Depth of shower maximum and mass composition of cosmic rays from 50 PeV to 2 EeV measured with the LOFAR radio telescope

We present an updated cosmic-ray mass composition analysis in the energy range 10^16.8 to 10^18.3 eV from 334 air showers measured with the LOFAR radio telescope and selected for minimal bias. In this energy range, the origin of cosmic rays is expected to shift from galactic to extragalactic sources. The analysis is based on an improved method to infer the depth of the maximum X_max of extensive air showers from radio measurements and air shower simulations. We show results of the average and standard deviation of X_max versus primary energy and analyze the X_max dataset at the distribution level to estimate the cosmic ray mass composition. Our approach uses an unbinned maximum likelihood analysis, making use of existing parametrizations of the X_max distributions per element. The analysis has been repeated for three main models of hadronic interactions. Results are consistent with a significant light-mass fraction, at best fit 23% to 39% protons plus helium, depending on the choice of hadronic interaction model. The fraction of the intermediate-mass nuclei dominates. This confirms earlier results from LOFAR, with systematic uncertainties on X_max now lowered to 7 to 9 g /cm² . We find agreement in mass composition compared to results from Pierre Auger Observatory, within statistical and systematic uncertainties. However, in line with earlier LOFAR results, we find a slightly lower average X_max. The values are in tension with those found at Pierre Auger Observatory but agree with results from other cosmic ray observatories based in the northern hemisphere.