Notes about Straggling ("landau") Functions

In a time-projection-chamber (TPC) charged particle identification can be achieved by measuring the ionization in a medium (gas or condensed) together with the measurement of the momentum or the energy of particles. A detailed understanding of the ionization process is needed. This can be studied theoretically by calculating energy-loss or straggling functions. Some of the results of such studies are given here. They can be used to assess the expected performance of a TPC and to apply necessary corrections. For TPCs with the geometry used in STAR, an accurate data analysis requires attention to track segmentation.