Étude des rayons cosmiques par Auger et LHAASO : R&D et analyse de données d'AugerPrime et simulations pour LHAASO

Cosmic rays are charged particles, as well as coproducts like photons and neutrinos, originated in cosmic-ray sources inside or outside the Galaxy. They arrive at the top of the Earth's atmosphere with primary energies of up to a few 10 EeV. When the cosmic rays enter the atmosphere, they interact with the molecules in the air and produce a large number of secondary particles, creating an extensive air shower (EAS). The ground-based observation of the EAS can be used to deduce the energy, the arrival direction, and the mass composition of cosmic rays. The Pierre Auger Observatory and the Large High Altitude Air Shower Observatory (LHAASO) are both EAS observatories aiming at solving open questions of cosmic-ray studies but focusing on different energy ranges, the highest-energy and the so-called knee (around few PeV) regions. Based on the experience gained during the operation of the Pierre Auger Observatory for more than 10 years, the Auger collaboration has proposed an upgrade project, called ''AugerPrime'', with the aim of increasing the sensitivity of the surface detector array to the primary mass of cosmic rays. Both observatories employ the so-called ''hybrid detector arrays'' composed of optical telescopes overlooking the longitudinal development and ground detector arrays sampling the signal densities in the lateral direction of the EAS. The ground detector arrays of both observatories are being constructed or upgraded to have various types of particle detectors (scintillator and water-Cherenkov detectors), which allow us to decompose the electromagnetic and muonic components of the EAS. In this thesis, a series of studies contributing to the AugerPrime and LHAASO projects are presented. Concerning the AugerPrime project, the present study includes R&D work of the scintillator detector and data analysis of the engineering array. For the LHAASO project, simulations of the wide field of view Cherenkov telescope array and a multivariate analysis of LHAASO-hybrid observations for the primary mass identification are presented.Les rayons cosmiques sont des particules chargées, ainsi que des coproduits comme les photons et les neutrinos, issus de sources de rayons cosmiques galactiques ou extragalactiques. Ils arrivent au sommet de l'atmosphère terrestre avec des énergies primaires allant jusqu'à quelques 10 EeV. Lorsque les rayons cosmiques entrent dans l'atmosphère, ils interagissent avec les molécules de l'air et produisent un grand nombre de particules secondaires, créant une gerbe atmosphérique (extensive air shower, EAS). Accompagné des particules secondaires, une émission de la lumière Cherenkov et de la lumière fluorescence est induite par le passage des particules dans l'atmosphère. L'Observatoire Pierre Auger et Large High Altitude Air Shower Observatory (LHAASO) sont des observatoires dédiés à la détection des gerbes atmosphériques dans le but de répondre aux questions ouvertes concernant les rayons cosmiques, mais se concentrant sur différentes gammes d'énergie, les plus hautes énergies et les énergies autour de quelques PeV. Après plus de 10 ans d'exploitation de l'Observatoire Pierre Auger, la collaboration Auger a proposé une amélioration des détecteurs de son réseau de surface, appelée "AugerPrime". Le but est d'augmenter la sensibilité à la masse des particules primaires en ajoutant un détecteur scintillateur sur le détecteur Cherenkov à eau. Les deux observatoires sont dits «hybrides» car composés de télescopes optiques observant le développement longitudinal des gerbes et des réseaux de détecteurs de surface échantillonnant leurs profils latéraux. Dans cette thèse, une série d'études contribuant aux projets AugerPrime et LHAASO sont présentées. En ce qui concerne le projet AugerPrime, la présente étude comprend le travail de recherche & développement des scintillateurs et l'analyse de données du réseau de tester. Pour le projet LHAASO, des simulations de télescopes Cherenkov et une analyse multivariée des observations hybrides pour l'identification des masses primaires sont présentées

First results from the AugerPrime engineering array

International audienceThe surface detector array of the Pierre Auger Observatory consists of 1660water-Cherenkov detectors (WCDs) which sample the charged particles and photonsof air showers initiated by cosmic rays of very high energy. With the AugerPrimeupgrade, the collaboration aims to increase the particle identificationcapability of the surface detectors. Scintillator surface detectors (SSDs) willbe added above the water-Cherenkov detectors and the stations will be equippedwith new electronics having better timing accuracy, higher sampling frequency,and increased processing capability. Furthermore, small photomultipliers will beadded to the WCDs to allow for an increase of the dynamic range of the signalreadout. In October 2016, an engineering array consisting of 12 AugerPrimedetector stations was installed within the existing array of the Observatory. Inthis contribution, we will discuss the first results from the AugerPrimeengineering array. In particular, the detector calibration in units of verticalequivalent muon (VEM) for WCD and minimum ionizing particle (MIP) for SSD.Furthermore, we will discuss the temperature dependence of the detectorparameters, show the lateral distribution function of showers measured with bothdetector types, and present the preliminary results of the study on signals fromdoublet stations

Time-dependent yield of the hydrated electron and the hydroxyl radical in D2_2O: a picosecond pulse radiolysis study

International audiencePicosecond pulse radiolysis measurements were performed in neat D2O and H2O in order to study the isotopic effect on the time-resolved yield of the hydrated electron and hydroxyl radical. First, the absorption band of the hydrated electron in D2O, e(D2O)(-), is measured between 250 and 1500 nm. The molar absorption coefficient of the solvated electron spectrum in D2O was determined using the isosbestic point method by scavenging the solvated electron using methyl viologen. The amplitude and shape of the absorption spectrum of the hydrated electron in D2O are different from those previously reported in the literature. The maximum of the hydrated electron in the D2O absorption band is ca. 704 nm with a molar absorption coefficient of (22 900 +/- 500) L mol(-1) cm(-1). Based on this extinction coefficient, the radiolytic yield of e(D2O)(-) just after the 7 ps electron pulse was determined to be (4.4 +/- 0.2) x 10(-7) mol J(-1), which coincides with the one for e(H2O)(-) in H2O. The time-dependent radiolytic yield of e(D2O)(-) was determined from a few ps to 8 ns. To determine the OD center dot radical yield, the contribution of the solvated electron and of the transient species produced by the electron pulse in the windows of the fused silica optical cell was taken into account for the analysis of the transient absorption measurements at 260 nm. Therefore, an appropriate experimental methodology is used for measuring low absorbance at two different wavelengths in ps pulse radiolysis. The yield of the OD center dot radical just after the 7 ps electron pulse was found to be (5.0 +/- 0.2) x 10(-7) mol J(-1). In the spurs of ionization, the decay rate of e(D2O)(-) is slower than e(H2O)(-), whereas the decay rate of OD center dot is similar to the one of OH center dot. Here, the established time-dependent yield of the solvated electron and the hydroxyl radical provide the foundation for improving the models used for spur reaction simulations in heavy water mainly for the chemistry of CANDU reactors

Primary particle identification with MVA method for the LHAASO project

International audienceThe LHAASO (Large High Altitude Air Shower Observatory) project, which is under construction at high altitude of 4400m a.s.l. in Sichuan, China,aims to observe the extensive air showers (EAS) induced by cosmic rays in the atmosphere. LHAASO consists of several large detector arrays including KM2A (1 km$^2$ array), WCDA (Water Cherenkov Detector Array) and WFCTA (Wide Field of view Cherenkov Telescope Array). By employing hybrid detection technique, LHAASO offers an accurate measurement of the cosmic-ray spectrum and composition around the knee region. Furthermore, the primary particle identification can be obtained by using Multivariate Analysis (MVA). In this contribution, we present the parameters that will be measured by various detectors of LHAASO in the EAS detection and discuss the performance of the MVA method for primary particle identification

Measurement of the knees of proton and H&He spectra below 1 PeV

International audienceGalactic Cosmic ray (CR) origin is still a mystery. Measuring the knees of the CR spectra for individual species is a very important approach to solve the problem. ARGO-YBJ and LHAASO-WFCTA[1] combined experiment made the first step by measuring the spectrum of hydrogen plus helium nuclei and finding the knee around 0.7 PeV[2]. A significant boost is expected by using LHAASO experiment[3] to measure the spectra and their knees for pure proton and other species in few years. The key is to separate the specific species from all CR samples. In this paper, a multi variate analysis (MVA) approach for the CR composition analysis in LHAASO experiment is discussed. Preliminary results of the analysis and expectations are presented

Study of light yield for different configurations of plastic scintillators and wavelength shifting fibers

International audienceIn the effort of the AugerPrime scintillator surface detector R&D; activity, we investigated the performances of different extruded and cast plastic scintillators that were read out with wavelength-shifting (WLS) optical fibers and then coupled to a PMT. In particular we compared the light yield of eighteen scintillator/fiber configurations, obtained combining eight different scintillator bars with six fiber types, in order to investigate which was satisfying the AugerPrime specifications in terms of light production ( >12 photoelectrons per minimum ionizing particle). In this paper, we present the results of the study on different scintillator bar geometries, scintillator production techniques, and wavelength-shifting optical fiber types. We also propose an effective way to optically couple the fibers to the PMT entrance window

Erratum: Combined fit of spectrum and composition data as measured by the Pierre Auger Observatory

We present a combined fit of a simple astrophysical model of UHECR sources to both the energy spectrum and mass composition data measured by the Pierre Auger Observatory. The fit has been performed for energies above 5 ⋅ 10(18) eV, i.e. the region of the all-particle spectrum above the so-called ankle feature. The astrophysical model we adopted consists of identical sources uniformly distributed in a comoving volume, where nuclei are accelerated through a rigidity-dependent mechanism. The fit results suggest sources characterized by relatively low maximum injection energies, hard spectra and heavy chemical composition. We also show that uncertainties about physical quantities relevant to UHECR propagation and shower development have a non-negligible impact on the fit results