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Researchers at the Pierre Auger Observatory have used fluctuations in muon numbers from cosmic ray-induced air showers to test whether Lorentz invariance—a fundamental symmetry of spacetime—is violated at extremely high energies near the Planck scale. This novel approach has produced the strongest constraints yet on Lorentz invariance violations in the hadronic sector, without requiring assumptions about cosmic ray composition. The method opens a new observational window into quantum gravity effects that cannot be accessed through other measurements.
Why it matters
These findings advance our understanding of fundamental physics by testing predictions from quantum gravity theories using cosmic ray observations. The technique provides an independent way to probe spacetime symmetries at energy scales that cannot be reached in terrestrial particle accelerators, potentially revealing new physics beyond the Standard Model.
arXiv:2602.14720v3 Announce Type: replace
Abstract: Quantum gravity theories often modify spacetime symmetries. In particular, Lorentz invariance may be violated when approaching the Planck scale. Although the scales at which interactions occur in extensive air showers induced by ultra-high-energy cosmic rays in the atmosphere are many orders of magnitude below the Planck scale, these violations might still be observable. In this work, the fluctuations in the number of muons in the extensive air showers measured at the Pierre Auger Observatory are exploited, for the first time, to constrain Lorentz invariance violations. The bounds derived in the hadronic sector are the strongest ever obtained, and do not rely on assumptions about the mass composition of ultra-high-energy cosmic rays. The fluctuations in the number of muons constitute a new and powerful observable to further explore Lorentz invariance in a region of the parameter space not accessible to other observables.
Source: Bounds on Lorentz invariance violation from muon fluctuations at the Pierre Auger Observatory