Results from Cosmic-Ray Experiments

1. Vasiliki Pavlidou Kavli Institute for Cosmological Physics, The University of Chicago Results from Cosmic-Ray Experiments

2. Outline • Ultra-high--energy cosmic-rays: issues and detection techniques • A hybrid experiment: the Pierre Auger Observatory • Recent results from cosmic ray experiments: GZK flux suppression; anisotropies at the highest energies • Sources of UHE neutrinos and expected fluxes • Neutrino detection techniques for UHE Cosmic-Ray Experiments • Current neutrino limits from Auger and HiRes • Outlook

3. UHECRs: the questions • Highest energy particles (> 1018 eV) • Spectrum? • Protons, heavier nuclei, photons? • Top-down or bottom-up? • Local or cosmological? • Sources? S. Swordy

4. Detecting UHECRs • Detection techniques: • detect fluorescent emittion generated by shower (Fly’s Eye, HiRes: Sokolsky Panofsky Prize talk, session E2) • detect shower footprint on ground(AGASA) Credit: Cosmus team (http://astro.uchicago.edu/cosmus)

5. The Pierre Auger Observatory of Ultra-high Energy Cosmic Rays: a hybrid experiment ~400 scientists from ~70 Institutions and 17 countries Currently: • 1550 tanks taking data • 24 fluorescence telescopes in 4 stations overlooking array AIM: 1600 tanks, 3,000km2

6. Advantages of the hybrid technique • Good statistics (100% duty cycle for SD) • Straight-forward aperture and exposure determination for SD • Model-independent energy calibration for FD  calibration of SD using hybrid events • Accurate geometry reconstruction for hybrid events (arrival direction determination <0.2 degrees)  calibration of SD (arrival direction accuracy typically < 1 degree).

7. Recent CR results and implications for neutrinos and astrophysics

8. 2.81 5.1 2.69 4.2 I. The CR spectrum at the highest energies • CR flux above 1019.6 eV suppressed! (HiRes, Auger) • But what does this mean?

9. E. Armengaudsims by A. Kravtsov Interpretation of CR flux suppression • Possible interpretations: • cosmological suppression due to energy losses on CMB (GZK cutoff) • accelerators running out of steam (cutoff in source spectrum) • How to tell: • Measure spectra of individual nearby sources (difficult!) • if suppression cosmological: highest energy (super-GZK) events only sample local universe (highly anisotropic)  sky distribution of super-GZK events might look anisotropic

10. AGN Cen A II. The UHECR Sky is anisotropic Pierre Auger Collaboration 2007, Science, 318, 939Pierre Auger Collaboration 2008, APh, 29, 188 Patrick Younk talk - session T8, Monday @ 3:30

11. Implications for Charged Particle Astrophysics • Flux suppression at highest energies is cosmological • Sources are extragalactic, and extend to cosmological distances • The highest-energy cosmic-ray sky is anisotropic, but nature of sources is still unclear • Intergalactic B-field small cosmic rays good messengers for mapping the nearby universe • Astrophysics! • UHECR source identification, study • Timely concurrent operation with gamma-ray, neutrino, and low-energy photon observatories • UHECR astronomy possible: time to build a bigger telescope! Auger North

12. Auger North • Planned location in Colorado, US • Full-sky coverage • Optimized for operation in energies where arrival directions are anisotropic • Sufficient exposure(~ 7 x South) to: • Detect individualsources • Calculate fluxes, spectra • Answer fundamentalquestions about nature’smost powerful accelerators, their physics, and their energy sources • Map the Galactic/intergalactic magnetic field! B. Siffert

13. n  p + e- + e ++ +   + e+ + e +  Implications for Neutrino Astrophysics • GZK cutoff observed cosmogenic neutrinos guaranteed! High Energy Proton sees Cosmic Microwave Background as High Energy Gamma Rays! p+cmb+ p + 0  n + + GZK, Photopion, or Cosmogenic Neutrinos

14. GZK neutrinos guaranteed but… • … flux and spectrum are model dependent • conversely: if we measure cosmogenic neutrino flux, we can constrain source models Allard et al ‘06

15. Other potential sources of UHE neutrinos • Astrophysical: Interactions of accelerated hadrons within possible sources (GRBs, AGN) • Exotic: Topological defects, superheavy dark matter L. Cazon fluxes from Protheroe 1999 review, arXiv:astro-ph/9809144

16. Neutrino detection in cosmic ray air shower experiments • Two Detection Channels: • Down-going neutrinos (all flavors) interacting in the atmosphere • Up-going tau neutrinos interacting in Earth crust -> Earth skimming neutrinos Lint () ~ 500 km (for >95 degrees: Earth opaque)‏ LEloss (~ 10 km (for e: much smaller) Ldecay () ~ 50 km (for : much larger)

17. Down-going neutrinos Old hadronic shower Young neutrino shower

18. Earth-skimming neutrinos

19. Auger (and HiRes) neutrino limits Pierre Auger Collaboration 2008, PRL submitted, arXiv:0712.1909(HiRes limits from: K. Martens for the HiRes Collaboration 2007, arXiv: 0707.4417)

20. Outlook • UHE CR results: flux suppression, anisotropies at highest energies • Charged particle astronomy possible; with increased statistics: source identification, measurement of flux, spectra • GZK cutoff observed: CR sources extragalactic, cosmological • Implications for neutrino astrophysics: cosmogenic neutrino flux guaranteed, flux associated with UHECR sources plausible • UHECR experiments capable of discriminating neutrino-like events • Current limits at ~10x GZK flux • Detection of cosmogenic neutrinos, neutrinos from UHECR sources in <10yr in most optimistic models (meaningful constraints guaranteed within same time)