dissertation
Gašper Kukec Mezek (Author), Andrej Filipčič (Mentor)

Abstract

Cosmic rays with energies above 10^18 eV, usually referred to as ultra-high energy cosmic rays (UHECR), have been a mystery from the moment they have been discovered. Although we have now more information on their extragalactic origin, their direct sources still remain hidden due to deviations caused by galactic magnetic fields. Another mystery, apart from their production sites, is their nature. Their mass composition, still uncertain at these energies, would give us a better understanding on their production, acceleration, propagation and capacity to produce extensive air showers in the Earth's atmosphere. Mass composition studies of UHECR try to determine their nature from the difference in development of their extensive air showers. In this work, observational parameters from the hybrid detection system of the Pierre Auger Observatory are used in a multivariate analysis to obtain the mass composition of UHECR. The multivariate analysis (MVA) approach combines a number of mass composition sensitive variables and tries to improve the separation between different UHECR particle masses. Simulated distributions of different primary particles are fitted to measured observable distributions in order to determine individual elemental fractions of the composition. When including observables from the surface detector, we find a discrepancy in the estimated mass composition between a mixed simulation sample and the Pierre Auger data. Our analysis results from the Pierre Auger data are to a great degree independent on hadronic interaction models. Although they differ at higher primary masses, the different models are more consistent, when combining fractions of oxygen and iron. Compared to previously published results, the systematic uncertainty from hadronic interaction models is roughly four times smaller. Our analysis reports a predominantly heavy composition of UHECR, with more than a 50% fraction of oxygen and iron at low energies. The composition is then becoming heavier with increasing energy, with a fraction of oxygen and iron above 80% at the highest energies.

Keywords

astroparticle physics;ultra-high energy cosmic rays;extensive air showers;mass composition;Pierre Auger Observatory;machine learning;multivariate analysis;

Data

Language: English
Year of publishing:
Typology: 2.08 - Doctoral Dissertation
Organization: UNG FPŠ - Graduate School
Publisher: [G. Kukec Mezek]
UDC: 539.1:524.1(043.3)
COBISS: 5364987 Link will open in a new window
Views: 3956
Downloads: 167
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Other data

Secondary language: Slovenian
Secondary title: Delčna sestava kozmičnih žarkov ekstremnih energij na observatoriju Pierre Auger
Secondary abstract: Kozmični žarki z energijami nad 10^18 eV, ki jih poimenujemo tudi kozmični žarki ekstremnih energij (UHECR), dosegajo energije trenutno nedosegljive trkalnikom delcev. Pri njihovem prehodu skozi Zemljino atmosfero tvorijo obširne atmosferske plazove sekundarnih delcev, ki jih detektiramo z obširnimi polji vodnih detektorjev Čerenkove svetlobe in detektorji fluorescenčne svetlobe. Zaradi posredne detekcije preko plazov sekundarnih delcev in uklanjanja kozmičnih delcev v galaktičnih magnetnih poljih, pa sta delčna sestava in izvori UHECR še odprti vprašanji. Določitev obeh bi nam omogočala boljši vpogled v njihov nastanek, pospeševanje, propagacijo in zmožnost tvorjenja plazov v Zemljini atmosferi. Raziskave delčne oziroma masne sestave UHECR temeljijo na razliki, ki jih ti povzročijo pri razvoju plazov sekundarnih delcev. V tem delu združimo izmerjene podatke obeh detekcijskih sistemov observatorija Pierre Auger v skupno analizo po večih spremenljivkah za določitev masne sestave UHECR. Tako imenovana multivariabilna analiza (MVA) združi več masno odvisnih spremenljivk in pripomore k izboljšani masni separaciji. Pri tem primerjamo porazdelitve simuliranih dogodkov z izmerjenimi porazdelitvami in s tem ocenimo delež posameznih delcev. Pri vključitvi spremenljivk detektorjev Čerenkove svetlobe prihaja do neskladja med podatki observatorija Pierre Auger in simulacijami. Masna sestava UHECR je pri ekstremnih energijah nezanesljiva zaradi odvisnosti od modelov hadronskih interakcij. Naši rezultati kažejo to modelsko odvisnost le pri težjih primarnih delcih, ki pa se močno zmanjša po združitvi deležev kisika in železa, ter je približno štirikrat manjša kot pri ostalih objavljenih rezultatih. Prav tako naši rezultati kažejo na predvsem težjo sestavo UHECR z več kot 50% deležem kisika in železa pri nizkih energijah, ter več kot 80% deležem kisika in železa pri najvišjih energijah.
Secondary keywords: astrofizika osnovnih delcev;kozmični žarki ekstremnih energij;obširni atmosferski plaz sekundarnih delcev;masna sestava;delčna sestava;observatorij Pierre Auger;strojno učenje;disertacije;Kozmični žarki;Disertacije;Delčna sestava;
URN: URN:SI:UNG
Type (COBISS): Doctoral dissertation
Thesis comment: Univ. v Novi Gorici, Fak. za podiplomski študij
Pages: VIII, 143 str.
ID: 11100089