In 2024, IFAE contributed in a major way to the reconstruction and calibration of the Virgo data, in the framework of the O4 commissioning efforts. The GW signal reconstruction code has evolved significantly during the years of the commissioning of O4 to include the signal recycling mirror, online noise suppression, and to increase its flexibility to adjust the optical response of the interferometer in real time. The new methods developed for O4 have shown in addition high robustness against glitches, compared to O3. IFAE members were at EGO to further improve Virgo’s sensitivity before the experiment resumed data taking in March 2024.
IFAE scientists spent significant periods at EGO, and participated in commissioning campaigns as well as in dedicated noise hunting activities related to magnetic injections. In particular, the study, via magnetic field injection campaigns, of potential couplings of interferometer elements with magnetics fields is crucial to determine the sensitivity of the experiment to the presence of Schumann fields.
In 2020, IFAE designed and built an instrumented baffle for Virgo to monitor the stray light around the main mirrors, and a first integration test took place at EGO. The baffle is built of super-mirror-polished and AR-coated stainless steel, equipped with 76 novel ultra-high-vacuum-compatible Si-based sensors and golden PCBs. The final installation in one of Virgo’s vacuum towers took place in spring 2021 and it has operated since then with great stability.
Following the success of the IMC instrumented baffle, the group has been immersed since 2021 in the design and construction of large instrumented baffles for Virgo large mirrors, in time for AdV+ phase II. Each baffle includes 120 sensors distributed in five concentric rings, operating with a DAQ readout rate of 1kHz, with the aim to correlate changes in the cavity field with glitches in the output signal of the interferometer. Both wired and wireless readouts are implemented. Dedicated simulations were performed to determine the spatial distribution of the sensors in the baffle and to adjust the dynamic ranges in the different detector rings. As in the case of the IMC, AR-coating is applied to the surfaces of baffles and sensors.
In 2024, Virgo finally decided to postpone the installation of new large masses in time for O5. This was justified by the new required intervention for implementing new stable recycling cavities, and a slower than expected progress in the development of new optical coatings to reduce thermal noise in the new mirrors. This event put on pause the development and implementation of the corresponding large payload on which the instrumented baffles need to be integrated to get suspended. An alternative location for the instrumented baffles, at the entrance of the vacuum tower hosting the mirror (at 1.3 m from the mirror) instead of suspended surrounding it, was found as a result of intense simulation studies indicating the additional noise due to vibrations was not a limitation. Results are collected at M. Andrés-Carcasona et al., Phys. Rev. D 111 (2025) 4, 042001.
The new location required significant modifications in the mechanical integration of the baffle: new supporting structure was implemented including springs to attach the baffle to the walls of the vacuum tube and for dumpling vibrations; new elements were incorporated in the backside of the baffle to avoid its exposure to scattered light from the nearby mirror. The electronics at the backside of the baffles were encapsulated and additional large (non-instrumented) baffles were envisaged behind the electronics to absorb the light from the mirror. Altogether, this postponed the conclusion of the final baffles to 2025 (see Figures 1 and 2).
An MoU participated by INFN, IFAE, and NIKHEF, has been signed with CERN to collaborate in the design of the ET infrastructure. IFAE plays a central role in the project, coordinated by CERN, for the design of the ET vacuum beam pipe and the baffling layout inside the pipes, where hundreds of baffles will have to be installed to reduce stray light noise. IFAE has been responsible for determining main physics requirements constraining the dimensions of the vacuum pipes, directly affecting the size of the ET tunnel and dominating the leading (related to civil-engineering) cost of the ET infrastructure. The work required complex simulations of the light propagation inside the ET optical cavities. Detailed calculations and simulations were carried out and the results are published in M. Andrés-Carcasona, et al., Phys. Rev. D 108, 102001 (2023). In 2024, the team worked towards extending the studies to include cryogenic areas close to the mirrors and misaligned cavities.
The ongoing R&D effort at CERN will involve the construction by 2025 of several prototypes, tens of meters long, serving as demonstrators, under ultra-high vacuum conditions and equipped with baffles provided by IFAE (see Figure 3).
In 2024, the group made enormous progress in the physics exploitation of the LVK data in all its research lines. Since 2018, a strong program was put in place 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. Recently, the program was extended towards the search for continuous GW signals. Altogether, this has translated into several publications in 2024, often with IFAE GW students as first authors and performed in close collaboration with the IFAE theory division, in top journals of the field. Most notably:
In 2024, the members of the group kept a good visibility in international conferences and workshops, including the most important conferences in the field. The group gave more than 30 contributions to conferences and workshops for both hardware and physics analysis.
IFAE already plays a unique role in the ground-based GW projects in Europe, being the only Spanish institution in both LIGO and Virgo Collaborations, that has already developed novel hardware for the current detectors, and being well placed in the R&D initiatives, initial design studies, and in the top-level governing bodies of the ET project.
IFAE coordinates the stray light control efforts in Virgo and ET. IFAE has been recently invited to participate as an observer in EGO’s council. Other governance positions inside the Virgo include participation in top level committees like the Virgo Steering committee, the joint EGO-Virgo Committee, the Virgo Organization committee, the LKV MoU committee, the IGWN formation committee, the LSC Council, the LVK climate change committee, the Virgo Speakers committee, or the Early Career Scientists committee.
IFAE organized the LVK meeting in September 2024 with the attendance of 400 persons coming to Barcelona and additional 400 attending the meeting online (https://lvkmeeting2024.ifae.es/). In November 2024, IFAE joined the LIGO Collaboration in the USA.
ET is a collaboration formed by about 1600 members from 200 institutions involving all the key players in Europe. Since 2022, Mario Martínez acts as coordinator for the Horizon-CSA INFRA-DEV ET-PP project, a pan-European 3.5 M€ project (12 M€ project total value) for the ET preparatory phase. Mario Martínez is a member of the ET Directorate and E. Coccia is the chair of the ET collaboration board.