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.

京都大学 / 0048 / 新制・課程博士 / 博士(理学) / 甲第20162号 / 理博第4247号 / 新制||理||1611(附属図書館) / 京都大学大学院理学研究科物理学・宇宙物理学専攻 / (主査)教授 川上 則雄, 教授 松田 祐司, 教授 前野 悦輝 / 学位規則第4条第1項該当 / Doctor of Science / Kyoto University / DFAM

We explore the Fermi surface of single crystals of YbPdBi, a cubic nonœntrosym­metric heavy fermion material, through angle-resolved magnetoresistanœ oscillations (AMRO). We characterize YbPdBi's magnetotransport, specific heat, neutron diffraction spectra, and magnetization. We observe an enhanœd Pauli paramagnetism at low tem­peratures, a Schottky-like anomaly with a strong dependenœ on the magnetic field, and a topological Hall effect in the magnetotransport. We probe the topology of YbPdBi and find evidenœ of Weyl semimetal behaviour in the magnetotransport. We found signs of a chiral anomaly in the magnetotransport, inde pendent of the angle between the current and magnetic field The observed chiral coefficients exhibit the expected temperature dependence characterized by µ = (2, 58±0,07) me V et 1-rv = (48 ± 1) m3 s-2. The transverse magnetoresistivity Pxy(H) presents two linear regimes of different slopes sep­arated by a graduai crossover, without saturation of the magnetization. This crossover's temperature dependenœ suggests it coincides with a reported T = 1 K peak in spe­cific heat. Using neutron diffraction, we find no evidenœ of magnetic ordering down to T = 0.1 K. An unexpected change in the symmetry of the AMRO measurements is observed, changing from two-fold to four-fold as the magnetic field was increased. This suggests that the Fermi surface of YbPdBi is changing with magnetic field strength. / Nous explorons la surface de Fenni des monocristaux de YbPdBi, un matériau cubique à fennions lourds non centrosymétriques, à travers des oscillations de magnétorésis­tance résolues en angle (AMRO). Nous caractérisons Je magnétotransport, la chaleur spécifique, les spectres de diffraction des neutrons et l'aimantation de YbPdBi. Nous observons un paramagnétisme de Pauli renforcé à basse température, une anomalie de type Schottky avec une forte dépendance au champ magnétique et un effet Hall topo­logique dans Je magnétotransport. Nous sondons la topologie de YbPdBi et trouvons des preuves du comportement de Weyl semimetal dans Je magnétotransport. Nous avons trouvé des signes d'une anomalie chirale dans Je magnétotransport, indépendamment de l'angle entre Je courant et Je champ magnétique. Les coefficients chiraux observés pré­sentent la dépendance attendue à la température caractérisée par µ= (2, 58±0,07) me V et 1-rv = (48 ± 1) m3 s-2. La magnétorésistivité transversale Pxy(H) présente deux ré­gimes linéaires de pentes différentes séparés par un croisement progressif, sans satura­tion de l'aimantation. La dépendance à la température de ce croisement suggère qu'il coïncide avec un pic T = 1 K signalé en chaleur spécifique. En utilisant la diffraction neutronique, nous ne trouvons aucune preuve d'ordre magnétique jusqu'à T = 0, I K. Un changement inattendu de la symétrie des mesures AMRO est observé, passant de deux à quatre fois à mesure que Je champ magnétique augmentait. Cela suggère que la surface de Fermi de YbPdBi change avec l'intensité du champ magnétique.