Infra-red enhanced loops in quadratic gravity

Abstract: It has been suggested that logarithmically enhanced infra-red loop corrections arising in theories with four derivatives correspond to a physical running of couplings, rendering quadratic gravity asymptotically free. We find that these effects depend on the gauge and on the field parameterisation. We compute physical on-shell amplitudes and find genuine infra-red log-enhanced loop corrections, that are process-dependent and cannot be absorbed into running couplings. As a byproduct, we derive the effective action of quadratic gravity at tree level, showing that its ghost does not contribute to effective operators and thereby does not violate positivity bounds.