Atmospherics effects on events measured by the Pierre Auger Observatory surface detector

Open Access
Author:
Criss, Adrienne Elizabeth
Graduate Program:
Physics
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
February 21, 2012
Committee Members:
  • Paul Sommers, Dissertation Advisor
  • Paul Sommers, Committee Chair
  • Stephane Coutu, Committee Member
  • Tyce De Young, Committee Member
  • Peter Istvan Meszaros, Committee Member
  • Jorge Osvaldo Sofo, Committee Member
Keywords:
  • cosmic rays
  • ultra-high energy
  • Pierre Auger Observatory
  • atmosphere
  • atmospheric effects
Abstract:
The nature of ultra high energy cosmic rays, nuclear particles from outer space with energies greater than 10^18 eV, is one of the big mysteries of physics due to the low flux of events that reach earth. This small flux necessitates the use of detectors with large collecting areas and excludes using satellite and balloon borne experiments which could detect cosmic rays directly. Instead, we must study the extensive air shower (EAS) caused by a primary cosmic ray as it traverses the atmosphere. When a cosmic ray enters the earth’s atmosphere it interacts to produce pions. The pions continue to interact until the energy per particle falls below a critical threshold, roughly 2000 MeV. While the charged pions produce muons and neutrinos, the neutral pions produce electromagnetic cascades of electrons, positrons, and gamma rays. This collection of particles is known as an EAS. The Pierre Auger Observatory was built to study these rare cosmic rays and answer fundamental questions about them, such as their origin and composition. Located in Argentina, the Pierre Auger Observatory detects the EASs created by primary cosmic rays. Since the development of EASs is affected by the atmosphere, it is important to understand these effects so that we can more accurately reconstruct the events. An accurate determination of the primary cosmic ray energy leads to the ability to make more sensitive anisotropy studies, for example. To this end, rate variations over annual and diurnal cycles are studied. We look at rate variations in comparison with atmospheric temperature and pressure. Using a modified Rayleigh analysis we show that the rate lags behind the temperature in both cases. A lag of 2 weeks is seen in the annual case; in the diurnal case a lag of 2 hours is visible. Both effects are seen at greater than a 3σ level using events with zenith angles less than 45 degrees. From the modified Rayleigh analysis, we also obtain coefficients which are used to normalize the atmospheric parameters to the rate. Since these coefficients measure the strength of the effect on the rate caused by that parameter, they can be used to remove the atmospheric effects. Using these atmospheric correction coefficients and phase lags, we correct the event energies. These corrections remove the spurious sinusoidal variations seen in the raw data. Redoing the analysis with the atmospheric corrected data, we find that the annual and diurnal rates are flat within uncertainties. This improves the upper limits on cosmic ray anisotropy in right ascension. No evidence of anisotropy is found in either the first or second harmonic over 5 differential energy bins. Anisotropy amplitudes of 1-3% are seen in the 4 ≤ E < 8 EeV and 8 EeV ≤ E energy ranges for both the first and second harmonic; the significances are less than 1.5σ in all cases though. At lower energies, the anisotropy amplitudes are generally even lower. The significances of detections are still below 3σ.