Chiral perturbation theory for nonzero chiral imbalance

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We construct the most general low-energy effective lagrangian including local parity violating terms parametrized by an axial chemical potential or chiral imbalance mu (5), up to O mml:mfenced close=")" open="("p4 ml:mfenced> order in the chiral expansion for two light flavours. For that purpose, we work within the Chiral Perturbation Theory framework where only pseudo-NGB fields are included, following the external source method. The O mml:mfenced close=")" open="("p2 mml:mfenced> lagrangian is only modified by constant terms, while the O mml:mfenced close=")" open="("p4 mml:mfenced one includes new terms proportional to mu 52 and new low-energy constants (LEC), which are renormalized and related to particular observables. In particular, we analyze the corrections to the pion dispersion relation and observables related to the vacuum energy density, namely the light quark condensate, the chiral and topological susceptibilities and the chiral charge density, providing numerical determinations of the new LEC when possible. In particular, we explore the dependence of the chiral restoration temperature T-c with mu (5). An increasing T-c(mu (5)) is consistent with our fits to lattice data of the ChPT-based expressions. Although lattice uncertainties are still large and translate into the new LEC determination, a consistent physical description of those observables emerges from our present work, providing a theoretically robust model-independent framework for further study of physical systems where parity-breaking effects may be relevant, such as heavy-ion collisions.
© The Authors. We are very grateful to A.A.Andrianov, V.A. Andrianov and J.Urrestarazu for useful discussions and comments. Work partially supported by research contracts FPA2016-75654C2-2-P, FPA2016-76005-C2-1-P, MDM-2014-0309 (Ministerio de Economa y Competitividad) and 2017SGR929 (Generalitat de Catalunya). This work has also received funding from the European Union Horizon 2020 research and innovation programme under grant agreement No 824093. A. V-R acknowledges support from a fellowship of the UCM predoctoral program.
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