Study of performance improvement for Air Shower Array with Surface Water Cherenkov Detectors

Ground-based observatories allow for the detection of the most energetic forms of cosmic radiation in a multi-messenger context, including cosmic rays with energies above 10 15 eV and gamma rays with energies above 10 10 eV. Current experiments, such as the Pierre Auger Observatory, and forthcoming ones, such as the Cherenkov Telescope Array (CTA), have to deal with interpreting extensive air showers to reconstruct the fluxes of particles arriving at Earth. In this scenario, the general purpose of this thesis is to contribute to the improvement of the techniques used to reconstruct the properties of the primary particle in measurements of extensive air showers. In particular, four individual studies based on the simulation of air showers are presented. First, a study of the fluctuations in the atmospheric depth at which a shower reaches its maximum (X max ) is performed. The best functional form to describe the simulated X max distributions is selected and, along with two alternative functions, parametrized. This study aims to improve the reconstruction of the relative abundances of primary masses in the ultra-high-energy regime (E 0 > 10 17 eV) and to provide a practical tool for any other studies depending on X max distributions. Second, the angular distribution of Cherenkov light emitted from extensive air showers is parametrized using simulations performed in a broad energy range: from 100 GeV to 1 EeV. For this, a functional form to describe the angular distribution of Cherenkov photons is built using analytical arguments, and its parameters are constrained to describe the simulated showers. The resulting parametrization can be used to reconstruct the properties of air showers both through fluorescence detectors and Imaging Atmospheric Cherenkov Telescopes (IACT). Third, a technique to measure X max through IACTs is developed. The proposed technique is shown to provide a resolution comparable to that of fluorescence detectors, allowing for mass composition studies around the knee in the cosmic-ray spectrum. Fourth, the problem of modeling hadronic interactions is visited in the context of the CTA observatory. An analysis based on simulations is performed to determine the extent to which image parameters reconstructed by IACTs are influenced by uncertainties in hadronic interaction models. Then, a computation of the uncertainty on the estimated CTA sensitivity curve due to hadronic interaction models is performed.

Potential for measuring the longitudinal and lateral profile of muons in TeV air showers with IACTs

Above 10\(^{15}\) eV, the CR flux drops below a few tens of particles per m\(^2\)-year. It is no longer possible to detect the incident particles above the atmosphere before they interact. Direct experiments (characterized by a small geometrical factor \(A\cdot \varDelta \varOmega \) [cm\(^2\) sr]) must be replaced by ground-based instruments that cover up to several thousands of km\(^2\), the extensive air shower (EAS) arrays. They use a completely different approach to CR measurements, started by pioneering experiments soon after World War II by Auger, Kohlhorster and Rossi. The EAS arrays are in most cases large area and long duration experiments studying, as accurately as possible, the nature, flux, mass, direction of primary CRs up to the highest energies. Air showers are initiated by primary CRs, through the interaction with a nucleus in the atmosphere. In addition to the hadronic component the decays of short-lived hadrons lead to a shower of particles: photons, electrons, and positrons constitute the electromagnetic (EM) component; muons and neutrinos constitute the penetrating component. All these particles travel at the speed of light in the atmosphere approximately along the direction of the primary CR. High energy primary \(\gamma \)-rays induce an almost pure EM cascade. In Sect. 4.3 we present a simple model for the EM cascade initiated by a \(\gamma \)-ray, which can be mathematically treated using differential transport equations of \(e^\pm \) and \(\gamma \) in the atmosphere. Some simple features, as derived from approximate solutions of the cascade equations, are presented. The cascades initiated by primary CR protons or nuclei have additional features. They are also characterized by large event-to-event fluctuations. Their description is today achieved using full Monte Carlo simulations, which follow the details of the development of the EM and muonic components. It is interesting, however, to obtain (Sect. 4.4) a first-order estimate of the quantities, which can be measured by experiments both for the muonic and the EM components. In the first sections of the chapter, we enumerate the main showers features (denoted as SF1, SF2,..., SF12)) which characterize EM and muonic component of the cascade initiated by \(\gamma \)-rays, protons, and heavier nuclei. These characteristics are confirmed by detailed Monte Carlo simulations of air showers in the atmosphere (Sect. 4.5), which are used by the experiments to interpret their observations. EAS arrays are installed on ground and are sometimes referred to as indirect detection experiments. Modern shower arrays employ complementary techniques (Sect. 4.6) such as scintillators, air Cherenkov detectors, etc. to measure simultaneously as many shower parameters as possible, in order to reduce the model dependence in the energy and mass number \(A\) determination. The features of the EM and muonic cascades will be used in Sects. 4.8, 4.9 and 4.10 to illustrate how indirect experiments can derive the CR flux and properties in the energy region around the knee of CRs.

We propose a new energy estimator to determine the energies of gamma-ray induced air showers based on the lateral distribution of extensive air showers in the energy range between 10 TeV and 1000 TeV. We carry out a detailed Monte Carlo simulation assuming the Tibet air shower array located at an altitude of 4,300 m above sea level. We define S50, which denotes the particle density at 50 m from the air shower axis, as a new energy estimator. Using S50, the energy resolution is estimated to be approximately 16 % at 100 TeV in the range of the zenith angle 𝜃 < 20∘. We find S50 giving a better energy resolution than 27 % for the air shower size (N e) and 30 % for the sum of detected particles ( $\sum \rho $ ), which have been used so far, at 100 TeV. We also compare the reconstructed age distributions of gamma-ray induced air showers and hadronic cosmic-ray induced air showers. The age parameter may help to discriminate between primary gamma rays and hadronic cosmic rays.

Experiments with high angular resolution that can detect gamma rays in the 100 TeV region will help study acceleration mechanisms of gamma-ray sources with morphological characteristics. In recent years, three groups using extensive air-shower (AS) arrays have reported detecting gamma rays in the 100 TeV region from celestial bodies in the galaxy. In extensive air shower experiments, an arrival direction of a cosmic ray is determined by estimating a shape of a front surface of an air shower based on the density distribution and detection time of secondary particles. The density of secondary gamma rays of an air shower is several times that of secondary electrons and positrons in an air shower and the difference increases as the distance from the shower axis increases. Therefore, increasing the sensitivity to detect secondary gamma rays plays an important role in determining the arrival direction of the shower. Monte Carlo simulation was used to investigate the effect of secondary gamma rays on the time determination accuracy of the air shower front. When an AS array capable of measuring secondary gamma rays with 100% sensitivity was installed at an altitude of 4,740 m, the angular resolution for a 500 TeV shower was improved by about 40% compared to an array that could observe only secondary electrons and positrons. The optimum shape of the water Cherenkov detector (WCD), which is highly sensitive to gamma rays, was investigated by Monte Carlo simulation. When a water tank with a radius of 4.5 m and PMTs with a diameter of 8 inches was used, it was found that about 19 PMTs (area ratio of approximately 0.01) were suitable for detection efficiency and timing response at a water depth of 1.6 m.

The Review of [formula omitted]-ray Astronomical Observing Techniques

On the potential of the imaging atmospheric Cherenkov technique for study of the mass composition of primary cosmic radiation in the energy region above 30 TeV

The Yakutsk Extensive Air Shower Array has been continuously operating for more than 50 years (since 1970) and up until recently it has been one of world's largest ground-based instruments aimed at studying the properties of cosmic rays in the ultra-high energy domain. In this report we discuss results recently obtained at the array - on cosmic rays energy spectrum, mass composition and directional anisotropy - and how they fit into the world data. Special attention is paid to the measurements of muonic component of extensive air showers. Theoretical results of particle acceleration at shocks are also briefly reviewed. Future scientific and engineering plans on the array modernization are discussed.

Ground-based gamma-ray astronomy has historically implemented two dramatically different techniques. One method employs Imaging Atmospheric Cherenkov Telescope(s) (IACT) that detect the Cherenkov light generated in the atmosphere by extensive air showers. The other method employs particle detectors that directly detect the particles that reach ground level — known as Extensive Air Shower (EAS) arrays. Until recently, the IACT method had been the only technique to yield solid detections of TeV gamma-ray sources. Utilizing water Chernkov technology, Milagro, was the first EAS array to discover new gamma-ray sources and demonstrated the power of and need for an all-sky high duty cycle instrument in the TeV energy regime. The transient nature of many TeV sources, the enormous number of potential sources, and the existence of TeV sources that encompass large angular areas all point to the need for an all-sky, high duty-factor instrument with even greater sensitivity than Milagro. The High Altitude Water Cherenkov (HAWC) Observatory will be over an order of magnitude more sensitive than Milagro. In this paper we will discuss the results from Milagro and the design of the HAWC instrument and its experimental sensitivity.

The Large High Altitude Air Shower Observatory (LHAASO) is a hybrid extensive air shower (EAS) array with an area of about 1 𝑘𝑚2 at an altitude of 4410 m a.s.l. in Sichuan province, China. It contains three sub-detectors: 1 𝑘𝑚2 array (KM2A) composed of 5195 electromagnetic particle detectors (ED) and muon detectors (MD); water Cerenkov detector array (WCDA) and 18 Wide Field-of-view Cerenkov Telescope Array (WFCTA). One of the main scientific goals is measuring the individual energy spectra of cosmic rays from ~30 TeV to a couple of EeV. The construction of the whole observatory will be completed by the end of 2021. In this paper, a new energy reconstruction method and result based on KM2A simulated nuclei events will be shown.

The detailed simulation of extended air showers (EAS) and their emission of Cherenkov and fluorescence light requires increasing computation time and storage volume with increasing energy of the primary particle. Given these limitations, it is currently challenging to optimize configurations of imaging air Cherenkov telescopes at photon energies beyond approximately 100 TeV. Additionally, the existing simulation frameworks are not capable of capturing the interplay of Cherenkov and fluorescence light emission at large zenith angle distances (≳ 70°), where the collection area of Cherenkov telescopes considerably increases. Here, we present EASpy, a framework for the simulation of EAS at large zenith angles using parametrizations for electron-positron distributions. Our proposed approach for the emission of fluorescence and Cherenkov light and the subsequent imaging of these components by Imaging Atmospheric Cherenkov Telescopes (IACTs) aims to provide flexibility and accuracy while at the same time it reduces the computation time considerably compared to full Monte Carlo simulations. We find excellent agreement of the resulting Cherenkov images when comparing results obtained from EASpy with the de-facto standard simulation tool CORSIKA and sim_telarray. In the process of verifying our approach, we have found that air shower images appear wider and longer with increasing impact distance at large zenith angles, an effect that has previously not been noted. We also investigate the distribution of light on the ground for fluorescence and Cherenkov emission and highlight their key differences to distributions at moderate zenith angles.

Since 1990, the Tibet AS$\gamma$ experiment has been observing gamma rays and cosmic rays with energies greater than several TeV using a surface air shower array. An underground muon detector (MD) array operating since 2014 enables us to significantly discriminate between gamma rays and cosmic rays by counting the number of muons in the air showers. However, discrimination with only the air shower array is challenging. We developed a convolutional neural network (CNN)-based method to improve the sensitivity of gamma-ray measurement data recorded by only the air shower array. The area-under-the-curve values of the CNN method for gamma rays generated by a Monte Carlo (MC) simulation assuming a gamma-ray source (Crab Nebula) were 0.75 at $\sim$10 TeV and 0.83 at $\sim$100 TeV. The detection significances of gamma rays were improved by factors of 1.232 $\pm$ 0.007 at $\sim$10 TeV and 1.557 $\pm$ 0.022 at $\sim$100 TeV. For verification, we applied the proposed method to experimental data including high-purity gamma-ray-like events in the direction of the Crab Nebula, acquired using both arrays. The distributions of gamma-ray-like properties obtained from the CNN were in good agreement with the MC Simulation, with reduced $\chi ^2$ values of 0.507–1.57, corresponding to an upper cumulative probability of 0.120–0.871.

We describe the design and operation of the first stage of the EAS‐UAP extensive air shower array, as a detector of very high energy cosmic rays (1016 > Eo > 1014eV). The array is located at the Campus of Puebla University. It consist of 18 liquid scintillator detectors, with an active surface of 1 m2 each and a detector spacing of 20 m in a square grid. One Auger Water Cherenkov detector is also included as part of the array. In this report we discuss the stability, the calibration, the arrival direction and lateral distribution function reconstruction capabilities of the detector array, as derived from the 10 detectors in operation in the first stage. Our results shows that the angular accuracy in arrival direction is less than 5.5° in the range from 20° to 60°. The measurements in the Water Cherenkov Detector show us the possibility to separete electromagnetic and muon component. The main characteristics of the array allow us also to use it as educational and training facility.

Recent developments in machine learning (ML) techniques present a promising new analysis method for high-speed imaging in astroparticle physics experiments, for example with imaging atmospheric Cherenkov telescopes (IACTs). In particular, the use of timing information with new machine learning techniques provides a novel method for event classification. Previous work in this field has utilised images of the integrated charge from IACT camera photomultipliers, but the majority of current and upcoming IACT cameras have the capacity to read out the entire photosensor waveform following a trigger. As the arrival times of Cherenkov photons from extensive air showers (EAS) at the camera plane are dependent upon the altitude of their emission, these waveforms contain information useful for IACT event classification. In this work, we investigate the potential for using these waveforms with ML techniques, and find that a highly effective means of utilising their information is to create a set of seven additional two dimensional histograms of waveform parameters to be fed into the machine learning algorithm along with the integrated charge image. This appears to be superior to using only these new ML techniques with the waveform integrated charge alone. We also examine these timing-based ML techniques in the context of other experiments.

Context. Measurement of diffuse γ-ray emission from the Milky Way with Imaging Atmospheric Cherenkov Telescopes (IACT) is difficult because of the high level of charged cosmic ray background and the small field of view. Aims. We show that such a measurement is nevertheless possible in the energy band 10−100 TeV. Methods. The minimal charged particle background for IACTs is achieved by selecting the events to be used for the analyses of the cosmic ray electrons. Tight cuts on the event quality in these event selections allow us to obtain a sufficiently low background level to allow measurement of the diffuse Galactic γ-ray flux above 10 TeV. We calculated the sensitivities of different types of IACT arrays for the Galactic diffuse emission measurements and compared them with the diffuse γ-ray flux from different parts of the sky measured by the Fermi Large Area Telescope below 3 TeV and with the astrophysical neutrino signal measured by IceCube telescope. Results. We show that deep exposure of existing IACT systems is sufficient for detection of the diffuse flux from all the Galactic Plane up to Galactic latitude |b| ∼ 5°. The Medium Size Telescope array of the CTA will be able to detect the diffuse flux up 30° Galactic latitude. Its sensitivity will be sufficient for detection of the γ-ray counterpart of the Galactic component of the IceCube astrophysical neutrino signal above 10 TeV. We also propose that a dedicated IACT system composed of small but wide-field-of-view telescopes could be used to map the 10−100 TeV diffuse γ-ray emission from across the whole sky. Conclusions. Detection and detailed study of diffuse Galactic γ-ray emission in the previously unexplored 10−100 TeV energy range is possible with the IACT technique. This is important for identification of the Galactic component of the astrophysical neutrino signal and for understanding the propagation of cosmic rays in the interstellar medium.