Constraints on Lorentz invariance violation from the extraordinary Mrk 421 flare of 2014 using a novel analysis method

The Lorentz Invariance Violation (LIV), a proposed consequence of certain quantum gravity (QG) scenarios, could instigate an energy-dependent group velocity for ultra-relativistic particles. This energy dependence, although suppressed by the massive QG energy scale EQG, expected to be on the level of the Planck energy 1.22 × 10¹⁹ GeV, is potentially detectable in astrophysical observations. In this scenario, the cosmological distances traversed by photons act as an amplifier for this effect. By leveraging the observation of a remarkable flare from the blazar Mrk\,421, recorded at energies above 100 GeV by the MAGIC telescopes on the night of April 25 to 26, 2014, we look for time delays scaling linearly and quadratically with the photon energies. Using for the first time in LIV studies a binned-likelihood approach we set constraints on the QG energy scale. For the linear scenario, we set $95%$ lower limits EQG>2.7×10¹⁷ GeV for the subluminal case and EQG> 3.6 ×10¹⁷ GeV for the superluminal case. For the quadratic scenario, the $95%$ lower limits for the subluminal and superluminal cases are EQG>2.6 ×10¹⁰ GeV and EQG>2.5×10¹⁰ GeV, respectively.