Surface Detector Array Research Articles

New air fluorescence detectors employed in the Telescope Array experiment

Since 2007, the Telescope Array (TA) experiment, based in Utah, USA, has been observing ultra high energy cosmic rays to understand their origins. The experiment includes a surface detector (SD) array and three fluorescence detector (FD) stations. The FD stations, installed surrounding the SD array, measure the air fluorescence light emitted from extensive air showers (EASs) for precise determination of their energies and species. The detectors employed at one of the three FD stations were relocated from the High Resolution Fly's Eye (HiRes) experiment. At the other two stations, newly designed detectors were constructed for the TA experiment. An FD consists of a primary mirror and a camera equipped with photomultiplier tube pixels. To obtain the EAS parameters with high accuracy, understanding the FD optical characteristics is important. In this paper, we report the characteristics and installation of the new FDs and the performances of the FD components. The results of the monitored mirror reflectance during the observation time are also described in this report.

The Auger Engineering Radio Array

High and ultra-high energy cosmic rays hitting the Earth's atmosphere cause extensive air showers (EAS). In recent years, these cosmic rays have been extensively studied at the Pierre Auger Observatory in Argentina. The EAS mainly consist of charged particles, especially electrons and positrons, which cause electro-magnetic emission in the MHz range by interaction with the Earth's magnetic field. To measure this radio emission, AERA, the Auger Engineering Radio Array, was deployed in October 2010 and commenced regular data acquisition in April 2011. AERA was designed as an engineering array for technology and methodology development towards future large-scale radio arrays. It will allow studies on the radio emission mechanism and the physics capabilities of the detection technique. AERA's unique site within the surface detector array (SD) of the Pierre Auger Observatory provides the possibility of coincident hybrid and super-hybrid EAS detection especially in overlap with the fluorescence telescopes Coihueco and HEAT. Besides a description of the setup, we present an overview of analyses of commissioning data taken between November 2010 and April 2011. Also, we show the first hybrid and self-triggered events detected with AERA in April 2011.

The scaler mode in the Pierre Auger Observatory to study heliospheric modulation of cosmic rays

The impact of the solar activity on the heliosphere has a strong influence on the modulation of the flux of low energy galactic cosmic rays arriving at Earth. Different instruments, such as neutron monitors or muon detectors, have been recording the variability of the cosmic ray flux at ground level for several decades. Although the Pierre Auger Observatory was designed to observe cosmic rays at the highest energies, it also records the count rates of low energy secondary particles (the scaler mode) for the self-calibration of its surface detector array. From observations using the scaler mode at the Pierre Auger Observatory, modulation of galactic cosmic rays due to solar transient activity has been observed (e.g., Forbush decreases). Due to the high total count rate coming from the combined area of its detectors, the Pierre Auger Observatory (its detectors have a total area greater than 16,000m2) detects a flux of secondary particles of the order of ∼108 counts per minute. Time variations of the cosmic ray flux related to the activity of the heliosphere can be determined with high accuracy. In this paper we briefly describe the scaler mode and analyze a Forbush decrease together with the interplanetary coronal mass ejection that originated it. The Auger scaler data are now publicly available.

Trigger Board for the Auger Surface Detector With 100 MHz Sampling and Discrete Cosine Transform

The surface detector array of the Pierre Auger Observatory contains 1600 water Cherenkov detectors spread over an area of 3000 km <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . The Cherenkov light is detected by three 9-inch photomultiplier tubes from which the signals of the anode and last dynode are digitized by 10 bit ADCs. The currently used Front-End Boards equipped with the ACEX <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">®</sup> and Cyclone <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">®</sup> FPGA are sampled with 40 MHz. New requirements from the Auger North (100 MHz) and AMIGA (80 MHz) specification as well as the proposal of new spectral triggers based on the 16-point Discrete Cosine Transform (DCT) impose a new Front End Boards with more powerful FPGA chip with a sufficient amount of DSP blocks. The DCT trigger allows recognition of ADC traces with a very short rise time and fast exponential attenuation related to a narrow, flat muon component of very inclined extensive air showers generated by hadrons and starting their development early in the atmosphere. Ten prototype boards equipped with Altera <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">®</sup> Cyclone <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">®</sup> III FPGA have been fabricated and successively tested in the lab and in real pampas conditions in six test surface detectors within April 19 - July 26, 2009. Boards contain only a single FPGA chip, which implements also the slow channel, in previous three generations supported by the external Dual-Port RAM. Tests confirmed full stability and high reliability of the digital part. Both lab and field tests confirm a high efficiency of the recognition of expected patterns of ADC traces.

Large scale anisotropy studies with the Pierre Auger Observatory

The Pierre Auger Observatory in Argentina measures air showers initiated by cosmic rays at EeV energy scales (1 EeV = 1018 eV). An important goal of the collaboration is to study the distribution of arrival directions of the primary particles and quantify potential anisotropies therein. Using data collected by the surface detector (SD) array of the Pierre Auger Observatory since 1 January 2004, we study large scale anisotropies in different energy windows. Modulations of the event rate within the period of a sidereal day are studied by means of a periodicity analysis accounting for local effects which may induce an artificial anisotropy. We present results for the event rate modulation in right ascension and derive upper limits for the equatorial component of a dipole amplitude (Auger Coll., 2011; Bonino, 2009).

The Status of the Telescope Array experiment

The purpose of The Telescope Array experiment is to identify origin of the ultra high energy cosmic rays. The Telescope Array is a hybrid detector consists of a surface detector array and air fluorescence detectors. This hybrid detector is observing extensive air showers to measure the energy spectrum, anisotropy and composition of Ultra High Energy Cosmic Rays. The detector construction has been completed in March 2008, and the hybrid observation with the full configuration has been running since that time. In this talk, the status of observation and our prospects are described.

The acceptance of surface detector arrays for high energy cosmological muon neutrinos

In order to search for ultra-high energy cosmological earth-skimming muon neutrinos by the surface detector array (SD) similar to one of the Pierre Auger Observatory (PAO), we propose to use the transition electromagnetic radiation at the medium interface induced by earth-skimming muons for triggering a few of aligned neighboring Cherenkov SD stations. Simulations of the acceptance of a modeling SD array have been done to estimate the detection probability of earth-skimming muon neutrinos.

The Pierre Auger Observatory scaler mode for the study of solar activitymodulation of galactic cosmic rays

Since data-taking began in January 2004, the Pierre Auger Observatory has been recording the count rates of low energy secondary cosmic ray particles for the self-calibration of the ground detectors of its surface detector array. After correcting for atmospheric effects, modulations of galactic cosmic rays due to solar activity and transient events are observed. Temporal variations related with the activity of the heliosphere can be determined with high accuracy due to the high total count rates. In this study, the available data are presented together with an analysis focused on the observation of Forbush decreases, where a strong correlation with neutron monitor data is found.

Search for first harmonic modulation in the right ascension distribution of cosmic rays detected at the Pierre Auger Observatory

We present the results of searches for dipolar-type anisotropies in different energy ranges above 2.5 × 10 17 eV with the surface detector array of the Pierre Auger Observatory, reporting on both the phase and the amplitude measurements of the first harmonic modulation in the right-ascension distribution. Upper limits on the amplitudes are obtained, which provide the most stringent bounds at present, being below 2% at 99% C. L. for EeV energies. We also compare our results to those of previous experiments as well as with some theoretical expectations.

Limits on the diffuse flux of ultra-high energy neutrinos using the Pierre Auger Observatory

The surface detector array of the Pierre Auger Observatory is sensitive to ultra-high energy neutrinos in the cosmic radiation. These particles can interact close to ground, both through charged and neutral currents in the atmosphere (down-going) and, for tau neutrinos, through the Earth-skimming mechanism (up-going) where a tau lepton is produced in the Earth's crust that can emerge and decay in the atmosphere. Both types of neutrino-induced events produce an inclined shower that can be identified by the presence of a broad time structure of signals in the water-Cherenkov detectors. Using data collected from the surface detector array of the Pierre Auger Observatory, we present the corresponding limits on the diffuse flux of ultra-high energy neutrinos.

RESULTS FROM THE TELESCOPE ARRAY EXPERIMENT

The Telescope Array (TA) is the largest hybrid detector in the northern hemisphere, which consists of an array of surface detectors (SD) and fluorescence detectors (FD), to explore the origin of ultra-high energy cosmic rays (UHECR) by measuring energy, arrival direction, mass composition, and other characteristics of UHECRs. Here we present the status and preliminary results based on the first data set of the experiment.

A phenomenological model of the muon density profile on the ground of very inclined air showers

Ultra-high energy cosmic rays generate extensive air showers in Earth’s atmosphere. A standard approach to reconstruct the energy of an ultra-high energy cosmic rays is to sample the lateral profile of the particle density on the ground of the air shower with an array of surface detectors. For cosmic rays with large inclinations, this reconstruction is based on a model of the lateral profile of the muon density observed on the ground, which is fitted to the observed muon densities in individual surface detectors. The best models for this task are derived from detailed Monte-Carlo simulations of the air shower development. We present a phenomenological parametrization scheme which allows to derive a model of the average lateral profile of the muon density directly from a fit to a set of individual Monte-Carlo simulated air showers. The model reproduces the detailed simulations with a high precision. As an example, we generate a muon density model which is valid in the energy range 10 18 eV < E < 10 20 eV and the zenith angle range 60 ° < θ < 90 ° . We will further demonstrate a way to speed up the simulation of such muon profiles by three orders of magnitude, if only the muons in the shower are of interest.

Measurement of the energy spectrum of cosmic rays above 1018 eV using the Pierre Auger Observatory

We report a measurement of the flux of cosmic rays with unprecedented precision and statistics using the Pierre Auger Observatory. Based on fluorescence observations in coincidence with at least one surface detector we derive a spectrum for energies above 1018 eV. We also update the previously published energy spectrum obtained with the surface detector array. The two spectra are combined addressing the systematic uncertainties and, in particular, the influence of the energy resolution on the spectral shape. The spectrum can be described by a broken power law E−γ with index γ=3.3 below the ankle which is measured at log10(Eankle/eV)=18.6. Above the ankle the spectrum is described by a power law with index 2.6 followed by a flux suppression, above about log10(E/eV)=19.5, detected with high statistical significance.

Neutrino identification with Auger and [formula omitted] flux limit

The surface detector array of the Pierre Auger Observatory is sensitive to Earth-skimming ν τ . The topological properties of such showers are quite different from hadronic events. Neutrino-induced showers are characterized by very elongated and asymmetric footprints and a significant electromagnetic component. Electrons and gammas produce broad timing signals, whereas hadronic showers give rise to signals mostly due to the surviving secondary muons with a shorter signal in time with respect to an electromagnetic signal. The data collected between 1st January 2004 and 31st August 2007 is used to place an upper limit on the diffuse flux of ν τ . Over this perio there is not a single event that fulfills the selection criteria. The limit for an E −2 differential energy spectrum at 90% C.L. is: E 2 d N ν τ / d E 1.3 × 10 − 7 GeV cm − 2 s − 1 sr − 1 .

Telescope Array Experiment

The Telescope Array experiment (TA) is a hybrid detector situated in the USA consisting of a surface detector array and fluorescence detectors for observing extensive air showers produced by ultra high energy cosmic rays. The construction of the TA was finished and data taking with the hybrid observation started in March 2008.

Trigger and aperture of the surface detector array of the Pierre Auger Observatory

The surface detector array of the Pierre Auger Observatory consists of 1600 water-Cherenkov detectors, for the study of extensive air showers (EAS) generated by ultra-high-energy cosmic rays. We describe the trigger hierarchy, from the identification of candidate showers at the level of a single detector, amongst a large background (mainly random single cosmic ray muons), up to the selection of real events and the rejection of random coincidences. Such trigger makes the surface detector array fully efficient for the detection of EAS with energy above 3 × 10 18 eV , for all zenith angles between 0 ∘ and 60 ∘ , independently of the position of the impact point and of the mass of the primary particle. In these range of energies and angles, the exposure of the surface array can be determined purely on the basis of the geometrical acceptance.

The 3rd generation Front-End cards of the Pierre Auger surface detectors: Test results and performance in the field

The surface detector array of the Pierre Auger Observatory comprises 1600 water Cherenkov detectors distributed over an area of 3000km2. The Cherenkov light is detected by three 9-in. photo-multiplier tubes from which the signals of the anode and last dynode are digitized by 10bit 40MHz FADCs. An Altera Cyclone FPGA is employed to generate different local triggers and to handle the data transfer to a communication board. After briefly discussing the design of the cards we present an autonomous test-bench, which has been set up in order to test the large number of boards prior to installation in the field. The qualification procedure and the results obtained in the laboratory are presented. Up to three years of operation in the field demonstrate a very good performance and reliability of the Front-End cards.

The present status of the Telescope Array experiment

The Telescope Array(TA) experiment located at western desert in Utah USA (N39.3,W112.9) is designed for observation of air shower from extreme high energy cosmic rays. The TA detector consists of 2 types of detector to enable a cross check on systematic difference from the two main methods of observation for the energy region. One is a Fluorescence detector (FD) for detecting fluorescence light from air shower and another is surface detector (SD) array for detecting air shower particles at ground level. Each SD consists of 2 layers of plastic scintillator with 3m 2 of surface and more sensitive to electromagnetic component in air shower. The full operation using 3FD stations and full SD array has started. Here we present the updated status of Telescope Array experiment.

Upper limit on the diffuse flux of UHE tau neutrinos

The Pierre Auger Observatory has been designed to study the cosmic rays of ultra-high energy by observing extensive atmospheric showers. It also has the capability of detecting ultra-high energy neutrinos by searching for very inclined showers with a significant electromagnetic component. The surface detector array of the Pierre Auger Observatory is sensitive to Earth-skimming tau-neutrinos ν τ that interact in the Earth's crust. For neutrinos in the energy range 2 × 10 17 eV E ν 2 × 10 19 eV , assuming a diffuse spectrum of the form E ν − 2 , data collected between 1 January 2004 and 31 August 2007 yield a 90% confidence-level upper limit on the diffuse ν τ flux of E ν 2 d N ν τ / d E ν 1.3 × 10 − 7 GeV cm − 2 s − 1 sr − 1 .

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

Radio detection of cosmic ray induced air showers has been demonstrated in dedicated setups in the past. Currently, radio detector setups are being tested at the Pierre Auger Observatory, in Malargüe, Argentina, for the detection of ultra high energy cosmic rays. These efforts have been rewarded with promising results. Radio signals from air showers have been measured in coincidence with the Auger surface detector array. One of the current data sets consists of 313 triggered radio events in coincidence with the surface detector of the observatory. These events have been used to make first analyses, which focus on the lateral distribution function of the radio signals and on the reconstruction of the arrival direction of cosmic rays. Furthermore, the radio background data have been studied. A periodic variation corresponding to the apparent motion of the galactic center has been observed. New, stand-alone, radio detectors have been developed and the next phase of testing at the Auger Observatory has started in May 2008. The aim is to show that an independent radio detector at the Auger Observatory with an area of about 20 km 2 is feasible.