The Virgo Collaboration

During 2021, the IFAE group in Virgo has made great progress in the construction of the new instrumented baffle for the upgrade of the experiment and remained deeply involved in the analysis of the LIGO/Virgo data. In addition, the group played a central role in the new Einstein Telescope (ET) project.

IFAE’s responsibilities within AdV+ relate to the understanding and control of the stray light, with an IFAE researcher coordinating the Virgo Stray Light Control (SLC) group. IFAE is instrumenting delicate parts of Virgo with many photo-sensors to monitor and control the stray light propagating throughout the interferometer. This project capitalizes on IFAE’s experience in detector development and brings new challenges to the institute, including precise optics, ultra-high vacuum, and infrared light detection, enhancing its technological capabilities. A first instrumented baffle was successfully installed in Virgo in spring 2021 (see Figure 1). Since then the IMC instrumented baffle has been operated showing good performance.

The data from the IMC baffle have been compared to the simulations providing an acceptable description of the measurements. The results are indicating the sensitivity of the instrument to defects in the mirrors and misalignments in the optical cavity. The results have been published in Ll. Mir et al., Classical and Quantum Gravity 39 115011 (2022). (see Figure 2). A long publication with all detector technical details is under preparation.

In 2019-2021, as part of the SLC responsibility, members of the IFAE team, in collaboration with LIGO-Caltech and Virgo-EGO scientists, continued the development of simulations of the light propagation in the interferometer. Special emphasis was put in the understanding of the light distribution surrounding the main mirrors in the suspended areas, as an important input for the design of the instrumented baffles. In particular, a detailed study was carried out for determining the light distribution in the IMC end-mirror area for different operating conditions, including severe misalignments. This was instrumental for defining the initial layout of the sensors in the instrumented baffle, as well as to determine the maximum light power the sensors would be exposed to. The results led to several presentations and an internal Virgo note, and then published in A. Romero et al., Classical and Quantum Gravity 38, no. 4 (2021).

In 2021, IFAE also took the responsibility to determine whether the apertures of the existing baffles in the cryotrap areas need to be replaced in preparation for the new O5 optics close to the large mirrors. This required detailed simulations of the coupling of the light with the baffle and the convolution with the anticipated baffle displacements. The results have been instrumental in deciding that no intervention is needed, saving efforts and a big risk to the collaboration in what otherwise would have involved a delicate intervention in Virgo’s vacuum pipes and towers. The results will be published in journals in 2022.

The group has put in place a strong program with three main pillars: the search and study of compact binary coalescence events with emphasis on fundamental physics related to tests of General Relativity and dark matter (DM) searches, with and without the use of deep learning techniques; the use of GW for cosmological tests; and the search for stochastic GW signals as probes of the early universe. A strong experiment–theory collaboration within IFAE has been established for this purpose. In addition, the group is a main actor in the understanding of the interferometer using detailed simulations. Altogether, this has translated into several publications with IFAE students as first authors, including three publications published in Phys. Rev. Lett. and Phys. Rev. D. in 2021/22 (for example, see results in Figures 3 and 4). In addition, IFAE has played an important role in the LIGO-Virgo collaboration papers related to the search for sub-solar mass binary candidates using matched-filtering and O3a data (submitted for publication in Phys. Rev. Lett.), stochastic GW searches using O3 data (now published in Phys. Rev. D ) and the determination of the Hubble constant using O2 data (now published in AstroPhysics Journal). For the latter, an IFAE researcher has been appointed as corresponding editor for the O3 version of the analysis now about to be submitted for publication.

IFAE has taken major responsibilities in the control of the stray light in the interferometer. One of the senior members of the group (Ll. M. Mir) acts as coordinator of the SLC working group in Virgo. M. Martínez is member of the Virgo Steering Committee, and member of the Virgo Organization Committee (VOC) with the charge of preparing new bylaws in the scenario of a fast expansion of the collaboration, thus bringing to the discussion the experience from the organization of very large HEP collaborations.

In the last three years, the IFAE group has also contributed to the preparation of the third-generation Einstein Telescope. IFAE engineers are part of the ET-pathfinder R&D platform working on cryogenic aspects and the re-design of the cryoshielding hosting the main mirrors. In addition, within the ET-SLC group, IFAE is playing a central role in the final design of ET, for those aspects related to stray light control and monitoring, which dictate important layout parameters of the experiment, as well as in the refined evaluation of the ET physics case and its sensitivity to GW signals. IFAE gained international visibility with Mario Martínez being member of the Steering Committee and co-convener of the SLC group. In 2020-2021, he led the Spanish scientific community efforts resulting in Spain formally supporting the ET candidature as ESFRI infrastructure in Europe, and its official recognition in 2021. Mario Martínez was appointed European coordinator of the new 3.5M€ Horizon Coordination and Support Actions four-years project, corresponding to the preparatory phase of the ET experiment.