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

THE PIERRE AUGER COLLABORATION, B. WUNDHEILER

PHYSICAL REVIEW D - PARTICLE AND FILDS

American Physical Society

Año: 2017

0556-2821

We present a new method for probing the hadronic interaction models at ultrahigh energy and extractingdetails about mass composition. This is done using the time profiles of the signals recorded with the water-Cherenkov detectors of the Pierre Auger Observatory. The profiles arise from a mix of the muon andelectromagnetic components of air showers. Using the risetimes of the recorded signals, we define a newparameter, which we use to compare our observations with predictions from simulations. We find, first,inconsistencies between our data and predictions over a greater energy range and with substantially moreevents than in previous studies. Second, by calibrating the new parameter with fluorescence measurementsfrom observations made at the Auger Observatory, we can infer the depth of shower maximum X max for asample of over 81,000 events extending from 0.3 to over 100 EeV. Above 30 EeV, the sample is nearly14 times larger than what is currently available from fluorescence measurements and extending the coveredenergy range by half a decade. The energy dependence of hXmax i is compared to simulations andinterpreted in terms of the mean of the logarithmic mass. We find good agreement with previous work andextend the measurement of the mean depth of shower maximum to greater energies than before, reducingsignificantly the statistical uncertainty associated with the inferences about mass composition.