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.

22 pages. Webinar presentation: https://youtu.be/WP4cXdYqLwY. v2: final version to appear on JHEP