During her time at DESY, scientist Saptaparna Bhattacharya took a very close look at events involving three bosons. Photo: DESY, Katerina Lipka
Superconducting cavities are at the heart of many particle accelerators, boosting particles to extremely high energies. Under the right conditions, however, the hollow niobium structures can also be used to probe phenomena that have hardly been explored so far. These include gravitational waves, which reach Earth from major galactic events, and axions, which may constitute the enigmatic dark matter but are extremely difficult to detect.
DESY researcher Krisztian Peters has now secured the exceptionally high sum of 3.4 million euros from the European Research Council (ERC) to develop a new detector within the project SONAR ("Superconducting Oscillator for Novel Astrophysical Radiation"), whose core components are two such cavities.
Together with his team, he aims to search for high‑frequency gravitational waves and ultra‑light axions in regions where no measurements exist so far. Any hint of axions or gravitational waves in this range would be a clear sign of hitherto unknown phenomena in the universe – and would amount to a scientific revolution. ERC Advanced Grants are among the most prestigious awards that individual researchers in Europe can receive; they recognise both the scientific excellence and track record of the researcher and the uniqueness and originality of the proposed project.
The science behind SONAR
Gravitational waves are tiny ripples in space that reveal extreme cosmic events which cannot be seen with telescopes. Scientists expect that not only the low‑frequency gravitational‑wave signals already detected by instruments such as LIGO exist in the universe, but also extremely high‑frequency ones, for which current detectors cover narrow frequency bands at best. It is precisely these high frequencies that SONAR is targeting. In this window, the team hopes to detect signals from sources that are so far purely hypothetical – and at the same time benefits from the fact that astrophysical background noise is particularly low here. A gravitational‑wave signal in this range would be a strong indication of something genuinely new, such as exotic compact objects, traces of the early universe or previously unknown particles beyond existing theories.
The fundamental theory of particle physics, the Standard Model, describes only a small fraction of what exists in the universe. In particular, it leaves open what dark matter is made of – the invisible mass whose gravity holds galaxies together. Even experiments at giant particle accelerators such as the Large Hadron Collider (LHC) at CERN near Geneva have found no evidence of dark matter or other deviations from the Standard Model yet. Axions are hypothetical particles that could solve the mystery of dark matter. Small, specialised axion experiments are of particular interest here because they can search within very specific mass and energy ranges. DESY is already running a highly sensitive axion experiment called ALPS II; SONAR will be a new, complementary experiment.
Technically, SONAR uses a superconducting microwave cavity in which electromagnetic waves circulate in two almost identical oscillation modes. A passing gravitational wave would very slightly stretch and squeeze the cavity, shifting energy from one mode to the other – a minute effect that ultra‑sensitive electronics can detect. The same setup can also be used to search for ultra‑light axions, which are prime candidates for the mysterious dark matter that must exist in the universe but has not yet been observed experimentally. If this technologically ambitious approach succeeds, SONAR would be among the first dedicated experiments to search for high‑frequency gravitational waves – and could open the door to completely new physics.
Concentrated expertise on campus
Krisztian Peters’ team will build on the experience they have already gained with a prototype cavity (link). “SONAR benefits enormously from the concentrated expertise gathered on the DESY–Universität Hamburg campus – we have specialists for accelerators, superconductivity, cryogenics, readout systems, theory, axions and much more,” says Krisztian Peters.
SONAR is DESY’s first ERC Advanced Grant. Competition was fierce: only 9.6 percent of all submitted proposals were selected. In total, 3329 proposals were submitted, and SONAR is one of 319 funded projects. Principal Investigators (PI) for ERC Advanced Grants must be active researchers with an outstanding track record and must be recognised leaders in terms of the originality and significance of their contributions. As a leading scientist in the particle physics division at DESY, Krisztian Peters focuses on the search for dark matter and other phenomena beyond the Standard Model of particle physics. He is also a key researcher in the “Quantum Universe” Cluster of Excellence at Universität Hamburg.
“My heartfelt congratulations to Krisztian Peters on this award,” says Ulrich Husemann, Director of Research in Particle Physics at DESY. “I am looking forward to SONAR, the new experiment on campus, which perfectly complements our research portfolio and builds on our expertise in particle physics, accelerator development and dark matter research.”
Over the five‑year project period, SONAR will fund two postdoctoral and four doctoral researchers and will see the construction of the two cavities. This includes the development of a control and readout system and of seismic isolation, commissioning in a cryostat, as well as data taking and analysis.
“I hope that SONAR will act as a technological pathfinder for the detection of cosmological gravitational waves in the high‑frequency regime, much as the early developments in laser interferometry paved the way for the first discovery of gravitational waves,” says Krisztian Peters.
