hyperon photo- and electroproduction are based on effective hadronic Lagrangians which contain interacting meson, baryon, and electromagnetic fields. These models do not have any explicit connection to the QCD and thus the number of free parameters depends on the amount of included resonances, which we tend to keep as small as possible. The main asset of these models, however, is that they can be easily applied in the calculations of electroproduction of hypernuclei. The plurality and diversity of theoretical models we have at hand gives us a unique opportunity to study and understand the photo- and electroproduction dynamics.
Production of charm mesons
Hadrons with heavy flavor provide new phenomena in the hadron physics. Many exotic hadrons such as the X, Y, Z states and the hidden-charm pentaquark Pc have been reported in the accelerator facilities. The properties of these states cannot be explained by the ordinary hadron picture, baryons as three quark states and mesons as quark-antiquark states, while they are expected to have exotic structures, e.g., the multiquark states and hadronic molecules. The heavy baryons can be good probes to investigate the diquark degrees of freedom inside the baryons, where the heavy baryon is interpreted as a bound state of a heavy quark and a light diquark for single-heavy baryons, and of a heavy diquark and a light quark for double-heavy baryons. The separation of the light degrees of freedom, i.e., the light-quark spins and orbital angular momenta, from the heavy-quark spins results in a novel symmetry in the heavy-quark sector, which is the heavy-quark spin symmetry. This symmetry appears because the spin-flip interaction of the heavy quark is suppressed by a factor of 1/mQ with the heavy-quark mass mQ.
Model selection tools
With the widespread use of machine-learning techniques, statistical methods, normally used in the analysis of experimental data, are now becoming essential tools in order to improve theoretical predictions.
Fitting a theoretical model to data involves finding the values of the parameters of the model that minimise a certain error function. Although the use of complex models, with a large number of parameters, may result in very low errors, it usually makes the minimisation procedure unstable, yielding many similar minima that correspond to wildly varying values of the parameters. Therefore, reducing the magnitude and / or the number of the model's parameters becomes particularly relevant. From a machine-learning perspective, this is a necessary step in order to prevent the parameters from overfitting the data, since an overfitted model has very limited predictive power.