1. The Pierre Auger Observatory Nicolás G. Busca Fermilab-University of Chicago FNAL User’s Meeting, May 2006

2. (1 particle per km 2 -century) FNALLHC Ultra High Energy CR (UHECR) spectrum Ankle Trans GZK events? by S. Swordy Cosmic Ray (CR) Spectrum The GZK puzzle: UHECR lose energy very quickly by collisions with the CMB, so they should come from nearby sources. But there are no known nearby sources of CR!

3. The Detection principle: observe the CR shower in the atmosphere. Such a low flux can only be detected with extremely large detectors: the atmosphere is an essential component! We try to observe 1 particle per km 2 per century.

4. The Pierre Auger Observatory Is the largest ground array detector of cosmic rays It’s goal is to study UHECR (E>10 18 eV) It’s design ensures high statistics and a experimentally based energy calibration Detector resolutions are also experimentally determined. Systematic uncertainties are traceable. What are UHECR? Where do they come from? Are they accelerated or do they come from super heavy particle decay? Shape of the spectrum… Questions addressed by Auger:

5. The Hybrid Detector Auger combines two detection techniques: Ground array of water cherenkov detectors (the surface detector or “SD”) The SD provides high statistics ~100% duty cycle. 24 fluorescence telescopes (the fluorescence detector or “FD”) The FD has only a 10% duty cycle: operates only on clear moonless nights. It provides a direct measurement of the primary energy.

6. PAO current deployment status 950 stations operating, >1200 deployed (no electronics yet) 18 fluorescence telescopes Aperture ~ 2000 km 2 sr Official data taking started in Jan. 2004 Expect deployment completion in 2007 60 km

7. The SD 1600 water cherenkov detectors, 3000 km 2 radio antenna solar panel battery box GPS electronics PMT purified water Water tank features: 12 tons of purified water 3 PMTs measure light deposited by shower particles electronics 25ns FADC self-calibration using atmospheric muon flux power supplied by solar panels GPS timing and positioning ~60km

8. An example SD event E ~ 80EeV Timing information is used to reconstruct the shower direction The SD samples a transverse section of the shower. Timing allows for a direction measurement. The signal amplitude characterizes the energy of the shower. Event 1225537 A fit to a lateral distribution function determines amplitude shower front SD stations 6km

9. The FD Sketch of a fluorescence telescope: spherical mirror camera diaphragm The camera: Array of 440 pmt (1.5deg each) 4 buildings with 6 telescopes each

10. An example hybrid event Highest signal tank pixels

11. SD-FD energy calibration Calibration curve from hybrid events:

12. The current Auger Spectrum (first 2 years of data) Features: No composition assumption for 3EeV<E<25EeV No assumption on hadronic interaction models Assumes no dramatic composition change occurs above 25EeV 30% energy systematic uncertainty at low energies and 50% at high energies (due to extrapolation of calibration curve)

13. other Results I didn’t have time to talk about: Sources and anisotropy studies: No excess from the Galactic Centre as claimed by AGASA and SUGAR (Lettesier-Selvon et al., 2005). No excess from galactic/super-galactic plane Photon Flux: < 26% at above 10EeV Auger as a gamma-ray burst observatory: Detection capabilities complementary to satellite detectors (Allard et al., 2005)

14. Some answers and prospects… Is there a GZK suppression in the spectrum? Not enough statistics to address this question yet. Aperture will multiply by ~3 once the observatory is finished. Where do CR come from? No evidence for individual sources, correlations with the galactic plane or global anisotropy. What are CR anyways? < 26% of photons above 10 19 eV. Prospects to study composition by exploiting other capabilities of the detector. Prospects Auger North: site selected, R&D phase.