At the present time it is unknown how the U(1)A anomaly of Quantum Chromodynamics behaves at high temperatures. We therefore want to look for thermal changes of the effects of the anomaly. For example, by studying the properties of the η' meson at high temperatures it would be possible to deduce important information on the axial anomaly, thanks to the deep connection between them. In this thesis the width of the η' as a function of the temperature is studied in the framework of large-Nc Chiral Perturbation Theory, at next-to-leading order, and in the corresponding Resonance Chiral Theory. We calculate the width increase due to scattering with particles from the heat bath, which we assume to consist of a pion gas. We compare the results obtained in both frameworks and as expected we find a smaller, but still consistent width increase when the more realistic resonance exchange is taken into account. The results suggest that the in-medium width of the η' may increase up to ΔΓ<img src="http://www.diva-portal.org/cgi-bin/mimetex.cgi?%5Capprox" /> 10 MeV at a temperature of T<img src="http://www.diva-portal.org/cgi-bin/mimetex.cgi?%5Capprox" /> 120 MeV. We find therefore a width increase of considerable size, comparable to the inverse lifetime of the fireball created in relativistic heavy-ion collisions. In other words, our results suggest that it may be possible to study experimentally how the properties of the η' change at high temperatures.

Title: Some aspects of low-energy QCD at the precision frontier Author: Tomáš Husek Department/Institute: Institute of Particle and Nuclear Physics Supervisor of the doctoral thesis: doc. RNDr. Karol Kampf, Ph.D. Abstract: This thesis concentrates on some low-energy aspects of QCD, namely on those which are connected to the electromagnetic decays of lightest neutral pseudoscalar mesons. Calculations of radiative corrections to neutral pion decays (the Dalitz decay and the rare decay) and a novel model for the pion electro- magnetic transition form factor are subjects discussed in the attached papers, which this work is based on. The associated theoretical aspects including Chiral Perturbation Theory or the large-Nc limit are introduced. We also discuss the complications which arise when the calculations of radiative corrections for η(′) Dalitz decays are performed. Some details about the collaboration with experi- ments which incorporate the calculation of the published corrections are provided. Last but not least, some techniques related to loop integrals are shown. Keywords: Chiral Perturbation Theory, large-Nc limit, radiative corrections, pion electromagnetic transition form factor 1