Dr Bastian Brandt (University of Frankfurt)
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.
Dr Anthony Francis (York University) Dr Bastian Brandt (University of Frankfurt) Mr Daniel Robaina (Institute for Nuclear Physics Mainz) Prof. Hartmut Wittig (University of Mainz) Prof. Harvey Meyer (University of Mainz) Prof. Owe Philisen (University of Frankfurt)