J. Coppens

Measurement of the depth of maximum of extensive air showers above 10(18) eV

We describe the measurement of the depth of maximum, Xmax, of the longitudinal development of air showers induced by cosmic rays. Almost four thousand events above 10^18 eV observed by the fluorescence detector of the Pierre Auger Observatory in coincidence with at least one surface detector station are selected for the analysis. The average shower maximum was found to evolve with energy at a rate of (106 +35/-21) g/cm^2/decade below 10^(18.24 +/- 0.05) eV and (24 +/- 3) g/cm^2/decade above this energy. The measured shower-to-shower fluctuations decrease from about 55 to 26 g/cm^2. The interpretation of these results in terms of the cosmic ray mass composition is briefly discussed.

The Fluorescence Detector of the Pierre Auger Observatory

The Pierre Auger Observatory is a hybrid detector for ultra-high energy cosmic rays. It combines a surface array to measure secondary particles at ground level together with a fluorescence detector to measure the development of air showers in the atmosphere above the array. The fluorescence detector comprises 24 large telescopes specialized for measuring the nitrogen fluorescence caused by charged particles of cosmic ray air showers. In this paper we describe the components of the fluorescence detector including its optical system, the design of the camera, the electronics, and the systems for relative and absolute calibration. We also discuss the operation and the monitoring of the detector. Finally, we evaluate the detector performance and precision of shower reconstructions.

Upper Limit on the Diffuse Flux of Ultrahigh Energy Tau Neutrinos from the Pierre Auger Observatory

The surface detector array of the Pierre Auger Observatory i s sensitive to Earthskimming tau-neutrinosντ that interact in the Earth’s crust. Tau leptons from ντ charged-current interactions can emerge and decay in the at mosphere to produce a nearly horizontal shower with a significant electromagneti c component. The data collected between 1 January 2004 and 31 August 2007 are used t o place an upper limit on the diffuse flux ofντ at EeV energies. Assuming an E ν differential energy spectrum the limit set at 90 % C.L. is E ν dNντ /dEν < 1.3 × 10 −7 GeV cm s sr in the energy range 2× 10eV < Eν < 2× 10 eV. The detection of Ultra High Energy (UHE) cosmic neutrinos at EeV (1 EeV≡ 10 eV) energies and above is a long standing experimental chall enge. Many experiments are searching for such neutrinos, and there are se veral ongoing efforts to construct dedicated experiments to detect them [Halzen et a l.(2002), Halzen(2007), Falcke et al.(2004)]. Their discovery would open a new windo w to the universe [Becker(2007)], and provide an unique opportunity to test fundamental parti cle physics at energies well beyond current or planned accelerators. The observati on of UHE Cosmic Rays (UHECRs) requires that there exist UHE cosmic neutrinos, ev en though the nature of the UHECR particles and their production mechanisms are s till uncertain. All models of UHECR origin predict neutrino fluxes from the decay of charged pions which are produced either in interactions of the cosmic rays in their sources, or in their subsequent interactions with background radiation fi elds. For example, UHECR protons interacting with the Cosmic Microwave Background ( CMB) give rise to the so-called ‘cosmogenic’ or GZK neutrinos [Berezinsky et al. (1969)]. The recently reported suppression of the cosmic ray flux above ∼ 4 × 10 eV [Abbasi et al.(2007), Yamamoto(2007), Pierre Auger Collaboration(2007a)] as we ll as the observed correlation of the highest energy cosmic rays with relatively nea rby extragalactic objects [Pierre Auger Collaboration(2007b)] both point to UHECR in teractions on the infrared or microwave backgrounds during extragalactic propagatio n. These interactions must result in UHE neutrinos although their flux is somewhat uncer tain since this depends on the primary UHECR composition and on the nature and cosmolog ical evolution of the sources as well as on their spatial distribution [Engel et al .(2001), Allard et al.(2006)]. Tau neutrinos are suppressed in such production processes r elative toνe or νμ, because they are not an end product of the charged pion decay c hain and far fewer are made through the production and decay of heavy flavours su ch as charm. Nevertheless, because of neutrino flavour mixing, the usual 1:2 ratio of νe to νμ at production is altered to approximately equal fluxes for all flavo urs after travelling cosmological distances [Learned et al.(1995)]. Soon after the discovery of neutrino oscillations [Fukuda et al.(1998)] it was shown that ντ entering the Earth just below the horizon (Earth-skimming) [Fargion(2002), Letessier-Sel von(2001), Feng et al.(2002)] can undergo charged-current interactions and produce τ leptons. Since aτ lepton can travel tens of kilometers in the Earth at EeV energies, it can emerge into the atmosphere and decay in flight producing an nearly horizontal extensive a r shower (EAS) above the detector. In this way the effective target volume for neu trinos can be rather large. The Pierre Auger Observatory [Abraham et al.(2004)] has bee n designed to measure UHECRs with unprecedented precision. Detection of UHE CRs is being achieved

Constraints on the origin of cosmic rays above 10(18) eV from large-scale anisotropy searches in data of the Pierre Auger Observatory

A thorough search for large-scale anisotropies in the distribution of arrival directions of cosmic rays detected above 10(18) eV at the Pierre Auger Observatory is reported. For the first time, these large-scale anisotropy searches are performed as a function of both the right ascension and the declination and expressed in terms of dipole and quadrupole moments. Within the systematic uncertainties, no significant deviation from isotropy is revealed. Upper limits on dipole and quadrupole amplitudes are derived under the hypothesis that any cosmic ray anisotropy is dominated by such moments in this energy range. These upper limits provide constraints on the production of cosmic rays above 10(18) eV, since they allow us to challenge an origin from stationary galactic sources densely distributed in the galactic disk and emitting predominantly light particles in all directions.A thorough search for large scale anisotropies in the distribution of arrival directions of cosmic rays detected above 1018 eV at the Pierre Auger Observatory is reported. For the first time, these large scale anisotropy searches are performed as a function of both the right ascension and the declination and expressed in terms of dipole and quadrupole moments. Within the systematic uncertainties, no significant deviation from isotropy is revealed. Upper limits on dipole and quadrupole amplitudes are derived under the hypothesis that any cosmic ray anisotropy is dominated by such moments in this energy range. These upper limits provide constraints on the production of cosmic rays above 1018 eV, since they allow us to challenge an origin from stationary galactic sources densely distributed in the galactic disk and emitting predominantly light particles in all directions. Subject headings: astroparticle physics — cosmic rays The large scale distribution of arrival directions of Ultra-High Energy Cosmic Rays (UHECRs) as a function of the energy is a key observable to provide further understanding of their origin. Above ≃ 0.25 EeV, the most stringent bounds ever obtained on the dipole component in the equatorial plane were recently reported, being below 2% at 99% C.L. for EeV energies (Auger Collaboration 2011a). Such a sensitivity provides some constraints upon scenarios in which dipolar anisotropies could be imprinted in the distribution of arrival directions as the result of the escape of UHECRs from the Galaxy up to the ankle energy (Ptuskin et al. 1993; Candia et al. 2003; Giacinti et al. 2012). On the other hand, if UHECRs above 1 EeV have already a predominant extragalactic origin (Hillas 1967; Blumenthal 1970; Berezinsky et al. 2006; Berezinsky et al. 2004), their angular distribution is expected to be isotropic to a high level. Thus, the study of large scale anisotropies at EeV energies would help in establishing whether the origin of UHECRs is galactic or extragalactic in this energy range. The upper limits aforementioned are based on first harmonic analyses of the right ascension distributions in several energy ranges. The analyses benefit from the almost uniform directional exposure in right ascension of any ground based observatory operating with high duty cycle, but are not sensitive to a dipole component along the Earth rotation axis. In contrast, using the large amount of data collected by the surface detector array of the Pierre Auger Observatory, we report in this letter on searches for dipole and quadrupole patterns significantly standing out above the background noise whose components are functions of both the right ascension and the declination (a detailed description of the present analysis can be found in (Auger Collaboration 2012)).

Interpretation of the depths of maximum of extensive air showers measured by the Pierre Auger Observatory

To interpret the mean depth of cosmic ray air shower maximum and its dispersion, we parametrize those two observables as functions of the first two moments of the

Anisotropy and chemical composition of ultra-high energy cosmic rays using arrival directions measured by the Pierre Auger Observatory

\ln A

Limit on the diffuse flux of ultrahigh energy tau neutrinos with the surface detector of the Pierre Auger Observatory

distribution. We examine the goodness of this simple method through simulations of test mass distributions. The application of the parameterization to Pierre Auger Observatory data allows one to study the energy dependence of the mean

Observation of radio signals from air showers at the Pierre Auger Observatory

\ln A

The rapid atmospheric monitoring system of the Pierre Auger Observatory

and of its variance under the assumption of selected hadronic interaction models. We discuss possible implications of these dependences in term of interaction models and astrophysical cosmic ray sources.To interpret the mean depth of cosmic ray air shower maximum and its dispersion, we parametrize those two observables as functions of the first two moments of the ln A distribution. We examine the goodness of this simple method through simulations of test mass distributions. The application of the parameterization to Pierre Auger Observatory data allows one to study the energy dependence of the mean ln A and of its variance under the assumption of selected hadronic interaction models. We discuss possible implications of these dependences in term of interaction models and astrophysical cosmic ray sources.

Upper limit on the diffuse flux of UHE tau neutrinos

The Pierre Auger Collaboration has reported. evidence for anisotropy in the distribution of arrival directions of the cosmic rays with energies E > E(th) = 5.5 x 10(19) eV. These show a correlation with the distribution of nearby extragalactic objects, including an apparent excess around the direction of Centaurus A. If the particles responsible for these excesses at E > E(th) are heavy nuclei with charge Z, the proton component of the sources should lead to excesses in the same regions at energies E/Z. We here report the lack of anisotropies in these directions at energies above E(th)/Z (for illustrative values of Z = 6, 13, 26). If the anisotropies above E(th) are due to nuclei with charge Z, and under reasonable assumptions about the acceleration process, these observations imply stringent constraints on the allowed proton fraction at the lower energies.