Time distribution of the muons observed with the Muon Detector of the Pierre Auger Observatory

DIEGO RAVIGNANI; JUAN MANUEL FIGUEIRA; ALEJANDRO ALMELA; MARÍA BELÉN ANDRADA; ANA MARTINA BOTTI; ANGEL CANCIO; ALBERTO ETCHEGOYEN; LUCIANO FERREYRO; ALAN FUSTER; BEATRIZ GARCÍA; NICOLÁS GONZÁLEZ; MATÍAS ROLF HAMPEL; EWA HOLT; JOHANNES HULSMAN; MARIELA JOSEBACHUILI; AGUSTÍN LUCERO; DIEGO MELO; SARAH MÜLLER; MATIAS PERLIN; MANUEL PLATINO; MATÍAS RONCORONI; FEDERICO SÁNCHEZ; CHRISTIAN SARMIENTO-CANO; DAVID SCHMIDT; MARINA SCORNAVACCHE; GAIA SILLI; ÁLVARO TABOADA; BRIAN WUNDHEILER

Otro; Pierre Auger Collaboration internal note; 2019

We synchronised the MD using events observed in coincidence with the SD thus enabling the muontiming in GPS coordinates without using a dedicated receiver. We implemented a two-step synchronisation procedure that allowed the combination of the muons observed by the modules located in the same array position with an uncertainty of only 8 ns.  Mixing muons from different modules is necessary to perform any time analysis as only few particles are usually detectedby each module. The new timing capability puts the MD on a par with the SD and the FD, which measure both the signal size and time. We applied the synchronisation to measure the time distribution of the muons reaching the MD using three different methods. With the average traces we measured the time spread of the MD and the SD signals as function of the distance to the shower axis up to 800 m and found that the MD signals are significantly narrower than SD ones. The relative widths of the MD and SD signals are consistent with the muons observed by the MD arriving more packed than electrons and gammas. We confirmed that the SD signals are wider than the MD ones with a method independent of the average traces that is based on the rise time of the signals. We also verified the growth of the MD signalspread with the distance using detector pairs, a method that resembles the one applied to estimate the time variance of the SD signals. The muon timing allowed us to reconstruct the shower geometry with the MD to find the arrival direction of the observed cosmic rays. We calculated the angular resolution of the MD applying the same method used for the SD. The timing of the muons will enable the measurement of the muon production depth with the MD. The MPD maximum will be a new composition observable,complementary of the Xmax from the FD and the rise time from the SD. With muon counters installed in all the in ll positions, the MD will target the energy region between 3x10^17eV and 10^19eV. The new data are expected to help establishing a consistent picture of the changes in the chemical composition at energies where the cosmic rays change from a Galactic to an extragalactic origin. We performed simulations from where we found that there is an excellent agreement between the muon time-distributions at 2.3 m underground of simulated and real events, and that both the real and simulated distributions can be properly described by an exponentially modified Gaussian function. Furthermore, we have also shown that the means and widths of simulated distributions as a function of the distance to the shower core are in good agreement with the ones obtained from real events.