Researchers from the international KM3NeT collaboration have used atmospheric neutrinos detected deep beneath the Mediterranean Sea to test a fundamental principle of physics, establishing new limits on possible violations of Lorentz invariance.
The international research team analysed 5,828 atmospheric neutrino candidate events recorded by KM3NeT/ORCA, an underwater neutrino telescope located in the Mediterranean Sea. The study found no evidence of Lorentz invariance violation but established new experimental limits on the possible size of such effects.
Lorentz invariance is a fundamental principle underlying Einstein’s theory of relativity. It states that the laws of physics remain the same for observers moving at constant velocities relative to one another. However, some theories attempting to reconcile gravity with quantum physics suggest that this principle may not always hold.
The findings, published in the Journal of Cosmology and Astroparticle Physics (JCAP), also provide the first direct constraints on certain possible differences in the behaviour of muon and tau neutrinos.
Investigating Whether the Laws of Physics Hold Across the Universe
The search for possible violations of Lorentz invariance is connected to efforts to develop a theory of quantum gravity, which would reconcile general relativity with quantum physics.
Some theoretical approaches suggest that spacetime, generally considered continuous, could instead consist of extremely small, discrete units. Such a possibility could have implications for fundamental physical laws, including Lorentz invariance.
“We still don’t know whether our spacetime is continuous or might be discrete,” explains Alba Domi, a researcher at the Erlangen Centre for Astroparticle Physics (ECAP), Friedrich-Alexander-Universität Erlangen-Nürnberg, and one of the study’s two corresponding authors.
“Looking for violations of Lorentz invariance is one way to investigate this possibility.”
Neutrinos offer a useful way to investigate these questions because they rarely interact with matter and can travel enormous distances, including through the Earth. They also undergo a phenomenon called neutrino oscillation, in which they change from one flavour, or type, to another as they travel.
Scientists can predict the probability of these transformations with high precision. Differences between observed behaviour and theoretical predictions could therefore indicate new physics, potentially including violations of Lorentz invariance.
Researchers Analyse Atmospheric Neutrino Data
To investigate possible deviations, the team examined 1.4 years of data collected using the first six detection units of KM3NeT/ORCA.
The researchers compared the observed neutrino energies and arrival directions with theoretical predictions to search for evidence of Lorentz invariance violation.
Although the analysis found no evidence of such violations, it established new limits on how large certain effects could be without conflicting with the observations.
The results are competitive with measurements from long-running experiments such as IceCube and Super-Kamiokande, despite being obtained using an early, partially constructed version of the detector.
The study also established the first direct limits on certain possible Lorentz invariance violation effects involving muon and tau neutrinos and their oscillations.
Expanding Detector Could Improve Future Investigations
At the time of the analysis, KM3NeT/ORCA was operating with only six detection units and was still under construction. The detector is being expanded towards its full planned configuration.
“These results show the potential of KM3NeT/ORCA to test fundamental principles of physics, even with a relatively small detector,” says Lukas Hennig, a PhD student at ECAP and the study’s other corresponding author.
“With more data and an expanding detector, we will be able to search for even smaller effects.”
Researchers also plan to extend their investigations to KM3NeT/ARCA, which focuses on higher-energy neutrinos arriving from distant astrophysical sources.
These particles could be particularly useful because some potential effects of Lorentz invariance violation may become more pronounced at higher energies and over very long distances.
“ARCA could be particularly well suited to this kind of research, but we don’t yet have enough astrophysical neutrino events to perform such an analysis,” explains Domi.
KM3NeT Uses Two Underwater Detectors to Study Neutrinos
KM3NeT is an international research infrastructure under construction in the Mediterranean Sea. It consists of two underwater neutrino telescopes, ORCA and ARCA, which use networks of sensitive light sensors to detect faint flashes produced when neutrinos interact with matter.
ORCA, or Oscillation Research with Cosmics in the Abyss, is located approximately 40 kilometres off the coast of Toulon, France, at a depth of around 2,450 metres. It studies neutrinos produced in Earth’s atmosphere, with a focus on their oscillations between different flavours.
ARCA, or Astroparticle Research with Cosmics in the Abyss, is located off the coast of Sicily, near Portopalo di Capo Passero, at a depth of approximately 3,500 metres. It focuses on higher-energy neutrinos originating from distant astrophysical sources and helps researchers investigate some of the Universe’s most energetic phenomena.
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