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 Arrival direction
Is there a cut off (GZK) ?
Arrival direction
Isotropic or correlated with astronomic sources ?
Mass composition
photons , protons, nuclei, neutrinos ?

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

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
If D>100 Mpc E< 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=1019 eV
E=1018 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
( )
( )
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. A unique and powerful design
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 Low population density.
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. The Fluorescence Detector
3.4 meter diameter
segmented mirror
Here are details of the fluorescence telescopes – the most important feature is the use of Schmidt optics.
The schematic shows features of the fluorescence detector.
The segmented mirror.
440 pixel camera.
That is the use of Schmidt with aperture stop and partial correct ring for reduced spherical aberration. The optical filter that passes the nitrogen fluorescence lines acts as a window to provide a closed and controlled environment.
440 pixel camera
Aperture stop and optical filter

25. Atmospheric Monitoring and Calibration
Absolute Calibration
Central Laser Facility
Atmospheric monitoring and fluorescence telescope calibration are crucial for precision shower energy determination.
Here are some of the several instruments used for atmospheric monitoring and calibration.
Fixed and stearable lasers at the Central Laser Facility in the center of the array can be seen from all of the fluorescence detectors.
In the future lidars on alt-az mounts will be used to regularly profile the atmosphere and will shot the path of taken by big event showers.
Other atmospheric monitoring devices include cloud monitors, horizontal attenuation monitors and radioson bearing balloons.
We have an end-to-end calibration of the fluorescence cameras. Here you see an illuminated dome that covers the telescope aperture.
Drum for uniform camera illumination – end to end calibration .
Lidar at each fluorescence eye

26. The Surface Array Detector Station
GPS antenna
Communications
antenna
Let me describe some of the details of the self-contained surface detector station. It a water cerenkov detector designed to be simple and robust. The Pampa Amarilla is in the foreground and the Andes are in the background. We spent a lot of time out here in the desert and have grown quite fond of it.
Electronics
enclosure
Solar panels
Battery box
3 – nine inch
photomultiplier
tubes
Plastic tank with
12 tons of water

28. Hybrid Event Θ~ 30º,E ~ 8x1018 eV Lateral density distribution
Flash ADC traces
Here is an example of a hybrid event at 8 EeV. On the upper right we see the 7 seven stations hit by the shower with their ADC traces on the left and the lateral distribution below.
The geometrical reconstruction of this event used both fluorescence detector and surface array information. The dashed red line is represents where the show/fluorescence detector plane hits the earth. As before the little arrow is where the surface array reconstructs the core position. Note that the SD core position falls exactly on the shower/detector plane – more on this later.
Lateral density distribution

29. Hybrid Event Θ~ 30º, E~ 8x1018eV Energy Tanks Time μ sec Pixels
Here is more data from the same event.
The plot on the left shows the development of the shower with shower max clearly visible.
To understand the plot on the lower right refer to the sketch which shows the angle x that is make by the advance of the angle X made by the shower with the fluorescence detector.
The plot shows the advance of X with time – time going from bottom to top. On the upper (or end) of the shower you see that projection of the event times of the surface detectors on the curve.
Angle Χ
Energy

30. Energy spectrum

31. The energy converter S(1000) at 38o 1 EeV 10 EeV 100 EeV Arisaka
Hybrid events
Compare ground parameter S(1000)
with the fluorescence detector energy.
Transfer the energy converter to the surface array only events.
Here is a plot of S(1000) – corrected for zenith angle - vs. hybrid reconstructed fluorescence energy from which the energy converter can be obtained.
Note that the systematic error grows when extrapolating this rule to 100 EeV!
Our rapidly increasing data set and reduced systematic uncertainties will be able to dramatically improve the precision of the energy scale.
It may be regarded as the s1000 measurement the shower would have produce if it had arrived at 38deg.
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 :
Xmax = (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: ,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: ,2007.

42. Sky map of Auger data set
Auger latitude = -36
Preference view to the
Galactic center.
Limited coverage in
Northern region
Here is the raw distribution of arrival directions for the data set on the sky in galactic coordinates.
Note the over sampling in the southern hemisphere where the detector is located and the hole in the north
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. Complete Auger South end 2007.
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.
Needs more work