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Spontaneous CP breaking in QCD and the axion potential: an effective Lagrangian approach
Tuesday 19 December 2017, 02:30pm - 03:30pm
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Description: Using the low-energy effective Lagrangian of QCD, valid for small quark masses and large number of colors, we study in detail the regions of parameter space where CP is spontaneously broken/unbroken for a vacuum angle theta= pi. In the CP broken phase there are first order phase transitions as one crosses theta=pi, while on the (hyper)surface separating the two regions, there are second order phase transitions signalled by the vanishing of the mass of the Nambu-Goldstone boson and by a divergent QCD topological susceptibility. 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 if the QCD parameters fall in the above mentioned CP broken region, in spite of the fact that the axion solve the strong-CP problem. These last results could be of interest for axionic dark matter calculations if the topological susceptibility of pure Yang-Mills theory falls off sufficiently fast when the temperature is increased towards the QCD deconfining transition.
Location Dipartimento di Scienze Fisiche 2G26