Nagoya University
Theoretical Elementary Particle Physics Laboratory

Date&Time Wed Feb 10 2021 (13:30 - 14:30)
Yu Hamada
Affiliation Kyoto Univ.
Title Gravitational instantons and anomalous chiral symmetry breaking
Abstract We study anomalous chiral symmetry breaking in two-flavour QCD induced by gravitational and QCD-instantons within asymptotically safe gravity within the functional renormalisation group approach. Similarly to QCD-instantons, gravitational ones, associated to a K3-surface connected by a wormhole-like throat in flat spacetime, generate contributions to the "t Hooft coupling proportional to exp(-1/gN) with the dimensionless Newton coupling gN. Hence, in the asymptotically safe gravity scenario with a non-vanishing fixed point coupling gN*, the induced "t Hooft coupling is finite at the Planck scale, and its size depends on the UV theory. Within this scenario the gravitational effects on anomalous U(1)A-breaking at the Planck scale could survive at low energy scales. In turn, fermion masses of the order of the Planck scale are inconsistent with observations. This constrains the allowed asymptotically safe UV-completion of the Gravity-QCD system. We map-out the parameter regime that is compatible with the existence of light fermions in the low-energy regime.
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