The $U_A(1)$ breaking in the chiral limit of two-flavour QCD
- Dr. Bastian BRANDT
We study the strength of the anomalous breaking of the $U_A(1)$ symmetry in the chiral limit of two-flavour QCD at the transition temperature using the difference of the pseudoscalar and scalar iso-vector screening masses. To this end we use simulations with non-perturbatively $O(a)$-improved Wilson fermions for three different pion masses in the range of 200-550MeV on lattices with a temporal extent of 16 and volumes of $32^3$ to $64^3$. We compare our results to the strong breaking at zero temperature and find that the effect of the breaking in the mass difference is reduced by at least a factor of five. This indicates a weak $U_A(1)$ breaking at the transition temperature and raises the question of its influence on the nature of the chiral transition. It is generally expected that a weaker breaking strenghtens the chiral phase transition.