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Pierre Auger Observatory

Cosmic ray study is essential to understand the astrophysics mechanisms in the universe. During the last three decades, several very energetic cosmic rays (more than 1020eV where observed by several detectors. Such an event is very scarce and not still explained. If we want to explain this phenomena, it's necessary to measure in a very precise way their energy distribution, their direction and to determine their nature.

The flux of such particles is extremely weak : its estimation is 0.02/km2/year. So, to obtain about a hundred events per year, we need to build a detector covering a very important ground area.
Cosmic rays are charged particles (usually a proton or a heavy nucleus) that constantly rain down on us from space. When a cosmic ray particle reaches Earth, it collides with a nucleus high in the atmosphere, producing many secondary particles, which share the original primary particle's energy. The secondary particles subsequently collide with other nuclei in the atmosphere, creating a new generation of energetic particles that continue the process, multiplying the total number of particles. The resulting particle cascade, called an extensive air shower, arrives at ground level with billions of energetic particles extending over a very large area.

Since 1993, an international regularly growing collaboration has for aim to build such a gigantic detector whose main objective is to determine the nature and the origin of these Ultra High Energy Cosmic Rays (UHECR).
This study had led to a Project Design Report describing a state of the art detector. This report, regularly updated as the technical options progressed and the physical aim of the project widened.

Observatory description

When totally built, the Pierre Auger Observatory will be a hybrid detector, made up of two giant arrays (more than 3 000 km2 each) and two sets of fluorescence detectors (Fly's Eye type) covering the same area as the array. Each array will be composed of about 1600 local stations (tank filled with water in which the particles are detected by the Cerenkov light emitted), spaced by 1.5 km.
UHECR origin being unknown, it is crucial to associate for this research two different sites in order to have an as uniform as possible covering of the sky. This covering is optimum with the chosen technique (which allows the showers reconstruction with almost any incident angles), and with sites located between 35° and 40° of latitude (north and south).

Employing these two complementary observation methods provides the Auger Observatory with high quality information about the types of particles in the primary cosmic rays. Comparing results from the different types of detectors also helps scientists reconcile the two sets of data and produce the most accurate results about the energy of primary cosmic rays. The fluorescence detectors are able to detect the total energy of an air shower, which is approximately equal to the energy of the primary cosmic ray. Total cosmic ray energy is more difficult to determine with the surface detectors, which sample a small fraction of the energy of an air shower.

The main technical difficulty for building such a kind of array is its gigantic size. It is impossible, for cost reasons, to link all these 1600 stations by electrical cables and so, we need them to be autonomous. This implies some constraints on the detector : The fact that the south site can explore, in a privileged way, the galaxy center, and the north site the extra galactic direction will allow to detected any tiny effect due to a particular source localization or magnetic field differences between galactic and extra galactic regions. During 1995 and 1996, a prospection team has evaluated about twenty candidate sites and visited twelve of them. It has presented its conclusions to the collaboration during two general meetings. Two sites has been choosen on topographic, atmospheric, physics criteria, but also considering the scientific and economic support proposed by the candidate countries. The two chosen sites are :

South site construction

For reason cited above, we started with the Argentinean Site construction, which was accepted in 1999 and whose financial schedule spread over the years 2000-2006. The Assembly Building began to be built in March 1999 and the 2000-2002 years were devoted to the first stage realization : the engineering array, whose area was slightly over the one of the largest array functioning then (AGASA in Japan). It was constituted of about 30 local stations, the central station, a fluorescence detector and two telecommunication stations. All the elements of this testing array were prototypes.

This first part of the project was a success and allow us to design the final detectors. Since the start of 2003 and until the end of 2007, we are producing and installing them.
At the moment (February 2007), the Pierre auger Observatory South Site is composed of:
The observatory is already the world greatest cosmic ray detector.

It will reach its final size at the end of 2007. However, the detector being modular, every adding of tank or fluorescence detector immediately contributes to the statistic increase.
According to the obtained results, the second site, in the northern hemisphere, could start to be built.

Collaboration and French participation

The Auger Project was first proposed in 1992 by Jim Cronin and Alan Watson. Today, more than 200 physicists from 55 institutions around the world are collaborating to build the southern site. The 15 participating countries are sharing the $50 million construction budget, each providing a minor part of the total cost. Alan Watson and Giorgio Matthiae are the current project spokespersons, and Jim Cronin is the spokesman emeritus.

The participating countries are :
The French participation to this collaboration involves laboratories coming from three different CNRS departments. The French groups were present at the very beginning of the project and several general important meetings were hold in Paris. Our teams have actively participated, since 1995, to the technical development phase of the project, with precise responsibilities. Among them, one can cite: We also have a main contribution to the event analyses, and we are interested in different fields: (this list is far from being exhaustive)

"A tragedy in physics is a beautiful theory ruined by an ugly fact"
(Old Chinese proverb)