Ultrahigh Energy Cosmic Rays at the Pierre Auger Observatory

1. Ultrahigh Energy Cosmic Rays at the Pierre Auger Observatory Carl Pfendner December 4, 2008

2. Knee Why Study UHECRs? after Gaisser • Measured spectrum extends to E > 1020 eV - the highest particle energies observed in the Universe. • Cosmic ray energy spectrum is nonthermal: • Energy distribution has no characteristic temperature. • Energies of the nonthermal Universe (up to 1020 eV) are well beyond the capabilities of thermal emission processes. • Source energy is given to a relatively small number of particles. Ankle

3. Knee Why Study UHECRs? after Gaisser • Origin at energies above GeV is unknown - no astrophysical object has ever been definitively identified as an accelerator of high energy nucleons. • Where and how are cosmic rays accelerated to these energies? • Traditional astrophysical sources or extreme solutions (topological defects, …) • The high energy end of the spectrum probes physics at energies still out of reach of any man-made accelerator - search for new physics… Ankle

4. Knee Why Study UHECRs? after Gaisser • Accessible to experiment: • Energy spectrum. • Chemical composition. • Arrival directions. • Astronomy with charged particles? Problems • Protons and nuclei are charged and therefore subject to deflection in Galactic and intergalactic magnetic fields (of unknown strength)! • Universe is opaque for cosmic rays above ~ 60 EeV… Ankle

12. electrons/positrons photons muons neutrons

13. Cosmic Rays Surface Detector Fluorescence Detector Pierre Auger Observatory

14. Surface Detector

15. 3 – nine inch PMTs Pierre Auger Observatory Surface Detector Array • Each detector station is a 11,000 liter tank filled with pure water. • 3 PMTs (9 inch) per tank measure Cherenkov light from shower particles. • Self-contained stations working on solar power. • Signal is transmitted to central station via radio signal.

16. Detection Techniques • Ground arrays sample the shower front arriving at ground level. • ~ 100% duty cycle. • Shower is sampled at one altitude only, development of the shower in the atmosphere is not seen.

17. Schmidt Design corrector lens (aperture x2) 440 PMT camera 1.5° per pixel segmented spherical mirror aperture box shutter filter UV pass safety curtain

18. Detection Techniques • Particles of the air shower cascade excite air molecules, which fluoresce in the UV (80% between 300 and 450 nm).. • Fluorescence light can be detected with photomultipliers observing the night sky - the shower is seen by a succession of tubes. • Air fluorescence detectors observe shower development in the atmosphere and provide a nearly calorimetric energy estimate (the amount of light is proportional to the number of particles in the shower). • Large instantaneous detector volume. • Operation on clear, moonless nights with good atmospheric conditions, so small duty cycle about 10%.

19. Fluorescence Detector Event • Signal and timing time

20. Pierre Auger Observatory Hybrid Detector • Auger combines a surface detector array (SD) and fluorescence detectors (FD). • 1600 surface detector stations with 1500 m distance. • 4 fluorescence sites overlooking the surface detector array from the periphery. • 3000 km2 area. • 1 Auger year = 30 AGASA years (SD). http://www.auger.org

21. Lateral density distribution Surface Detector Event

22. The Hybrid Approach • Shower-detector-plane (SDP) gives one part of shower direction. • Position of the shower in the SDP from PMT times. • Need to fit for shower impact parameters Rp and angle 

23. GZK Suppression • Cosmic rays interact with the 2.7 K microwave background. • Protons above ~ 51019 eV suffer severe energy loss from photopion production. • Proton (or neutron) emerges with reduced energy, and further interaction occurs until the energy is below the cutoff energy. • Greisen-Zatsepin-Kuz’min Suppression

24. pair production energy loss pion production energy loss pion production rate GZK Suppression

25. GZK Suppression? HiRes AGASA 25% syst. error M. Takeda et al., PRL 81 (1998) 1163 HiRes Collab., PRL 100 (2008) 101101

26. GZK Suppression • The predicted “end to the cosmic ray spectrum” was (finally) observed by the High Resolution Fly’s (HiRes) detector operated between 1997 and 2006 in Utah… • … after the Akeno Giant Air Shower Array (AGASA) (1984 - 2003) had previously cast doubt on it. • HiRes has ~ 5  evidence for suppression in the spectrum. • Confirmed with Auger data. 25% syst. error 25% syst. error 25% syst. error HiRes Collaboration, PRL 100 (2008) 101101

27. Comparison AGASA (surface array) HiRes (fluorescence telescopes) Auger (hybrid)

28. Auger Chemical Composition

29. Cosmic Ray Astronomy Question • The existence of the GZK suppression suggests that sources of the highest energy cosmic rays can be at large distances (= extragalactic). • Is it possible to observe the closest sources (sources within the GZK “horizon”) directly? • Searches for point sources with previous experiments have been unsuccessful. • Magnetic field might scramble arrival directions even for the closest sources. • The number of sources might be large and sources might generally be weak. • From past experience, the signal is expected to be weak, and first evidence for sources might come from a statistical analysis rather than from a direct source search. Possible examples are: • Searches for clustering of cosmic ray arrival directions. • Searches for correlations with known astrophysical source classes (AGN, BL Lacs, …)

30. Cosmic Ray Astronomy? Dolag, Grasso, Springel and Tkachev, astro-ph/0310902 0º 5º

31. Skymap • 472 AGN with z < 0.018 (red crosses), 27 cosmic ray arrival directions with 3.1º circle, color indicates relative exposure, position of CenA (white cross). Auger Collaboration, Science 318 (2007) 938

32. Summary • Anisotropy studies are continuing • Despite setbacks • Chemical Composition studies are inconclusive but ongoing • Auger and HiRes observe a steepening of the spectrum at around 60 EeV consistent with the GZK suppression (Auger in a mostly mass- and model-independent analysis).