Weyl fermion creation by cosmological gravitational wave background at 1-loop

• DOI: 10.1007/jhep01(2025)023
• Publication Date: 2025-01-03T04:28:50Z
• Authors: Azadeh Maleknejad; Joachim Kopp
• Keywords: Conformal gravity

A bstract Weyl fermions of spin $$ \frac{1}{2} <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mfrac> <mml:mn>1</mml:mn> <mml:mn>2</mml:mn> </mml:mfrac> </mml:math> minimally coupled to Einstein’s gravity in 4 dimensions cannot be produced purely gravitationally an expanding Universe at tree level. Surprisingly, as we showed a recent letter [1], this changes gravitational 1-loop when cosmic perturbations, like wave background, are present. Such background introduces new scale, thereby breaking the fermions’ conformal invariance. This leads non-vanishing self-energy for and induces their production. In paper, present extended study mechanism, explicitly computing effect using in-in formalism. We work radiation-dominated era fixed background. Gravitational wave-induced fermion production has rich phenomenological consequences. Notably, if eventually acquire mass, assuming realistic — potentially detectable backgrounds, mechanism can explain abundance dark matter Universe. More generally, gravitational-wave induced freeze-in is generating other feebly interacting fermions, e.g. right-handed neutrinos. show that loop level dominate over conventional tree-level superheavy sizable part parameter space ( https://github.com/koppj/GW-freeze-in/ ).