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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 a.a.watson@leeds.ac.uk. OVERVIEW Why there is interest in cosmic rays > 10 19 eV The Auger Observatory Description and discussion of measurements:-

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Latest Results on the Highest Energy Cosmic Rays

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

  2. 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. Flux of Cosmic Rays 1 particle m-2 s-1 Air-showers ‘Knee’ 1 particle m-2 per year 25 decades in intensity 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 theONLYfirm 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. * 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 Aim: Find properties of UHECR with unprecedented precision First discussions in 1991(Jim Cronin and Alan Watson)

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

  12. 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

  13. GPS Receiver and radio transmission Fluorescence Detector site

  14. Telecommunication system

  15. Lateral density distribution Zenith Angle ~ 48º Energy ~ 7 x 1019 eV 18 detectors triggered S km An example of an event recorded with the Cherenkov detectors

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

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

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

  20. 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: 4440 (HiRes stereo: 307 > 5 x 1019 eV: 59 : 19 > 1020 eV: 3 : 1) HiRes Aperture: x 4 at highest energies x 10 AGASA

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

  22. 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

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

  24. 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

  25. Searching for Anisotropies

  26. 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

  27. 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

  28. 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 (??)

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

  30. Some Longitudinal Profiles measured with Auger

  31. Xmax Resolution

  32. Mean Xmax from 3754 events 138 71 34 685

  33. Xmax rms for same events 685 138 71 34

  34. Accepted by PRL: 29 Jan 2010

  35. 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?

  36. 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)

  37. 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 2006 14

  38. 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

  39. Prospects from LHCf

  40. 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

  41. 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

  42. Back Up Slides

  43. The essence of the hybrid approach Preciseshower geometryfrom degeneracy given by SD timing Essential step towards high quality energy and Xmax resolution Times at angles, χ, are key to finding Rp

  44. 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°

  45. 12/15 events close to AGNs in Veron-Cetty Catalogue

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

  47. Using Veron-Cetty AGN catalogue First scan gave ψ < 3.1°, z < 0.018 (75 Mpc) and E > 56 EeV Each exposure was 4500 km2 sr yr 6 of 8 ‘misses’ are with 12° of galactic plane

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

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