DATA: 14092017
SEZIONE DI FIRENZE Seminario Gabriele Veneziano (CERN):
Using the wellknown lowenergy effective Lagrangian of QCD  valid for small (nonvanishing) quark masses and a large number of colors  we study in detail the regions of
parameter space where CP is spontaneously broken/unbroken for a vacuum angle Îž = Ï?.
In the CPbroken region there are first order phase transitions as one crosses Îž = Ï? , while on the (hyper)surface separating the two regions, there are second order phase transitions signaled by the vanishing of the mass of a pseudo NambuGoldstone boson and by a divergent
QCD topological susceptibility. The second order point sits at the end of a first order line associated with the CP spontaneous breaking, in the appropriate complex parameter plane.
When the effective Lagrangian is extended by the inclusion of an axion these features of QCD imply that standard calculations of the axion potential have to be revised when the QCD parameters fall in the above mentioned CPbroken region, in spite of the fact that the axion solves the strongCP problem. These latter results could be of interest for axionic dark matter calculations if the topological susceptibility of pure YangMills theory falls off sufficiently fast when temperature is increased towards the QCD deconfining transition.
