Determination of the Stray Light-Induced Noise from the Baffle in the Cryogenic Trapping Area of Advanced Virgo in O5

Abstract

As part of the Advanced Virgo upgrade for the O5 observation run in 2026, a more powerful laser and larger end mirrors in the Fabry–Perot cavities will be installed. The new optical configuration will increase the laser beam waist in the cryogenic trapping area close to the end towers. This could require enlarging the apertures in the vacuum pipe, now dictated by the presence of a baffle, in order to avoid beam clipping and noise due to coupling with baffle vibrations, potentially leading to a significant background to the gravitational-wave signals. This is a delicate operation that would require displacing the cryotrap and the end tower. In this study, we compute the expected back-scattering from the existing cryogenic trap baffle and compare it to the expected Virgo sensitivity in O5 to determine whether the existing hardware configuration constitutes a threat for the future performance of the detector.