1. Latest Results on the Highest Energy Cosmic Rays
Higgs-Maxwell Workshop: Edinburgh: 10 Feb 2010
Latest Results on the Highest Energy Cosmic Rays
Alan Watson
University of Leeds

2. Are there hints of new particle physics?
OVERVIEW
Why there is interest in cosmic rays > 1019 eV
The Auger Observatory
Description and discussion of measurements:-
Energy Spectrum
Arrival Directions
Primary Mass
Are there hints of new particle physics?

3. 25 decades in intensity 11 Decades in Energy Flux of Cosmic Rays
1 particle m-2 s-1
Air-showers
25 decades
in intensity
‘Knee’
1 particle m-2 per year
Ankle
1 particle km-2 per year
S Swordy
(Univ. Chicago)
LHC
11 Decades
in Energy

4. Why the interest in studying very high energy CR?
(i) Are there excesses from some regions of sky?
– can there be a cosmic-ray astronomy?
Deflections in magnetic fields:
at ~ 1019 eV: still ~ 10° in Galactic magnetic field
- depending on the direction
For interpretation, and to help deduce the B- fields, we
really need to know Z (we try to infer A)

5. (ii) Can anything be learned from the shape of the spectrum?
Steepening above 5 x 1019 eV predicted
Greisen-Zatsepin-Kuz’min – GZK effect (1966)
γ2.7 K + p  Δ+  n + π+ or p + πo
(sources of photons and neutrinos) or
γIR/2.7 K + A  (A – 1) + n
(IR background more uncertain)
These reactions lead to the ONLY firm
predictions in cosmic rays

6. Interaction Length of protons as function of energy
Leads to attenuation on scale of ~ 100 Mpc
Taylor and Aharonian 2008

7. (iii) How are the particles accelerated?
Synchrotron Acceleration (e.g. CERN)
Emax = ZeBRc
Diffusive Shock Acceleration
Emax = kZeBRc, with k<1
(e.g. Shocks in AGNs, near Black Holes……?)
Observed at interplanetary shocks………

8. B R * Magnetar Emax = kZeBRβc k < 1 Hillas 1984 ARA&A B vs R
Synchrotron Losses B
Colliding Galaxies R

9. To summarise:
Particles of energy near predicted GZK-steepening could
tell us about sources within 70 – 100 Mpc
IF particles are protons, the deflections are expected to be
small enough above ~ 5 x 1019 eV that point sources might
be seen – provided there are not too many.
So, measure:
- energy spectrum - to test prediction
- arrival direction distribution - explore
- mass composition – for interpretation

10. The Pierre Auger Collaboration
*Croatia
Czech Republic
France
Germany
Italy
Netherlands
Poland
Portugal
Slovenia
Spain
United Kingdom
Argentina
Australia
Brasil
*Bolivia
Mexico
USA
*Vietnam
*Associate Countries
~330 PhD scientists from
~100 Institutions and 18 countries
Aim: Find properties of UHECR with unprecedented precision
First discussions in 1991 (Jim Cronin and Alan Watson)

11. Arrays of water- → Cherenkov detectors
Auger Observatory is a HYBRID Detector
Nitrogen fluorescence
as at Fly’s Eye and HiRes
Fluorescence →
AND
Arrays of water → Cherenkov detectors 11

13. 1390 m above sea-level or ~ 875 g cm-2
Area of Lancashire
West Yorkshire
Inside M25
30 x area of Paris
Rhode Island, USA
1390 m above sea-level or ~ 875 g cm-2

14. GPS Receiver
and radio transmission
Fluorescence
Detector site

15. Telecommunication system

16. Zenith Angle ~ 48º Energy ~ 7 x 1019 eV
18 detectors triggered
Lateral density distribution S
Now I would like to show you the observatory in action.
Here is an event of fairly high energy – about 70 EeV - at a moderate zenith angle of 48 degrees. Note that an EeV corresponds to 1 x 10^18 eV.)
At the left are the flash ADC traces of the detector stations that participated in the event. The horizontal axis is in nanoseconds while the vertical axis is in vertical equivalent muons. The three colors are the response to the three PMTs. As you see the signal goes out to 3 mircoseconds.
Here is the pattern of hits on the array with the size of the dot proportional to the log of the signal in the tank. The red arrow showed the reconstructed direction and core position.
The last frame shows the lateral distribution of detector signals from the core position. km
An example of an event recorded
with the Cherenkov detectors

17. Fluorescence telescopes: Number of telescopes: 24
Mirrors: 3.6 m x 3.6 m with field of view 30º x 30º, each telescope is equipped with 440 photomultipliers.
May 3, 2009 17

18. shower-detector plane
FD reconstruction
Signal and timing
Direction & energy
Pixel geometry
shower-detector plane

19. A Hybrid Event
Energy Estimate
from area under
curve
(2.1 ± 0.5) x 1019 eV
must account for
‘missing energy’

20. f = Etot/Eem
1.17 f
1.07
Etot (log10(eV))

21. Results from Pierre Auger Observatory
Data-taking started on 1 January 2004 with
125 (of 1600) water-Cherenkov detectors
6 (of 24) fluorescence telescopes
more or less continuous operation since then
Exposure = 12,790 km2 sr yr
> 1019 eV: (HiRes stereo: 307
> 5 x 1019 eV: : 19
> 1020 eV: : 1)
HiRes Aperture: x 4 at highest energies
x 10 AGASA

22. Auger Energy Calibration
785 EVENTS
S(1000)
log E FD(eV)

24. Energy Spectrum from Auger Observatory
Accepted
Physics Letters B
4 Feb 2010
SD + FD
Schuessler
HE 0114
Above 3 x 1018 eV, the exposure is energy independent: 1% corrections in overlap region
Five-parameter fit: index, breakpoint, index, critical energy, normalization 24

25. Energy Estimates are
model and mass dependent
Takeda et al. ApP 2003

26. For the few events above 1020 eV
Auger (3) and HiRes stereo (1)
Integral flux is (2.36 ± 1.9/1.1) x 10-4 km-2 sr-1yr-1
11 AGASA events
(6.35 ± 1.9) x 10-3 km-2 sr-1 yr-1
a factor of more than 25
Even a factor of x 2 increase in Auger energies
would not be enough to explain difference
Consensus is that Auger and HiRes have got it right

27. Searching for Anisotropies

28. Situation as at November 2007: Science and Astroparticle Physics
ANISOTROPY
Situation as at November 2007: Science and Astroparticle Physics
Correlation with VCV catalogue of AGN for < 3.1°, <75 Mpc and E > 5.5 x 1019 eV
Cen A
27 events
Now recognised as a non-optimum catalogue: event number has doubled

29. Comparison with Swift-BAT AGN density map
The Auger Sky above 60 EeV
Comparison with Swift-BAT AGN density map
5° of smoothing
Simulated data sets based on isotropy (I) and Swift-BAT model (II) compared to data (black line/point). 29

30. Indications on Mass Composition
Anisotropy surely suggests a large proton fraction
Most unexpected result from Pierre Auger Observatory so far points in another direction
Could be indicative of interesting new physics (??)

31. How we try to infer the variation of mass with energy
photons
Xmax
< 2% above 10 EeV
protons
Data ? Fe
Energy per nucleon is crucial
Energy

32. Some Longitudinal Profiles measured with Auger

33. Xmax Resolution

34. Mean Xmax from 3754 events
685

35. Xmax rms for same events
685

36. Accepted by PRL: 29 Jan 2010

37. Some of the outstanding questions:-
Is the spectrum suppression the GZK effect?
Why does AGASA find such a different spectrum?
How can anisotropy and mass data be reconciled?
Could there be something wrong with particle physics
at trans-LHC energies?

38. Need to reconcile:
Anisotropy
- but Xmax suggests diminishing fraction of protons
AGASA result on spectrum
Could cross-section (p-air) be high?
Could leading particle take very little energy?
Could the multiplicity be unexpectedly high?
These features would give
Xmax higher in atmosphere than current models
Reduce fluctuations in Xmax
Flatten particle distribution close to shower axis (AGASA)

39. The p-p total cross-section
LHC measurement of sTOT expected to be at the
1% level
– very useful in the extrapolation up to UHECR energies
10% difference in measurements of
Tevatron Expts:
(log s)
James L. Pinfold IVECHRI

40. LHCf: an LHC Experiment for Astroparticle Physics
LHCf: measurement of
photons and neutral pions
and neutrons in the very forward
region of LHC
Add an EM calorimeter at
140 m from the Interaction
Point (IP1 ATLAS)
For low luminosity running

41. Prospects from LHCf

42. Some of the outstanding questions:-
Is the spectrum suppression the GZK effect?
Why does AGASA find such a different spectrum?
How can anisotropy and mass data be reconciled?
Could there be something wrong with particle physics?
OR:
Cosmic Rays are rather isotropic even above 5 x 1019 eV
They are mainly Fe nuclei
The suppression marks an acceleration limit

43. Next steps:
Run Auger South until at least 2015
Build Auger North (x7 AS) in South East Colorado
Go into space: JEM-EUSO on ISS and free-flyer in 2020s?
There are still lots of questions to answer

44. Back Up Slides

45. The essence of the hybrid approach
Precise shower geometry from
degeneracy given
by SD timing
Essential step towards high quality energy and Xmax resolution
Times at angles, χ , are key to finding Rp

46. Angular Resolution from Central Laser Facility
355 nm, frequency tripled, YAG laser,
giving < 7 mJ per pulse: GZK energy
Mono/hybrid rms 1.0°/0.18°

47. 12/15 events close to AGNs in Veron-Cetty Catalogue

48. Test Using Independent Data Set
8/13 events lined up as before: chance 1/600

49. Period total AGN hits Chance Probability 1 Jan 04 - 26 May 2006 15 12
Using Veron-Cetty AGN catalogue
First scan gave ψ < 3.1°, z < (75 Mpc) and E > 56 EeV
Period
total
AGN
hits
Chance
Probability
1 Jan 04
- 26 May 2006 15 12
3.2
1st Scan
27 May 06 – 31 August 2007 13 8
2.7
1.7 x 10-3
Each exposure was 4500 km2 sr yr
6 of 8 ‘misses’ are with 12° of galactic plane

50. Nature has been unkind (?)
AND
we chose a poor catalogue

51. A clear message from the Pierre Auger Observatory is
that we made it too small
Rate of events that seem to be anisotropically distributed
is only ~ 2 per month

52. Swift-Bat catalogue
Top: Flux-Weighted
Bottom: Unweighted
Smearing takes account of
angular resolution and deflections
Maximum Likelihood fit