Određivanje sastava kozmičkog zračenja iz mjerenja površinskim detektorima opservatorija Pierre Auger

The Pierre Auger Observatory started collecting data in 2004. The obtained dataset is of unprecedented quality and quantity enabling information about ultra high energy cosmic ray flux and origin. However, due to very limited duty cycle of Fluorescence Detector, the reliable information about UHECR mass compostion in the region of the highest primary energies is still missing. Therefore, the accent has been put on finding a method to infer mass composition from measurements of the Surface Detector which duty cycle is unlimited. Two approaches were investigated for obtaining mass composition information from the SD measurements. Correlation between muon content sensitive observables and structural and temporal properties of signal recorded in SD stations was studied in details. Muon fraction estimator was researched and determined. Its performance was studied on wide range of air shower observables and potential applications. It was applied to the PAO SD data and estimate of muon signal was obtained. Comparison to simulations of different hadronic interactions models was perfomed. The deficit of predicted muon signal was confirmed and quantified with respect to obtained estimated value. Detailed study of correlation between the depth of the shower maximum and SD observables related to shower front was performed. The Xmax estimator based on SD observables was determined and its performance was studied. In addition, the empirical start time variance model was obtained and applied in reconstruction of shower geometry related SD observables.Više od stotinu godina od njihova otkrića, kozmičko zračenje i njegova svojstva još uvijek pobuđuju značajno zanimanje znanstvenika diljem svijeta. Štoviše, posebnu znanstvenu nepoznanicu predstavljaju karakteristike kozmičkog zračenja najviših energija, onih iznad 1018 eV, koje svojim iznosom nadilazi najveće dostupne energije na ubrzivačima čestica koje su ljudi danas sposobni napraviti, poput velikog hadronskog sudarivača (LHC) na Institutu “European Organization for Nuclear Research” (CERN) u Švicarskoj. I upravo je radi toga, ali i drugih razloga, pitanje o prirodi i izvorima tih čestica najvećih energija ikad zabilježenih u Prirodi, uvršteno početkom ovog stoljeća među “11 najvećih neriješenih pitanja fizike” u časopisu “Nature”

Performance and systematic uncertainties of CTA-North in conditions of reduced atmospheric transmission

The Cherenkov Telescope Array (CTA) is the next-generation stereoscopic system of Imaging Atmospheric Cherenkov Telescopes (IACTs). In IACTs, the atmosphere is used as a calorimeter to measure the energy of extensive air showers induced by cosmic gamma rays, which brings along a series of constraints on the precision to which energy can be reconstructed. The presence of clouds during observations can severely affect Cherenkov light yield, contributing to the systematic uncertainty in energy scale calibration. To minimize these systematic uncertainties, a calibration of telescopes is of great importance. For this purpose, the influence of cloud transmission and altitude on CTA-N performance degradation was investigated using detailed Monte Carlo simulations for the case where no action is taken to correct for the effects of clouds. Variations of instrument response functions in the presence of clouds are presented. In the presence of clouds with low transmission ($\leq$ 80%) the energy resolution is aggravated by 30% at energies below 1 TeV, and by 10% at higher energies. For higher transmissions, the energy resolution is worse by less than 10% in the whole energy range. The angular resolution varies up to 10% depending both on the transmission and altitude of the cloud. The sensitivity of the array is most severely reduced at lower energies, even by 60% at 40 GeV, depending on the clouds' properties. A simple semi-analytical model of sensitivity degradation has been introduced to summarize the influence of clouds on sensitivity and provide useful scaling relations