Ultra high energy particles arrive at Earth constantly. They provide a beam at energies higher than any man-made accelerator, but at a very low rate. Two large experiments, the Pierre Auger Observatory and the Telescope Array experiment, have been taking data for several years now covering together the whole sky. I summarize the most recent measurements from both experiments, I compare their results and, for a change, I highlight their agreements.
Deep Dive into Ultra High Energy Cosmic Rays.
Ultra high energy particles arrive at Earth constantly. They provide a beam at energies higher than any man-made accelerator, but at a very low rate. Two large experiments, the Pierre Auger Observatory and the Telescope Array experiment, have been taking data for several years now covering together the whole sky. I summarize the most recent measurements from both experiments, I compare their results and, for a change, I highlight their agreements.
Ultra high energy cosmic rays (UHECRs) are subatomic particles that reach the Earth with macroscopic energies (UHE > 10 18 eV). Obviously, accelerating particles to energies of the order of 10 6 TeV is not easy, and the flux of UHECRs is extremely low: only a few particles per year per km 2 reach us. Thus, experiments above the Earth's atmosphere are not feasible. Instead, UHECRs are observed via their interactions with the Earth's atmosphere. When the primary particle enters the top of the atmosphere, it produces a cascade of secondary particles, a.k.a. an extensive air shower. In the process, an enormous amount of energy is deposited in the atmosphere, and billions of secondary particles reach the ground. One detection technique consist of particle counters on the ground to measure the secondary particles, and to infer the properties of the primary particle. In collider terms, UHECRs provide beams of particles with energies of millions of TeV, our experiments are fixed target experiments, and the atmosphere is our calorimeter. The center-of-mass of the first interactions is of the order of hundreds of TeV.
Another detection technique uses the fact that part of the energy deposited in the atmosphere by UHE-CRs is re-emitted isotropically as ultra-violet light. The process is very inefficient, and a primary particle of 10 20 eV is seen from the ground as a 50 W light bulb crossing the atmosphere at the speed of light.
There are currently two large experiments taking statistically significant data at ultra-high energies. Namely, the Pierre Auger Observatory (Auger) in Argentina, and the Telescope Array experiment (TA) in Utah, USA. In this paper I present the most recent measurements produced by these two large International collaborations. Most of these results have been presented recently at the International Cosmic Ray Conference in Beijing, China.
A powerful feature of the design of both Auger and TA is the capability of observing air showers simulta-neously by the two different but complementary techniques mentioned in the Introduction.
Both experiments consist of a large array of particle detectors on the ground. This surface array measures the particle densities as the shower strikes the Earth. Due to its large area and almost 100% duty cycle, it provides the statistics needed to study these rare particles.
On dark moonless nights, air fluorescence1 telescopes record the development of the shower that results from the interaction of the primary particle with the upper atmosphere. By recording the light produced by the developing air shower, fluorescence telescopes can make a nearly calorimetric measurement of the energy. This energy calibration can then be transferred to the surface array with its 100% duty factor and large event gathering power. The energy determination is therefore done with minimal reliance on either numerical simulations, or assumptions about the composition, or interaction models. (But as anyone that has ever participated in the energy calibration of a calorimeter can understand, the calibration of a time dependent calorimeter of thousands of cubic kilometers is not a trivial task.)
The Pierre Auger Observatory features an array of over 1600 water Cherenkov detectors spread over 3000 km 2 , and arranged on a triangular grid, with the sides of the triangles being 1.5 km [1]. Four fluorescence detector (FD) stations, each containing six fixed telescopes designed to detect air-fluorescence light, overlook the ground array. The surface detector (SD) stations measure the density distribution of the air shower cascade as it strikes the ground while the FD telescopes measure the light produced by atmospheric nitrogen excited by the cascading shower.
XXXI PHYSICS IN COLLISION, Vancouver, BC Canada, August 28 -September 1, 2011 This combined approach is called the hybrid detection technique.
The primary purpose of the FD is to measure the longitudinal profile of showers recorded by the SD whenever it is dark and clear enough to make reliable measurements of atmospheric fluorescence from air showers. The integral of the longitudinal profile is used to determine the shower energy, and the speed of shower development is indicative of the primary particle’s mass. The hybrid detector has better angular resolution than the surface array alone.
The Auger site in Argentina is in the Province of Mendoza near the city of Malargüe. The site is located at a latitude of 35 • south with a mean altitude of 1400 m a.s.l. The site is a relatively flat alluvial plain, sufficiently large to easily encompass the required 3000 km 2 footprint of the array. In Fig. 1 I compare the size of the Auger array with the area around the LHC. There are convenient elevated positions on the edge of the array that allow placement of the four FD stations slightly above ground level. Each SD station is a cylindrical water tank of 3.7 m in diameter and 1.2 m deep, holding 12 metric tons of filtered highly deionized
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