Ultra High Energy Cosmic Rays at Pierre Auger Observatory

1. Ultra High Energy Cosmic Rays at Pierre Auger Observatory Hernán Wahlberg Universidad Nacional de La Plata

2. Outline • Physics motivation • Previous detection of UHECR • SD: AGASA • FD: HiRes • The Pierre Auger Observatory (PAO) • Physics results from PAO • Energy spectrum • Composition • Anisotropy • Conclusion and future prospects

3. What do we want to know? • Energy spectrum • Is there a cut off (GZK) ? • Arrival direction • Isotropic or correlated with astronomic sources ? • Mass composition • photons , protons, nuclei, neutrinos ?

4. Energy spectrum 1 m-2 second-1 1st Knee 2nd Knee 1 m-2 year-1 Ankle 1 km-2 year-1 Ultra-High Energy Cosmic Rays E (eV)

5. The Extreme Universe AGN Pulsar SNR GRB Radio Galaxy

6. Possible acceleration sites E ~ b Z BmGLkpc • Several possible accelerators in nature up to 1020 eV • Bottom-Up: Fermi acceleration • Extremely difficult to accelerate above 1020 eV • Top-Down: Decay of super heavy relics from early Universe -> photons and neutrinos predicted

7. Interaction of UHE protons • The Greisen-Zatsepin-Kuzmin cut-off • Dominant mechanisms for energy loss p + 2.7k +  p + 0 n + + If particles are observed > 5 x 1019 eV, then they must be local (GZK cut-off) Interaction of protons with intergalactic radiation fields.

8. Where they should not come from… Proton mean energy vs. propagation distance • Constraint on the proximity of UHECR sources. • Modification of the spectrum. GZK energy cut-off IfD>100 MpcE< 100 1018 eV regardless of the initial energy. If UHECR are due to know stable particles the must come for our vicinity.

9. Is it possible to do particle astronomy? Trajectory of protons in the Galaxy Galactic Magnetic Field ~ 2 µG E=1018 eV E=1019 eV E=1020 eV We can do point-source-search astronomy with UHECR

10. Detecting UHECR

11. Shower development

12. Detectors Fluorescence light emitted by the atmospheric nitrogen excited by the shower passage 10 % duty cycle Longitudinal profile is measured Cherenkov light is emitted as relativist muons and electrons pass through the water 100 % duty cycle Lateral profile is measured

13. Previous experiments and measurements

14. AGASA - Surface Detector Array • 100 km2 scintillator array • Operation 1991 – 2004 • Measure via footprint on ground. Akeno Giant Air Shower Array • High duty-cycle. • Exposure is easily estimated • Self-calibration with • atmospheric muons. • X Energy measurement • relies on assumptions • about interaction models.

15. Energy spectrum by AGASA Top-down? Bottom-up with GZK Cutoff New analysis! M. Teshima

16. HiRes – Fluorescence Detectors High Resolution Fly’s Eye (Utah) • Nearly calorimetric energy measurement. X Low duty-cycle. X Aperture is not easily determined. X Atmospheric uncertainty X Fluorescence yield. HiRes 1 HiRes 2 C. Finley

17. Energy Spectrum by HiRes (1999-2004) (1996-2005) Consistent with GZK Cutoff C. Finley

18. Energy Spectrum AGASA (SD) HiRes (FD) Ralph Engel

19. The Pierre Auger Observatory A new cosmic ray observatory designed for a high statistics study of the Highest Energy Cosmic Rays The Collaboration Argentina Mexico Australia Netherlands Bolivia* Poland Brazil Slovenia Czech Rep. Spain France UK Germany USA Italy Vietnam* *Associate 63 Institutions, 369 Collaborators

20. Hybrid instruments A unique and powerful design Surface detector array +fluorescence detectors Calorimetric energy calibration form fluorescence detector transferred to the event gathering power of the surface array. A complementary set of mass sensitive shower parameters. Different measurement techniques force understanding of systematic uncertainties 4 Eyes (6x4 telescopes) 1600 Water Tanks 1.5 km spacing 3000 km2

21. Full sky coverage N Northern Auger in Colorado Southern Auger in Argentina S • Low population density. • Favourable atmospheric conditions (clouds, rain, light, aerosol).

22. The southern location (Malargüe–Argentina)

23. Six Telescopes looking at 30o x 30o each

24. 3.4 meter diameter segmented mirror 440 pixel camera Aperture stop and optical filter The Fluorescence Detector

25. Atmospheric Monitoring and Calibration Atmospheric Monitoring Absolute Calibration Central Laser Facility Drum for uniform camera illumination – end to end calibration . Lidar at each fluorescence eye

26. GPS antenna Communications antenna Electronics enclosure Solar panels Battery box 3 – nine inch photomultiplier tubes Plastic tank with 12 tons of water The Surface Array Detector Station

28. Hybrid Event Θ~ 30º,E ~ 8x1018 eV Lateral density distribution Flash ADC traces

29. Hybrid Event Θ~ 30º, E~ 8x1018eV Tanks Time μ sec Pixels Angle Χ Energy

30. Energy spectrum

31. The energy converter S(1000) at 38o • Hybrid events • Compare ground parameter S(1000) • with the fluorescence detector energy. • Transfer the energy converter to the surface array only events. 1 EeV 10 EeV 100 EeV Arisaka

32. Preliminary energy spectrum  E3 (2006)

33. Mass composition

34. Primaries and shower development photons protons iron Xmax R N° particles Δt

35. Hadrons vs. photons • Separating photon showers from events initiated by nuclear primaries is much easier than distinguishing light and heavy primaries! <2005: Upper limits to the photon fraction only with ground arrays. AUGER: Photon discrimination with Xmax using HYBRID events. Best limit so far!

36. Real data vs. photon simulation Data Set: Hybrid events (Jan04 – Feb 06) E>1019 eV 29 events satisfy the selection criteria. For each event, high statistics shower simulations. Dg Differences Dg between photon prediction and data range from 2.0 to 3.8 standard deviations. Event 1687849: Xmax = 780 + 28(stat) + 23(syst) g cm-2 MC photons : <Xmax>= 1000 g cm-2 , rms=71 g cm-2

37. Photon fraction upper limit (E>10EeV) Astropart.Phys.27:155-168,2007.

38. Arrival direction

39. Angular resolution of Auger SD AGASA Crucial for anisotropy studies >1 EeV >3 EeV Hybrid angular resolution 0.5 º (mean) >10 EeV HiRes (Stereo)

40. Anisotropy around galactic enter AGASA 4.5 σ SUGAR 2.9 σ

41. Auger around galactic centre Astropart.Phys.27:244-253,2007.

42. Sky map of Auger data set Auger latitude = -36 Preference view to the Galactic center. Limited coverage in Northern region If super-GZK events come from a finite set of local sources in the North we could miss them… Galactic Coordinates

43. Conclusions • Pierre Auger Observatory status • SD: 30 times larger than AGASA. (>3/4 complete) • FD: 4 stations of HiRes-like telescopes. (4/4 complete.) • Hybrid observation is giving critical information to determine the energy and composition. • First estimate of the energy spectrum. GZK feature? • Upper limit to photon fraction using FD technique for the first time. New limits soon with SD technique. • Anisotropy studies • no hints of anisotropies in the region of the GC. • No excess of events from the GP or SGP.

44. Future plans • Complete Auger South end 2007. • Use rapidly expanding data set to enable • Improvement in the energy assignment. • High statistics study of the spectrum in the GZK region. • Anisotropy studies and point source searches. • Composition studies. • Reduce systematic uncertainties. • Exploit events beyond a zenith angle of 60º. • search for neutrinos • Begin work on Auger North.