Inferences on mass composition and tests of hadronic interactions from 0.3 to 100 EeV using the water-Cherenkov detectors of the Pierre Auger Observatory

We present a new method for probing the hadronic interaction models atultrahigh energy and extracting details about mass composition. This isdone using the time profiles of the signals recorded with thewater-Cherenkov detectors of the Pierre Auger Observatory. The profilesarise from a mix of the muon and electromagnetic components of airshowers. Using the risetimes of the recorded signals, we define a newparameter, which we use to compare our observations with predictionsfrom simulations. We find, first, inconsistencies between our data andpredictions over a greater energy range and with substantially moreevents than in previous studies. Second, by calibrating the newparameter with fluorescence measurements from observations made at theAuger Observatory, we can infer the depth of shower maximumXmax for a sample of over 81,000 events extending from 0.3 toover 100 EeV. Above 30 EeV, the sample is nearly 14 times larger thanwhat is currently available from fluorescence measurements and extendingthe covered energy range by half a decade. The energy dependence of⟨Xmax⟩ is compared to simulations and interpretedin terms of the mean of the logarithmic mass. We find good agreementwith previous work and extend the measurement of the mean depth ofshower maximum to greater energies than before, reducing significantlythe statistical uncertainty associated with the inferences about masscomposition.