Bounds on Lorentz invariance violation from muon fluctuations at the Pierre Auger Observatory

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.

Figures (9)

• Figure 1: Mean lifetime of the neutral pion as a function of the energy for the Lorentz invariant case and for different strengths and orders of LIV.
• Figure 2: Average number of muons at ground vs primary energy in LI and LIV cases.
• Figure 3: Relative fluctuation of the number of muons vs primary energy; data (black points, from PierreAuger:2021qsd; the statistical uncertainty is indicated by the error bars, while the total systematic uncertainty by the square brackets) are compared to expectations from LI and LIV ($\eta=-10^{-3}$) models.
• Figure 4: Mixed relative fluctuations obtained using the parameterisations considering $\eta$ in the range $[-10^{-1},-10^{-16}]$ as a function of the primary energy, corresponding to the maximum with respect to $f$. Each colour corresponds to a different violation strength (right axis, where the arrow indicates the direction of increasing violation strength). The measured relative fluctuations in the number of muons are shown with black points PierreAuger:2021qsd.
• Figure 5: Relative differences in shower maximum (left panel) and deposited energy (right panel) as a function of $\log_{10}(E/\mathrm{eV})$ for different LIV values, using the EPOS-LHC hadronic interaction model. Both are expressed in percent.